CVE-2026-41887 Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)
An authenticated administrator can exploit path traversal in LESS theme settings to read arbitrary files or perform server-side request forgery.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-41887?
CVE-2026-41887 is a medium-severity path traversal vulnerability in flarum framework, published on 8 May 2026. An attacker needs administrator access to exploit it, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-41887?
4 open source packages ship an affected version, including flarum/core.
| Package | Affected | Fixed in |
|---|---|---|
flarum/core Packagist | All versions | 1.8.16 |
flarum/core Packagist | All versions | 1.8.16 |
flarum/core Packagist | From 2.0.0-beta.1 | 2.0.0-rc.1 |
flarum/core Packagist | From 2.0.0-beta.1 | 2.0.0-rc.1 |
How do I fix CVE-2026-41887?
Upgrade flarum/core to a fixed version for your release line: 1.8.16 or 2.0.0-rc.1.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-41887 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-41887 exists.
Sources
Also known as GHSA-xjvc-pw2r-6878.