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Fencer Vulns
medium 4.9

CVE-2026-41887 json

Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)

published 2026-04-22 6 mo ago modified 2026-10-26 13 d ago also GHSA-xjvc-pw2r-6878 , EUVD-2026-28804

cvss
4.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 41%
public exploits
0
none known

Summary

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.

Patch / fix

  • packagist flarum/core 1.8.16, 2.0.0-rc.1

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
6 mo ago

Exploit prediction

0.47% chance of exploitation in the next 30 days, higher than 41% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 4.9 CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N
GitHub 3.1 4.9 CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
packagist flarum/core
<1.8.16
>=2.0.0-beta.1, <2.0.0-rc.1
1.8.16
2.0.0-rc.1

Affected products

  • flarum framework

Classification

Weakness
  • CWE-22 Improper Limitation of a Pathname to a Restricted Directory
  • CWE-918 Server-Side Request Forgery

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.