CVE-2026-55367

High CVSS 7.5

Published 2026-11-04 · last changed 2026-11-04

The fix

We hold no Microsoft remediation data for CVE-2026-55367, which normally means it is not a Windows CVE. For non-Microsoft software the fix is the vendor's own updated release, and the affected versions are listed below.

What it is

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.

Sources

NVD record for CVE-2026-55367

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