CVE-2025-57804: Python-Hyper h2

Medium severity, CVSS 6.9. EPSS: 1.7% chance of exploitation in the next 30 days.

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

Affected products

• Python-Hyper h2: before 4.3.0 (fixed in 4.3.0)

Published 2025-08-25. Last modified 2026-06-17.