GHSA-xjvc-pw2r-6878 Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)

• Packagist flarum/core: < 1.8.16 → fixed in 1.8.16 • Packagist flarum/core: >= 2.0.0-beta.1, < 2.0.0-rc.1 → fixed in 2.0.0-rc.1

Summary 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) ''`, an attacker can read arbitrary files reachable by the PHP process (local file inclusion) or trigger outbound HTTP(S) requests (server-side request forgery). Impact An attacker who has compromised — or legitimately obtained — an administrator account can: • Read arbitrary local files reachable by the PHP process (e.g. `/etc/passwd`, `.env`, config files containing database credentials, OAuth secrets, API keys). • 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). The 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. This is a privilege-escalation vulnerability: a forum administrator is not intended to have host-level file read or access to internal network resources. Example payload Submitted via `POST /api/settings` with an admin session: ```json { "theme_primary_color": "#4D698E;@import (inline) '/etc/passwd';" } ``` The 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`. Patches • `flarum/core` 1.8.16 — fix for the 1.x branch. • `flarum/core` 2.0.0-rc.1 — fix for the 2.x branch. The 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`. Workarounds If upgrading is not immediately possible: • Ensure administrator accounts are protected with strong, unique passwords and (where supported) two-factor authentication. • Restrict administrator access to trusted users only. • Review the forum's public `forum.css` for unexpected content that could indicate prior exploitation. There is no configuration-level mitigation on affected versions — the fix requires the upgraded code. Resources • CVE-2023-27577 — the original vulnerability whose patch was incomplete. • GHSA-vhm8-wwrf-3gcw — the original advisory. Credit Reported to the Flarum Foundation by William (Liam) Snow IV (@LiamSnow), discovered during a graduate-level network security lab at Worcester Polytechnic Institute.