CVE-2026-55213 (CVSS 7.5)

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.

Is your site exposed to CVE-2026-55213?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0034

Probability of exploitation

0.27%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-789 CWE-789

References

Other References

https://github.com/h2o/h2o/commit/edd7a120bfc4af11ac0cbebce2a43cc1f93f9af1 https://github.com/h2o/h2o/security/advisories/GHSA-432c-8xmj-frmq

Frequently Asked Questions

What is CVE-2026-55213? +

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. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-55213? +

CVE-2026-55213 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0034, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-55213? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-19204

A client may send a WebSocket frame with an unknown opcode and a very large declared payload length, causing Jetty …

CVE-2026-12185

In Bouncy Castle for Java before 1.85, BKS/UBER keystore allocates from untrusted lengths before integrity check. This issue also affects …

CVE-2026-58060

In Bouncy Castle for Java before 1.85, HSS public-key level count unbounded, enabling huge allocation on verify. This issue also …

CVE-2026-12852

In Bouncy Castle for Java before 1.85, MLS wire decoder allocates attacker-declared opaque length before bounds check.

CVE-2026-14682

In Bouncy Castle for Java before 1.85, Possible OOM from unbounded up-front allocation on a definite-length read. This issue also …

CVE-2025-26618

Erlang is a programming language and runtime system for building massively scalable soft real-time systems with requirements on high availability. …

CVE-2026-55213