Werkzeug is one of Python's most commonly used WSGI (Web Server Gateway Interface) web application libraries. It's especially popular for its built-in debugging tools, which help developers track down bugs right from the browser. But in May 2024, a critical vulnerability — CVE-2024-34069 — was revealed in Werkzeug’s debugger. This flaw meant that, under the right circumstances, a remote attacker could execute arbitrary Python code right on a developer’s machine, with the debugger only exposed on localhost!

In this post, I’ll offer a plain-English breakdown of what happened, how the exploit works, and how to stay safe. You’ll find example code, attack scenarios, and resources to dive deeper.

What is Werkzeug?

Werkzeug (“tool” in German) is at the heart of several big Python web frameworks — most famously Flask. As a developer, you might use its built-in debugger to interactively inspect variables and run Python snippets when things go wrong. Normally, this debugger is only visible on your computer (localhost), and it’s protected by a PIN.

The Vulnerability: CVE-2024-34069

CVE-2024-34069 exposes a logic flaw in how Werkzeug’s built-in debugger verifies connections to its "interactive console." Normally, the debugger only responds to local requests (like from 127...1). But the vulnerability allows an attacker who:

Controls a domain (plus at least one subdomain),

- Tricks you into visiting a crafted page on their domain and subdomain (say, evil.com and sub.evil.com),

And guesses a URL in your app that triggers the debugger (such as an error page),

…to execute code on your computer — even though your debugger *seems* only available locally.

Why is this dangerous?

It means your "local-only" debugger can be hijacked from outside your network, putting your files, credentials, and environment at risk.

You run a development server with debugging enabled

from flask import Flask

app = Flask(__name__)
app.debug = True

@app.route("/")
def index():
    # The following will crash and trigger the debugger
    return 1 / 

if __name__ == "__main__":
    app.run()

You access it at http://localhost:500/ and see the classic debug page.

2. Attacker’s Setup

The attacker hosts a site at, e.g., attacker.com and a subdomain at sub.attacker.com. On both, they serve a page containing malicious JavaScript exploiting browser SameSite/CORS rules and your browser's handling of cookies and localhost requests.

Example: The Attacker’s Script

While a working exploit script is complex and beyond the scope of this demo, here's a simplified version the attacker might use to try credentials or PIN brute force:

// Not a real attack script but illustrates the communication attempt
fetch("http://localhost:500/?__debugger__=yes", {
    credentials: "include"
}).then(response => response.text())
  .then(data => {
    // Try to brute-force the PIN or leverage timing attacks
    // Or prompt the user to enter a PIN via phishing
  });

3. Social Engineering & PIN Theft

Now the attacker needs you to visit both their domain and their subdomain and, crucially, enter the debugger PIN. Often, this step is achieved through phishing or by tricking you into thinking you need to enter the PIN for legitimate reasons.

4. Remote Access

Once the attacker has your PIN and makes requests from the right domain/subdomain combo, they can interact with your local Werkzeug debugger session, running arbitrary Python code.

Who’s Impacted?

- Any developer running Werkzeug (or a Werkzeug-based server, like Flask) with the built-in debugger turned on.

Anyone working in *debug mode* with code that may raise errors accessible via HTTP routes.

Even though the attack scenario seems convoluted, experienced attackers can automate the process or target popular projects and open source contributors.

Fixing the Issue

Werkzeug patched this issue in version 3..3. The fix tightens checks around domain requests and the interaction between the PIN and debugger session.

Disable Debug Mode in Production:

Never expose the debugger (app.debug = True) outside your own computer or on public/production servers.

Protect Your PIN:

Never share your Werkzeug debugger PIN. If you suspect it’s leaked, restart your server to get a new one.

References (Read More)

- Werkzeug Release 3..3 with CVE-2024-34069 fix
- NVD CVE-2024-34069 record
- Werkzeug official documentation
- Flask debug mode security FAQ

Conclusion

CVE-2024-34069 is a timely reminder that tools meant for developer convenience can open unintended security holes — even if they *seem* safe to use on your own machine.

Always keep your dependencies up to date, use debug servers only on trustworthy networks, and treat all “local” services as potentially vulnerable.
Stay secure, happy coding!


*This post is exclusive, written in simple language, and summarizes the most important facts about CVE-2024-34069. Reach out if you have further questions or concerns about securing your Python web apps.*

Timeline

Published on: 05/06/2024 15:15:23 UTC
Last modified on: 06/04/2024 17:41:07 UTC