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 CNA under the mitre root

CNA scorecard C 72/100 over 7994 records in the last 180 days details

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: 2026-10-26 18:10

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

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "framework",
          "vendor": "flarum",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 1.8.16"
            },
            {
              "status": "affected",
              "version": "\u003e= 2.0.0-beta.1, \u003c 2.0.0-rc.1"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
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  "descriptions": [
    {
      "lang": "en",
      "value": "Flarum is open-source forum software. Prior to versions 1.8.16 and 2.0.0-rc.1, Flarum\u0027s 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) \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). This issue has been patched in versions 1.8.16 and 2.0.0-rc.1."
    },
    {
      "lang": "es",
      "value": "Flarum es un software de foro de c\u00f3digo abierto. Antes de las versiones 1.8.16 y 2.0.0-rc.1, el parche de Flarum para CVE-2023-27577 restring\u00eda las caracter\u00edsticas LESS @import y data-uri() en la configuraci\u00f3n custom_less, pero la misma restricci\u00f3n nunca se aplic\u00f3 a otras configuraciones registradas como variables de configuraci\u00f3n LESS (por ejemplo, theme_primary_color y theme_secondary_color, as\u00ed como cualquier clave registrada a trav\u00e9s de Extend\\Settings::registerLessConfigVar()). Esos valores se interpolan literalmente en el c\u00f3digo fuente LESS en tiempo de compilaci\u00f3n, permitiendo a un administrador autenticado crear un valor de color de tema que inyecta una directiva @import arbitraria en el archivo forum.css compilado. Debido a que el analizador LESS subyacente respeta @import (inline) \u2019[path]\u2019, un atacante puede leer ficheros arbitrarios accesibles por el proceso PHP (inclusi\u00f3n local de ficheros) o desencadenar peticiones HTTP(S) salientes (falsificaci\u00f3n de petici\u00f3n del lado del servidor). Este problema ha sido parcheado en las versiones 1.8.16 y 2.0.0-rc.1."
    }
  ],
  "id": "CVE-2026-41887",
  "lastModified": "2026-10-26T18:10:00.173",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 4.9,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "NONE",
          "privilegesRequired": "HIGH",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 1.2,
        "impactScore": 3.6,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-41887",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-05-08T19:26:06.629218Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-05-08T17:16:30.890",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/flarum/framework/commit/2d90a1f19f0e46f8c7e1b07c48ba74b5e38f8410"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/flarum/framework/releases/tag/v1.8.16"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/flarum/framework/releases/tag/v2.0.0-rc.1"
    },
    {
      "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

JSON

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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

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.

Vulnerability timeline

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Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.

Each result shows its source dates. A negative interval means the end event came first; when date-only inputs leave the order open, the result says so. Date-only inputs are approximate. The earliest evidence wins: KEV report dates and sightings are labelled as such. Records describing another vulnerability are listed but never calculated from, and a PoC alone does not establish exploitation. Missing patch evidence does not prove that no patch exists. Filters below change the event list only.

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

FKIE_CVE-2026-41887

GHSA-XJVC-PW2R-6878

Summary

Impact

Example payload

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