GitLab Forking Restriction Bypass (CVE-2025-3396): Anatomy of an Authorization Flaw

Introduction

Intellectual property controls are only as strong as their weakest enforcement point. In July 2025, a subtle but impactful flaw in GitLab's group forking restrictions allowed project owners to sidestep organizational boundaries, risking code leakage and compliance failures. For security teams relying on GitLab's group-level policies to contain sensitive projects, this vulnerability is a wake-up call: API endpoints must be as rigorously protected as the UI.

About GitLab: GitLab is a leading DevOps platform used by millions of developers and thousands of organizations worldwide. It offers both open-source (Community Edition) and commercial (Enterprise Edition) solutions, powering CI/CD pipelines, code collaboration, and secure software delivery for enterprises and startups alike.

Technical Information

Vulnerability Mechanism

CVE-2025-3396 is rooted in an incorrect authorization check on GitLab's project forking API endpoint. When a group enables the Prevent project forking outside current group setting, it intends to restrict forks to within the group hierarchy—crucial for intellectual property containment. However, the API endpoint:

POST /api/v4/projects/:id/fork

failed to properly validate the namespace parameter when invoked by a project owner. By crafting a request specifying a destination namespace outside the allowed group (such as a personal namespace), an attacker with project owner privileges could create forks in unauthorized locations, bypassing group policy.

Exploitation Flow

1. Attacker Requirements:
   • Must be an authenticated project owner within a group that has forking restrictions enabled.
   • Must have API access (personal access token or session).
2. Attack Steps:
   • Craft a POST request to /api/v4/projects/:id/fork with a namespace parameter pointing to an external or personal namespace.
   • The server, lacking proper authorization checks, allows the fork to be created outside the group.

Root Cause:

• The authorization logic did not enforce group-level forking restrictions when handling API requests, specifically failing to validate the destination namespace against group policy.
• Classified as CWE-863 (Incorrect Authorization).

Affected Configurations:

• GitLab EE and CE with group-level forking restrictions enabled.
• Project owner roles with API access.

No user interaction is required (UI:N), and the attack can be performed remotely (AV:N) with low complexity (AC:L).

Patch Information

In the latest GitLab patch release (versions 18.1.2, 18.0.4, and 17.11.6), several critical security vulnerabilities have been addressed to enhance the platform's security posture.

Authorization Bypass Prevention

• CVE-2025-3396: Authenticated project owners could bypass group-level forking restrictions by manipulating API requests. The fix involved reinforcing permission checks within the API endpoints to ensure that group-level policies are consistently enforced, regardless of the request source.

Other Notable Fixes in This Release:

• XSS mitigation (CVE-2025-6948)
• Additional authorization fixes (CVE-2025-4972, CVE-2025-6168)
• rsync utility update

Patch Source:

• GitLab Patch Release Advisory (2025-07-09)

Detection Methods

Detecting vulnerabilities like CVE-2025-3396 requires a combination of automated tools and manual verification processes:

• Automated Scanning:
  • Use vulnerability scanners such as Nessus. Nessus Plugin ID 241691 detects GitLab instances vulnerable to CVE-2025-3396 by checking version numbers.
• Manual Verification:
  • Review GitLab instance version and group forking settings.
  • Audit API logs for suspicious fork requests specifying external namespaces.
• Monitoring:
  • Implement logging and alerting for API activities related to project forking and group-level permissions.

Affected Systems and Versions

• Products: GitLab Community Edition (CE) and Enterprise Edition (EE)
• Vulnerable Versions:
  • 13.3 up to (but not including) 17.11.6
  • 18.0 up to (but not including) 18.0.4
  • 18.1 up to (but not including) 18.1.2
• Vulnerable Configurations:
  • Groups with Prevent project forking outside current group enabled
  • Project owner users with API access

Vendor Security History

GitLab has a well-established track record for security responsiveness:

• Maintains a public bug bounty program (HackerOne)
• Typically releases patches within weeks of disclosure
• Previous vulnerabilities include similar authorization and access control issues, which have been addressed in regular patch cycles
• Transparent advisories and detailed documentation for each security release

References

• NVD: CVE-2025-3396
• HackerOne Report
• GitLab Patch Release Advisory (2025-07-09)
• Nessus Plugin 241691
• CWE-863: Incorrect Authorization

Source: This report was created using AI

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