CVE-2026-55213 h2o CVE debrief

Who should care

Users of the h2o HTTP server, especially those using musl libc, should be aware of this vulnerability and take steps to update to the patched version to prevent potential crashes and denials of service. System administrators and security teams responsible for managing h2o deployments should prioritize updating to the patched version. Additionally, developers and maintainers of systems that rely on h2o should review the vulnerability and ensure that their deployments are updated to prevent potential disruptions.

Technical summary

The h2o HTTP server, supporting HTTP/1.x, HTTP/2, and HTTP/3, is vulnerable to a crash when processing QPACK instructions over HTTP/3. The lib/http3/qpack.c file allocates a large on-stack buffer (approximately 800 KB) using alloca, which exceeds the default pthread stack size in musl libc. This results in a segmentation fault when the guard page is touched. The issue was addressed with commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1. The vulnerability affects users of the h2o HTTP server, especially those using musl libc.

Defensive priority

High priority should be given to updating the h2o server to the patched version, especially for environments using musl libc, to mitigate potential crashes and service disruptions.

Recommended defensive actions

• Update h2o server to the version containing commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 or later.
• Review and adjust stack size settings if necessary, though this might not be a recommended long-term solution.
• Monitor server stability and logs for any signs of crashes or segmentation faults.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The CVE record and NVD detail provide information on the vulnerability. The source item and references offer additional context and the fix for the issue. Evidence is limited, and defenders should verify the vulnerability's impact on their systems, especially those using musl libc. The fix was introduced in commit edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1. Users should review their deployments and update to the patched version to prevent potential crashes and denials of service. Limited evidence suggests that the vulnerability is exploitable over HTTP/3.

Official resources