CVE-2026-44453 h2o is vulnerable to musl libc stack overflow

High 7.5 Published 16 Jul 2026 Updated 17 Jul 2026

An attacker can crash the h2o HTTP server by triggering a stack overflow via large static file paths, causing a denial of service.

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-2026-44453?

CVE-2026-44453 is a high-severity denial of service vulnerability in h2o, published on 16 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 6b5370d, h2o is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path on stack, by calling alloca. The maximum size of the memory allocated using alloca can be as huge as ~600KB, which exceeds the default pthread stack size used by musl libc (128KB). If the amount of memory allocated by alloca exceeds the stack size, the h2o server crashes with a segmentation fault, while it tries to touch the guard page. This issue has been fixed by commit 6b5370d.

Weakness: CWE-789, CWE-770

Which versions are affected by CVE-2026-44453?

It affects 1 product.

Products include H2o H2o.

How do I fix CVE-2026-44453?

No fix has been published for CVE-2026-44453 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-2026-44453 being exploited?

There is no known exploitation of CVE-2026-44453 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-44453, GHSA-rf9v-m59p-mq84, UBUNTU-CVE-2026-44453.