Buffer overflow in H2o
CVE-2026-55368
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8, H2O's access-log formatter could write past its log-line buffer when an affected custom access-log format was configured. The allocation errors affected %{error}x, %q when the query string was empty, and numeric elements whose temporary terminating NUL reached the end of the buffer. A matching request could crash the server, while the effect of the one-byte NUL overwrite beyond a crash was not established. This vulnerability is fixed with commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.
Vulnerability class: Buffer Overflow
Published .
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
- H2o — versions < 48b7ae12107fd2b118e4641da49599e9aac8edd8
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-55368?
- CVE-2026-55368 is a high-severity vulnerability in H2o, classified under Out-of-bounds Write. CVSS score: 8.1/10. Published 2026-11-04.
- How severe is CVE-2026-55368?
- High severity. CVSS v3 base score is 8.1 out of 10.