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Product: h11, version: < 0.16.0

CVE-2025-43859

Severity: CRITICAL

Description: h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

CVSS Score: 9.1

Affected Products:

  • python-hyper h11 - Versions: < 0.16.0

References: