---
id: CVE-2026-55213
title: 'h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3'
summary: >-
  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-s…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cwe:
  - CWE-789
published: '2026-07-10'
updated: '2026-07-10'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-55213'
references:
  - url: 'https://github.com/h2o/h2o/commit/edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1'
    label: security-advisories@github.com
  - url: 'https://github.com/h2o/h2o/security/advisories/GHSA-432c-8xmj-frmq'
    label: security-advisories@github.com
tags:
  - nvd
epss: 0.00488
epssPercentile: 0.39324
ingestedAt: '2026-07-11T20:15:26.889Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.