CVE-2026-44435: h2o Quicly

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

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit 937d0e9, an assertion failure is raised when the total number of valid handshake messages received over a CRYPTO stream of a single packet number space exceeds 32KB, causing a Denial of Service. This issue has been fixed by commit 937d0e9.

Affected products

• h2o Quicly: before 2026-05-29 (fixed in 2026-05-29)

Published 2026-07-16. Last modified 2026-08-06.