HTTP/1.1 Parsing Vulnerability in h11 by Python Hyper

CVE-2025-43859

What is CVE-2025-43859?

CVE-2025-43859 is a vulnerability identified in h11, a Python implementation of HTTP/1.1 developed by Python-hyper. This flaw involves a leniency in the parsing of line terminators within chunked-coding message bodies, which can create conditions that lead to request smuggling vulnerabilities. Organizations utilizing h11 in their applications could face significant risks if their implementation interacts poorly with buggy reverse proxies, potentially allowing malicious actors to manipulate HTTP requests in unexpected ways.

Technical Details

The vulnerability arises from the way h11 processes HTTP/1.1 chunked-coding messages prior to version 0.16.0. Specifically, faulty parsing can occur that enables an attacker to exploit the lenient handling of line terminators, leading to more complex HTTP requests being smuggled past an application’s security measures. Patch version 0.16.0 addresses this parsing issue, but effective mitigation requires that either h11 or the related reverse proxy is corrected, as problems in both components create the exploitable conditions.

Potential Impact of CVE-2025-43859

1. Request Smuggling Attacks: This vulnerability could facilitate request smuggling attacks, allowing malicious requests to be processed in unintended ways, potentially bypassing security protocols and allowing further exploitation of the application.

2. Data Leakage: Exploiting the vulnerability could lead to unauthorized access to sensitive information or data leakage, as attackers could manipulate requests to access protected resources or deliver malicious payloads.

3. Service Disruption: By successfully exploiting the request smuggling technique, attackers may be able to disrupt service for legitimate users, resulting in denial-of-service conditions and damaging the trustworthiness of the affected applications.

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