high7.5
CVE-2026-54340
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3.
Impact
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
| Vendor | Product | Affected versions |
|---|---|---|
| H2o | H2o | from 2026-05-29 before 2026-06-04 |