Request Smuggling Vulnerability in Python h11 Package
Summary
DEBIAN-CVE-2025-43859: h11 is a Python implementation of HTTP/1.1. Affects python-h11 versions < 0.14.0-1.1~deb12u1. Fixed in 0.14.0-1.1~deb12u1. How to fix ↓
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Background
The python-h11 package is a Python implementation of HTTP/1.1, designed to facilitate the handling of HTTP requests and responses in a straightforward manner. It is widely used in web applications for its simplicity and efficiency. However, prior to version 0.16.0, h11 exhibited leniency in parsing line terminators in chunked-coding message bodies, making it susceptible to request smuggling vulnerabilities under specific conditions.
Vulnerability Detail
The vulnerability arises from a flaw in the way h11 handles line terminators in chunked-coding message bodies. This leniency can lead to request smuggling, where an attacker can manipulate the HTTP requests sent to a server, potentially allowing them to bypass security measures or execute unauthorized actions. The issue is particularly concerning when h11 is used in conjunction with buggy reverse proxies, as the exploitation of this vulnerability requires a combination of both components being flawed.
How To Fix
Minimum fixed version
0.14.0-1.1~deb12u1
This is the first release that contains the fix.
sudo apt-get install --only-upgrade python-h11Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a workaround, ensure that your application does not rely solely on h11 for HTTP processing, and consider using a more robust reverse proxy that does not exhibit the same vulnerabilities. Additionally, implementing strict validation of incoming requests can help mitigate the risk.
Conclusion
The python-h11 package has a critical vulnerability related to request smuggling due to its lenient parsing of HTTP message bodies. It is crucial for users to update to the patched version to safeguard their applications. Additionally, utilizing tools like Vulert can help monitor and alert for such vulnerabilities in real-time.
See every vulnerability recorded for python-h11 →
FAQ
The vulnerability is a request smuggling issue caused by lenient parsing of line terminators in chunked-coding message bodies.
You can check if your application is affected by using the Vulert playground at Vulert Scanner, which allows you to monitor for open-source vulnerabilities without requiring any signup.
Exploitation of this vulnerability can lead to unauthorized access, data breaches, or the ability to bypass security measures in web applications.
Yes, the vulnerability has been patched in version 0.16.0 of python-h11. It is recommended to update to the latest version to mitigate the risk.
If an immediate update is not possible, consider implementing strict request validation and using a more secure reverse proxy as a temporary measure.
References
Related vulnerabilities
Recent vulnerabilities recorded for python-h11.
