VulnerabilityAnalyzed

CVE-2026-54340

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3.

HIGH 7.5EPSS 0.49%

Does this matter?

High impact if exploited, but EPSS currently rates exploitation as unlikely (0.49%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.

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.

CVSS 3.1: 7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS: 0.49% probability · 41th percentile
CISA KEV: Not listed
Weakness: CWE-400, CWE-770
Affected: h2o/h2o
Source: security-advisories@github.com

References

https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063Patch
https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9Third Party Advisory

Source: NVD record, EPSS from FIRST.org, KEV from CISA. Refreshed daily.