CVE-2026-44453 h2o CVE debrief

Who should care

Users of the h2o HTTP server, operators, platform administrators, vulnerability management teams, and security teams should be aware of this vulnerability and take steps to mitigate it. This includes updating to the latest version of h2o and ensuring that the server is properly configured. Affected deployments should be reviewed for potential exposure, and compensating controls should be implemented while remediation is scheduled and verified.

Technical summary

The h2o HTTP server is vulnerable to a Denial of Service attack when calling alloca under certain conditions. When serving static files, h2o builds the file path on the stack using 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. The issue has been fixed by commit 6b5370d.

Defensive priority

High

Recommended defensive actions

• Update to the latest version of h2o
• Ensure that the server is properly configured
• Monitor server logs for potential attacks
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-07-16T23:16:17.423Z and has not been modified since then. The NVD entry is currently 7.5 HIGH. The h2o HTTP server is vulnerable to a Denial of Service attack when calling alloca under certain conditions. The vulnerability occurs when serving static files and building the file path on the stack using alloca, which can lead to a segmentation fault and crash the server. The issue has been fixed by commit 6b5370d. Evidence limits suggest that defenders verify the affected scope and severity, and review the supplied official advisory or CVE record to validate vendor guidance.

Official resources