CVE-2026-41887

Vulnerability Description

Flarum is open-source forum software. Prior to versions 1.8.16 and 2.0.0-rc.1, 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). This issue has been patched in versions 1.8.16 and 2.0.0-rc.1.

CVSS Score

Related Weaknesses (CWE)

References

• https://github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8
• https://github.com/flarum/framework/releases/tag/v1.8.16
• https://github.com/flarum/framework/releases/tag/v2.0.0-rc.1
• https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878
• https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878