Vulnerability-Lookup

CVE-2026-41887 (GCVE-0-2026-41887)

Vulnerability from cvelistv5 – Published: 2026-05-08 15:50 – Updated: 2026-05-08 19:26

Title

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

Summary

Flarum is open-source forum software. Prior to versions 1.8.16 and 2.0.0-rc.1, 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). This issue has been patched in versions 1.8.16 and 2.0.0-rc.1.

Severity

4.9 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-08 19:26 UTC

CWE

CWE-22 - Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-918 - Server-Side Request Forgery (SSRF)

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/flarum/framework/security/advi…	x_refsource_CONFIRM
https://github.com/flarum/framework/commit/2d90a1…	x_refsource_MISC
https://github.com/flarum/framework/releases/tag/…	x_refsource_MISC
https://github.com/flarum/framework/releases/tag/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
flarum	framework	Affected: < 1.8.16 Affected: >= 2.0.0-beta.1, < 2.0.0-rc.1	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-41887

Vulnerability from fkie_nvd - Published: 2026-05-08 17:16 - Updated: 2026-06-17 10:47

Severity

4.9 (Medium) - CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8410
	security-advisories@github.com	https://github.com/flarum/framework/releases/tag/v1.8.16
	security-advisories@github.com	https://github.com/flarum/framework/releases/tag/v2.0.0-rc.1
	security-advisories@github.com	https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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      "source": "security-advisories@github.com",
      "url": "https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/flarum/framework/security/advisories/GHSA-xjvc-pw2r-6878"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-22"
        },
        {
          "lang": "en",
          "value": "CWE-918"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-XJVC-PW2R-6878

Vulnerability from github – Published: 2026-04-22 20:34 – Updated: 2026-05-13 13:30

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 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/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.

Severity

4.9 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N

Show details on source website

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{
  "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

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Confirmed: The vulnerability has been validated from an analyst's perspective.
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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.

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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.
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Action not permitted

CVE-2026-41887 (GCVE-0-2026-41887)

FKIE_CVE-2026-41887

GHSA-XJVC-PW2R-6878

Summary

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Example payload

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