| Name | CVE-2026-54340 |
| Description | 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. |
| Source | CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more) |
Vulnerable and fixed packages
The table below lists information on source packages.
| Source Package | Release | Version | Status |
|---|
| h2o (PTS) | bullseye | 2.2.5+dfsg2-6 | vulnerable |
| bookworm | 2.2.5+dfsg2-7 | vulnerable |
The information below is based on the following data on fixed versions.
| Package | Type | Release | Fixed Version | Urgency | Origin | Debian Bugs |
|---|
| h2o | source | (unstable) | (unfixed) | | | |
Notes
[bookworm] - h2o <postponed> (Minor issue, DoS; 2.2.5 h2o_hpack_parse_headers() bounds neither the decoded field count nor retained header state, but amplification requires many deliberately stalled HTTP/2 streams)
[bullseye] - h2o <postponed> (Minor issue, DoS; 2.2.5 h2o_hpack_parse_headers() bounds neither the decoded field count nor retained header state, but amplification requires many deliberately stalled HTTP/2 streams)
https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9
Fixed by (merge commit): https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063