Out-of-bounds Read in H2o
CVE-2026-44452
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.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.004 (34.0th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 34th percentile — 34.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: 0.000
Affected products
- H2o — versions < 8dc37cb
Weakness classification (CWE)
References
- github.com/h2o/h2o/commit/8dc37cb1e6171f7f772667618ea440696fed82c3 (Patch)
- github.com/h2o/h2o/security/advisories/GHSA-w68q-rqwx-7wvq (Third Party Advisory, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-44452?
- CVE-2026-44452 is a medium-severity vulnerability in H2o, classified under Out-of-bounds Read. CVSS score: 5.9/10. Published 2026-07-16.
- How severe is CVE-2026-44452?
- Medium severity. CVSS v3 base score is 5.9 out of 10.