{
  "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": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-22",
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-22T20:34:52Z",
    "nvd_published_at": null,
    "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": "2026-04-22T20:34:52Z",
  "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": "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)"
}