CVE-2026-44452: h2o

Medium severity, CVSS 5.9. EPSS: 0.4% 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 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length hostname while trying to copy the hostname, assuming that it is NULL-terminated. This is a potential denial-of-service attack vector in sense that it might trigger segmentation violation. This issue has been fixed by commit 8dc37cb.

Affected products

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

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