GHSA-XJVC-PW2R-6878
Vulnerability from github – Published: 2026-04-22 20:34 – Updated: 2026-05-13 13:30Summary
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).
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:
{ "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/core1.8.16 — fix for the 1.x branch.flarum/core2.0.0-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.cssfor 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.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 1.8.15"
},
"package": {
"ecosystem": "Packagist",
"name": "flarum/core"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.8.16"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 2.0.0-beta.8"
},
"package": {
"ecosystem": "Packagist",
"name": "flarum/core"
},
"ranges": [
{
"events": [
{
"introduced": "2.0.0-beta.1"
},
{
"fixed": "2.0.0-rc.1"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-41887"
],
"database_specific": {
"cwe_ids": [
"CWE-22",
"CWE-918"
],
"github_reviewed": true,
"github_reviewed_at": "2026-04-22T20:34:52Z",
"nvd_published_at": "2026-05-08T17:16:30Z",
"severity": "MODERATE"
},
"details": "## Summary\n\nFlarum\u0027s 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) \u0027\u003cpath\u003e\u0027`, 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 \u2014 or legitimately obtained \u2014 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 \u2014 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) \u0027/etc/passwd\u0027;\" }\n```\n\nThe setting is stored verbatim, interpolated into the LESS source on the next CSS compile, and the target file\u0027s contents appear in `/assets/forum.css`.\n\n## Patches\n\n- **`flarum/core` 1.8.16** \u2014 fix for the 1.x branch.\n- **`flarum/core` 2.0.0-rc.1** \u2014 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\u0027s public `forum.css` for unexpected content that could indicate prior exploitation.\n\nThere is no configuration-level mitigation on affected versions \u2014 the fix requires the upgraded code.\n\n## Resources\n\n- [CVE-2023-27577](https://nvd.nist.gov/vuln/detail/CVE-2023-27577) \u2014 the original vulnerability whose patch was incomplete.\n- [GHSA-vhm8-wwrf-3gcw](https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw) \u2014 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.",
"id": "GHSA-xjvc-pw2r-6878",
"modified": "2026-05-13T13:30:21Z",
"published": "2026-04-22T20:34:52Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/flarum/framework/security/advisories/GHSA-vhm8-wwrf-3gcw"
},
{
"type": "WEB",
"url": "https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2023-27577"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-41887"
},
{
"type": "WEB",
"url": "https://github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8410"
},
{
"type": "PACKAGE",
"url": "https://github.com/flarum/framework"
},
{
"type": "WEB",
"url": "https://github.com/flarum/framework/releases/tag/v1.8.16"
},
{
"type": "WEB",
"url": "https://github.com/flarum/framework/releases/tag/v2.0.0-rc.1"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N",
"type": "CVSS_V3"
}
],
"summary": "Flarum: Path traversal in LESS parser via theme color settings (incomplete fix for CVE-2023-27577)"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.