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.
| Product | Vendor Advisory | Solution File | Added | Published |
|---|---|---|---|---|
| Freebsd | — | Upgrade h2o | Jun 15, 2026 | Jun 11, 2026 |
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