Vulnerability 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 Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| H2O | H2O | >= 2026-05-29, < 2026-06-04 |
Related Weaknesses (CWE)
References
- https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063Patch
- https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9Third Party Advisory
FAQ
What is CVE-2026-54340?
CVE-2026-54340 is a vulnerability with a CVSS score of 7.5 (HIGH). 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...
How severe is CVE-2026-54340?
CVE-2026-54340 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-54340?
Check the references section above for vendor advisories and patch information. Affected products include: H2O H2O.