CVE-2026-55213 h2o: musl libc stack overflow (QPACK)
A remote attacker can crash the h2o HTTP/3 server by sending a crafted QPACK instruction that triggers a stack overflow via alloca.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-55213?
CVE-2026-55213 is a high-severity denial of service vulnerability in h2o, published on 10 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-789
Which versions are affected by CVE-2026-55213?
How do I fix CVE-2026-55213?
No fix has been published for CVE-2026-55213 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55213 being exploited?
There is no known exploitation of CVE-2026-55213 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-55213, GHSA-432c-8xmj-frmq, UBUNTU-CVE-2026-55213.