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.

• CWE-770

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

Published

Nov 4, 2026

Last Modified

Nov 5, 2026

Status

Received

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