CVE-2025-43859: Python-Hyper h11

Critical severity, CVSS 9.1. EPSS: 0.6% chance of exploitation in the next 30 days.

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 (fixed in 0.16.0)

Published 2025-04-24. Last modified 2026-06-17.