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.

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)

EPSS: 0.005 (40.4th percentile) — read the EPSS interpretation.

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.

Affected products

H2o — versions < 9265bdd

Weakness classification (CWE)

References

security-advisories@github.com (x_refsource_CONFIRM, Third Party Advisory)
security-advisories@github.com (Patch, x_refsource_MISC)

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.