CVE-2026-54340: h2o

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.

Affected products

• h2o h2o: from 2026-05-29, before 2026-06-04 (fixed in 2026-06-04)

Published 2026-07-17. Last modified 2026-08-05.