CVE-2026-71554: Python-Hyper h2

Medium severity, CVSS 5.3. EPSS: 0.4% chance of exploitation in the next 30 days.

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, providing a request smuggling primitive. This issue is fixed in version 4.4.1.

Affected products

• Python-Hyper h2: before 4.4.1 (fixed in 4.4.1)

Published 2026-08-06. Last modified 2026-09-18.