CVE-2026-41887

MEDIUM4.9EPSS 0.01%

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

發布日:2026/4/22修改日:2026/5/13

描述

## Summary Flarum'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()`). 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). ## 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](https://nvd.nist.gov/vuln/detail/CVE-2023-27577) — the original vulnerability whose patch was incomplete. - [GHSA-vhm8-wwrf-3gcw](https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw) — the original advisory. ## Credit Reported 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.

受影響套件(1)

Packagist/flarum/corefrom 0, < 1.8.16

CVSS 分數

來源	版本	嚴重程度	向量
osv	CVSS 3.1	MEDIUM4.9	CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

描述

受影響套件(1)

CVSS 分數

參考連結(8)