CVE-2026-54340
highpriority · track · 0EPSS 0.005 · 41 %ileEUUSEPSS · scored 2026-10-23
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
Technical severity, 0–10. One row per scoring source.
base score 0–10 · higher = more severe · hover a vector to decode⚠ sources disagree · medium–high · 6.9–7.5
ssvc
A stakeholder decision — act, attend, or track — from exploitation and impact.
weaknesses
The CWE weakness classes this flaw belongs to.
affected products (2)
Products and version ranges known to be affected.
also known as (2)
Other names and identifiers for this entity.
sources
Which sources assert this record, and when.
references (6)
External citations — patches, advisories, analysis.
- https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063cnnvd
- https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9cnnvd
- https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063enisa_euvd
- https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9enisa_euvd
- https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063Patchnvd
- https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9Third Party Advisorynvd
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