What is CVE-2026-55367?

CVE-2026-55367 is a high-severity vulnerability in H2o, classified under Allocation of Resources Without Limits or Throttling. CVSS score: 7.5/10. Published 2026-11-04.

How severe is CVE-2026-55367?

High severity. CVSS v3 base score is 7.5 out of 10.

Resource exhaustion in H2o

CVE-2026-55367

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to keep buffering received bytes without a per-Capsule limit while waiting for the remainder. Deployments that enabled proxy.connect-udp and accepted CONNECT-UDP requests could exhaust server memory and lose availability. This vulnerability is fixed with commit 4212ce114506a073505fd389db16ad2ba4e582a0.

Published 2026-11-04 · last modified 2026-11-05.

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

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

H2o — versions < 4212ce114506a073505fd389db16ad2ba4e582a0

Weakness classification (CWE)

CWE-770 — Allocation of Resources Without Limits or Throttling

References

Frequently asked questions

What is CVE-2026-55367?: CVE-2026-55367 is a high-severity vulnerability in H2o, classified under Allocation of Resources Without Limits or Throttling. CVSS score: 7.5/10. Published 2026-11-04.
How severe is CVE-2026-55367?: High severity. CVSS v3 base score is 7.5 out of 10.