GitHub Security Advisory: GHSA-xjvc-pw2r-6878
Package: flarum/core (COMPOSER)
Severity: MODERATE (CVSS 4.9)
CWE: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal'), CWE-918: Server-Side Request Forgery (SSRF)
Vulnerable versions: <= 1.8.15
Patched in: 1.8.16

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

Details:
## 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://git