flarum/framework

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

• Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
• Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

1. Attacker Setup: Hosts server at evil.host.com.
2. Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
3. Victim Interaction: User clicks link to https://evil.host.com.
4. Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
5. Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

1. Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
2. Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

• ❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
• ✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

• Check PSL status: https://publicsuffix.org/list/

Impact

• Account Takeover: Attacker gains authenticated session access.
• Data Exposure: Email, private messages, and other personal data exposed.
• Exploitable Only If:
  • Parent domain is not PSL-listed.
  • Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

1. Session Token Rotation:

   // After authentication:
   invalidateOldSession();
   const newToken = generateToken();
   

2. Cookie Scoping (already in place):

   // Restrict cookies to explicit subdomain:
   "Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";
   

3. Public Suffix Registration:
   Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

• Attacker controls authoritative subdomain (e.g., evil.host.com).
• Parent domain (host.com) is not PSL-listed.
• Session tokens persist post-authentication.

References

• RFC 6265: HTTP Cookie Handling
• Public Suffix List

Impact

The Flarum /logout route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. Sample: example.com/logout?return=https://google.com. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation.

Some ecosystem extensions modifying the logout route have already been affected. Sample: https://discuss.flarum.org/d/22229-premium-wordpress-integration/526

Patches

The vulnerability has been fixed and published as flarum/core v1.8.5. All communities running Flarum should upgrade as soon as possible to v1.8.5 using:

composer update --prefer-dist --no-dev -a -W

You can then confirm you run the latest version using:

composer show flarum/core

Workarounds

Some extensions modifying the logout route can remedy this issue if their implementation is safe. In any case we recommend updating to 1.8.5.

References

For any questions or comments on this vulnerability, please visit https://discuss.flarum.org/

For support questions, create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.

Impact

The Flarum forum software is affected by a vulnerability that allows an attacker to conduct a Blind SSRF attack or disclose any file on the server, even with a basic user account on any Flarum forum. By uploading a file containing a URL and spoofing the MIME type, an attacker can manipulate the application to execute unintended actions. The vulnerability is due to the behavior of the intervention/image package, which attempts to interpret the supplied file contents as a URL, which then fetches its contents. This allows an attacker to exploit the vulnerability to perform SSRF attacks, disclose local file contents, or conduct a blind oracle attack.

Patches

This has been patched in Flarum v1.8.

Workarounds

As a temporary workaround for the SSRF aspect of the vulnerability, one can disable PHP's allow_url_fopen which will prevent the fetching of external files via URLs.

Credits

Adam Kues - Assetnote