High 7.5Published Jul 16, 2026

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

Updated under an hour ago (Oct 28, 2026, 12:23 AM ET)

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Every CVE for h2o

Exploited in the last 30 days

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: 7.5 HIGH (v3.1, CNA)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness (CWE): CWE-400, CWE-770
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: GitHub_M
NVD analysis: Analyzed
Record last updated: Jul 17, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

h2o h2o h2o:h2o (from the name)	before 9265bdd	CNA record
h2o h2o h2o:h2o	from 2026-05-29 before 2026-06-04	NVD

References

References

https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9	x-refsource-confirm, third-party-advisory
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063	x-refsource-misc, patch

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.