Vulnerability · Published Jul 16, 2026 · Assigned by GitHub_M

CVE-2026-54340h2o has HTTP/2 state amplification

High 7.5

In plain English

A high-severity uncontrolled resource consumption in h2o from h2o. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 43% of all known vulnerabilities).

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 57.4% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	h2o	no fix listed	DEBIAN-CVE-2026-54340
Ubuntu:24.04:LTS	h2o	no fix listed	UBUNTU-CVE-2026-54340

Affected products (as reported)

Vendor	Product	Versions
h2o	h2o	< 9265bdd

Weakness: CWE-400, CWE-770 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9 x_refsource_CONFIRM
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063 x_refsource_MISC