CVE-2023-41419 - Privilege Escalation in Gevent's WSGIServer

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What is CVE-2023-41419?

Security researchers have identified a serious vulnerability in the Gevent library (before version 23.9.), widely used for running Python web servers. The flaw — tracked as CVE-2023-41419 — allows a remote attacker to escalate privileges through the WSGIServer component by sending a specially crafted script. In simple words, an attacker could get more access on your server than they should.

In this post, we’ll break down what went wrong, show how this can be exploited, and give you practical tips to protect your projects.

Background: What is Gevent and WSGIServer?

Gevent is a popular Python networking library that enables asynchronous I/O. Among its features is the WSGIServer, an HTTP server commonly used for hosting Python web applications using the WSGI standard.

If you deploy Flask, Django, or other WSGI apps with Gevent, you’re likely affected unless you’re running Gevent version 23.9. or later.

The Vulnerability Explained

CVE-2023-41419 happens because the WSGIServer component did not safely handle crafted input, which lets an attacker send a script that causes the server to execute code with higher privileges.

This is especially dangerous if you run your server as root, or if there are different trust domains in your app.

How an Attacker Can Exploit CVE-2023-41419

The root cause is improper sanitization or validation of WSGI script input in gevent.pywsgi.WSGIServer. Attackers can send HTTP requests with payloads that are executed by the server, giving them elevated access.

Suppose you’re running a simple WSGI app with Gevent

from gevent.pywsgi import WSGIServer

def app(environ, start_response):
    start_response('200 OK', [('Content-Type', 'text/html')])
    return [b"Hello, World!"]

http_server = WSGIServer(('...', 808), app)
http_server.serve_forever()

On vulnerable versions, an attacker could send a specially crafted HTTP request (not shown here for safety reasons) that abuses certain headers or WSGI environment fields, triggering unintended server-side code execution.

Exploit Example

Let’s imagine the attacker sends headers that get executed by the WSGIServer due to improper sanitization:

GET / HTTP/1.1
Host: your-server
X-WSGI-Script: |malicious_code|

Inside a vulnerable handler, the server improperly executes the contents of X-WSGI-Script because of lack of input sanitization. In real-world attacks, the payload might be Base64 encoded or obfuscated.

> Disclaimer: For safety, we do not provide real-world malicious Python payloads. The key lesson is that user input flows directly to code execution in the vulnerable code path.

Patch & Mitigation

Fixed in Version: 23.9.
Release notes: Gevent Release Notes

pip install -U gevent

`
- Never Run as Root:
Always deploy your servers under restricted user accounts.
- Use WAF or Layer 7 Filtering:
Block suspicious requests and limit who can reach your server.
- Security Reviews:
Regularly audit code and dependencies for vulnerabilities.

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### Official References

- NVD Entry: CVE-2023-41419
- Gevent GitHub Issues
- Original Patch

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### Summing Up

If you use Gevent’s WSGIServer, CVE-2023-41419 is a real risk — it lets remote attackers run code or escalate privileges using crafted input. The fix is simple: upgrade your Gevent install now and encourage others to do the same.

Stay safe and keep your libraries up-to-date!

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*This technical deep dive was made exclusively for our readers seeking to understand vulnerabilities in popular Python frameworks. Pass it on to fellow developers and keep your software secure!*

Timeline

Published on: 09/25/2023 12:15:11 UTC
Last modified on: 12/08/2023 20:51:32 UTC