CVE-2026-55213: h2o

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1, when h2o processes a QPACK instruction sent from the peer over HTTP/3, lib/http3/qpack.c might allocate an on-stack buffer as large as approximately 800 KB by calling alloca, which exceeds the default pthread stack size used by musl libc and causes the h2o server to crash with a segmentation fault while touching the guard page. This issue is fixed in commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1.

Affected products

• h2o h2o: before edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 (fixed in edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1)

Published 2026-07-10. Last modified 2026-07-13.