Vulnerability · Published Jul 10, 2026 · Assigned by GitHub_M

CVE-2026-55213h2o: musl libc stack overflow (QPACK)

High 7.5

In plain English

A high-severity security flaw in h2o from h2o. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 42% of all known vulnerabilities).

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 57.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	h2o	no fix listed	DEBIAN-CVE-2026-55213
Ubuntu:24.04:LTS	h2o	no fix listed	UBUNTU-CVE-2026-55213

Affected products (as reported)

Vendor	Product	Versions
h2o	h2o	< edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1

Weakness: CWE-789 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

https://github.com/h2o/h2o/security/advisories/GHSA-432c-8xmj-frmq x_refsource_CONFIRM
https://github.com/h2o/h2o/commit/edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 x_refsource_MISC