{"id":"GHSA-xjvc-pw2r-6878","aliases":["CVE-2026-41887"],"title":"Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)","titleSource":"OSV","descriptionSource":"OSV","declaredSeverity":"MODERATE","description":"## Summary\n\nFlarum's patch for [CVE-2023-27577](https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw) 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()`).\n\nThose 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).\n\n## Impact\n\nAn attacker who has compromised — or legitimately obtained — an administrator account can:\n\n- **Read arbitrary local files** reachable by the PHP process (e.g. `/etc/passwd`, `.env`, config files containing database credentials, OAuth secrets, API keys).\n- **Trigger outbound HTTP/HTTPS requests** from the Flarum host, enabling SSRF against internal services and cloud metadata endpoints such as `http://169.254.169.254/` (AWS IMDSv1, GCP, Azure).\n\nThe contents of the attacker-controlled import are embedded into the compiled `forum.css`, which is publicly served — so the attacker can retrieve whatever was read simply by fetching the CSS file.\n\nThis is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources.\n\n### Example payload\n\nSubmitted via `POST /api/settings` with an admin session:\n\n```json\n{ \"theme_primary_color\": \"#4D698E;@import (inline) '/etc/passwd';\" }\n```\n\nThe setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file's contents appear in `/assets/forum.css`.\n\n## Patches\n\n- **`flarum/core` 1.8.16** — fix for the 1.x branch.\n- **`flarum/core` 2.0.0-rc.1** — fix for the 2.x branch.\n\nThe fix extends the existing `@import` / `data-uri()` validation in `Flarum\\Forum\\ValidateCustomLess::whenSettingsSaving` to every dirty setting whose key is registered as a LESS config variable, not just `custom_less`.\n\n## Workarounds\n\nIf upgrading is not immediately possible:\n\n- Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication.\n- Restrict administrator access to trusted users only.\n- Review the forum's public `forum.css` for unexpected content that could indicate prior exploitation.\n\nThere is no configuration-level mitigation on affected versions — the fix requires the upgraded code.\n\n## Resources\n\n- [CVE-2023-27577](https://nvd.nist.gov/vuln/detail/CVE-2023-27577) — the original vulnerability whose patch was incomplete.\n- [GHSA-vhm8-wwrf-3gcw](https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw) — the original advisory.\n\n## Credit\n\nReported to the Flarum Foundation by **William (Liam) Snow IV** ([@LiamSnow](https://github.com/LiamSnow)), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.","severity":"medium","score":4.9,"published":"2026-04-22T20:34:52.000Z","modified":"2026-05-13T13:55:21.933Z","withdrawn":false,"knownExploited":false,"affected":[{"ecosystem":"Packagist","name":"flarum/core","sources":[{"name":"OSV · GHSA-xjvc-pw2r-6878","url":"https://osv.dev/vulnerability/GHSA-xjvc-pw2r-6878"}],"ranges":[{"events":[{"introduced":"0"},{"fixed":"1.8.16"}],"type":"ECOSYSTEM"}],"versions":["v0.1.0-beta","v0.1.0-beta.10","v0.1.0-beta.11","v0.1.0-beta.11.1","v0.1.0-beta.12","v0.1.0-beta.13","v0.1.0-beta.14","v0.1.0-beta.14.1","v0.1.0-beta.15","v0.1.0-beta.16","v0.1.0-beta.2","v0.1.0-beta.3","v0.1.0-beta.4","v0.1.0-beta.5","v0.1.0-beta.6","v0.1.0-beta.7","v0.1.0-beta.7.1","v0.1.0-beta.7.2","v0.1.0-beta.8","v0.1.0-beta.8.1","v0.1.0-beta.8.2","v0.1.0-beta.9","v1.0.0","v1.0.1","v1.0.2","v1.0.3","v1.0.4","v1.1.0","v1.1.1","v1.2.0","v1.2.1","v1.3.0","v1.3.1","v1.4.0","v1.4.1","v1.5.0","v1.6.0","v1.6.1","v1.6.2","v1.6.3","v1.7.0","v1.7.1","v1.7.2","v1.8.0","v1.8.1","v1.8.10","v1.8.11","v1.8.12","v1.8.13","v1.8.14","v1.8.15","v1.8.2","v1.8.3","v1.8.4","v1.8.5","v1.8.6","v1.8.7","v1.8.8","v1.8.9"],"fixed":["1.8.16"]},{"ecosystem":"Packagist","name":"flarum/core","sources":[{"name":"OSV · GHSA-xjvc-pw2r-6878","url":"https://osv.dev/vulnerability/GHSA-xjvc-pw2r-6878"}],"ranges":[{"events":[{"introduced":"2.0.0-beta.1"},{"fixed":"2.0.0-rc.1"}],"type":"ECOSYSTEM"}],"versions":["v2.0.0-beta.1","v2.0.0-beta.2","v2.0.0-beta.3","v2.0.0-beta.4","v2.0.0-beta.5","v2.0.0-beta.6","v2.0.0-beta.7","v2.0.0-beta.8"],"fixed":["2.0.0-rc.1"]}],"sourceAffected":[],"references":["https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw","https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878","https://nvd.nist.gov/vuln/detail/CVE-2023-27577","https://nvd.nist.gov/vuln/detail/CVE-2026-41887","https://github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8410","https://github.com/flarum/framework","https://github.com/flarum/framework/releases/tag/v1.8.16","https://github.com/flarum/framework/releases/tag/v2.0.0-rc.1"],"sources":[{"name":"OSV","id":"GHSA-xjvc-pw2r-6878","modified":"2026-05-13T13:55:21.933564Z","url":"https://osv.dev/vulnerability/GHSA-xjvc-pw2r-6878"}],"assessments":[{"source":"OSV","vector":"CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N","score":4.9}],"products":[],"remediation":[],"kev":{"status":"not-listed","checkedAt":"2026-10-28T23:25:34.350Z"},"coverage":[{"source":"NVD","status":"not-checked"},{"source":"GHSA","status":"not-checked"},{"source":"EUVD","status":"not-checked"},{"source":"EPSS","status":"not-checked"}],"evidence":[]}