CVE-2026-55213CWE-789

h2o: musl libc stack overflow (QPACK)

High · published July 10, 2026

CVSS v3.1
7.5
EPSS
0%
Percentile
41.1
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v3.1
7.5 · HIGH
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS v4.0
Not supplied
EPSS
0.00488 · 41.1th percentile
Weakness
CWE-789 · Memory Allocation with Excessive Size Value
Published
2026-07-10T20:49Z
EPSS history
Timeline
  • 10 JUL 20:49Z
    h2o: musl libc stack overflow (QPACK)
    cvelistv5