flarum/core

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

Summary

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.

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

Flarum's patch for CVE-2023-27577 restricted the @import and data-uri() LESS features in the custom_less setting, but the same restriction was never applied to other settings registered as LESS config variables (for example theme_primary_color and theme_secondary_color, as well as any key registered via Extend\Settings::registerLessConfigVar()).

Those values are interpolated verbatim into the LESS source at compile time, allowing an authenticated administrator to craft a theme-color value that injects an arbitrary @import directive into the compiled forum.css. Because the underlying LESS parser honours @import (inline) '<path>', an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery).

Impact

An attacker who has compromised — or legitimately obtained — an administrator account can:

• Read arbitrary local files reachable by the PHP process (e.g. /etc/passwd, .env, config files containing database credentials, OAuth secrets, API keys).
• Trigger outbound HTTP/HTTPS requests from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as http://169.254.169.254/ (AWS IMDSv1, GCP, Azure).

The contents of the attacker-controlled import are embedded into the compiled forum.css, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.

This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.

Example payload

Submitted via POST /api/settings with an admin session:

{ "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" }

The setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in /assets/forum.css.

Patches

• flarum/core 1.8.16 — fix for the 1.x branch.
• flarum/core 2.0.0-rc.1 — fix for the 2.x branch.

The fix extends the existing @import / data-uri() validation in Flarum\Forum\ValidateCustomLess::whenSettingsSaving to every dirty setting whose key is registered as a LESS config variable, not just custom_less.

Workarounds

If upgrading is not immediately possible:

• Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.
• Restrict administrator access to trusted users only.
• Review the forum's public forum.css for unexpected content that could indicate prior exploitation.

There is no configuration-level mitigation on affected versions — the fix requires the upgraded code.

Resources

• CVE-2023-27577 — the original vulnerability whose patch was incomplete.
• GHSA-vhm8-wwrf-3gcw — the original advisory.

Credit

Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.