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Vulnerability · Published May 8, 2026 · Assigned by GitHub_M

CVE-2026-41887Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)

Medium 4.9

In plain English

A medium-severity path traversal in framework from flarum. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)4.9Medium
Exploit odds (30 days)0.5%top 59.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Packagistflarum/core1.8.16GHSA-xjvc-pw2r-6878
Packagistflarum/core2.0.0-rc.1GHSA-xjvc-pw2r-6878

Affected products (as reported)

VendorProductVersions
flarum framework < 1.8.16>= 2.0.0-beta.1, < 2.0.0-rc.1

Weakness: CWE-22, CWE-918 · Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

References