CVE-2026-55367

high

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3.

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.

CVSS

7.5

7.5 (CVSS v3.1)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Severity source: NVD CVSS v3.1 (security-advisories@github.com)

Timeline (UTC)

Published: 2026-11-04 22:18

Modified: 2026-11-04 22:18

NVD status: Received

Last synced 2 hours ago

Verified against the source API 2026-11-04 22:37 UTC

Exploitation

Not listed in CISA KEV (as of last catalog sync)

Absence from KEV does not mean a CVE is not exploited. Exploitation status: unknown (no authoritative evidence in our sources).

Affected products

Not reported by the source (unknown — not "none").

Weakness classification (CWE)

CWE-770

Source: NVD / CISA. “NVD-CWE-Other” and “NVD-CWE-noinfo” mean NVD could not assign a specific weakness.

Mitigation & advisories

No vendor advisory or patch reference tagged by NVD. Check the references below.

References

https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0 security-advisories@github.com
https://github.com/h2o/h2o/pull/3754 security-advisories@github.com
https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c security-advisories@github.com

Data sources

NVD CVE API 2.0 — fetched 2026-11-04 22:37 UTC · record ↗

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