CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-04-24 · updated 2026-06-17
Weakness: CWE-444

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.

Affected products

python-hyper h11: before 0.16.0

References

https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed
https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj

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