Resource exhaustion in H2o
CVE-2026-54340
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.
Vulnerability class: DoS (Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.005 (41.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 41st percentile — 41.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: 0.000
Affected products
- H2o — versions < 9265bdd
Weakness classification (CWE)
References
- github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9 (Third Party Advisory)
- github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063 (Patch)
Frequently asked questions
- What is CVE-2026-54340?
- CVE-2026-54340 is a high-severity vulnerability in H2o, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2026-07-17.
- How severe is CVE-2026-54340?
- High severity. CVSS v3 base score is 7.5 out of 10.