What’s Vulnerable › Vulnerabilities

CVE-2026-44452: h2o is vulnerable to heap overrun

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-16 · updated 2026-08-06
• Weakness: CWE-125

Description

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 8dc37cb, when h2o receives a ClientHello message over TLS or QUIC and it contains a zero-length SNI extension, the h2o server runs over the zero-length hostname while trying to copy the hostname, assuming that it is NULL-terminated. This is a potential denial-of-service attack vector in sense that it might trigger segmentation violation. This issue has been fixed by commit 8dc37cb.

Affected products

• h2oai/h2o-3: before 2026-05-29 — fixed in 2026-05-29

How to fix

Upgrade to a fixed version: h2oai/h2o-3 2026-05-29. Check the vendor advisory for mitigations.

References

• https://github.com/h2o/h2o/commit/8dc37cb1e6171f7f772667618ea440696fed82c3
• https://github.com/h2o/h2o/security/advisories/GHSA-w68q-rqwx-7wvq

Get alerted when CVE-2026-44452 or the next vulnerability affects your stack