PT-2026-57321 · H2O · H2O

Published 2026-07-10 · Updated 2026-07-10 · CVE-2026-55213

Mitre

CVSS v3.1

7.5

High

Base vector

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

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.

Fix

Weakness Enumeration

CWE-789

Related Identifiers

CVE-2026-55213

Affected Products

H2O

References · 3

• https://nvd.nist.gov/vuln/detail/CVE-2026-55213 · Security Note
• https://github.com/h2o/h2o/security/advisories/GHSA-432c-8xmj-frmq · Note
• https://github.com/h2o/h2o/commit/edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 · Note