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

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Summary

CVECVE-2026-41887
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-08 17:16:30 UTC
Updated2026-05-12 16:45:18 UTC
DescriptionFlarum 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.

Risk And Classification

Primary CVSS: v3.1 4.9 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

EPSS: 0.000110000 probability, percentile 0.013390000 (date 2026-05-11)

Problem Types: CWE-22 | CWE-918 | CWE-22 CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') | CWE-918 CWE-918: Server-Side Request Forgery (SSRF)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary4.9MEDIUMCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N
3.1CNADECLARED4.9MEDIUMCVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
High
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Flarum Framework affected < 1.8.16 Not specified
CNA Flarum Framework affected >= 2.0.0-beta.1, < 2.0.0-rc.1 Not specified

References

ReferenceSourceLinkTags
github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
github.com/flarum/framework/releases/tag/v1.8.16 [email protected] github.com
github.com/flarum/framework/releases/tag/v2.0.0-rc.1 [email protected] github.com
github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8410 [email protected] github.com
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