Vulnerability · Published Nov 4, 2026 · Assigned by GitHub_M

CVE-2026-55368h2o: buffer overflow when using the %{error}x log directive

High 8.1

In plain English

A high-severity out-of-bounds write in h2o from h2o. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).

Severity (CVSS 3.1)8.1High

Exploit odds (30 days)0.3%top 74.8% of all CVEs

Exploited in the wildNo reportsnot in CISA KEV

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	h2o	no fix listed	DEBIAN-CVE-2026-55368
Ubuntu:24.04:LTS	h2o	no fix listed	UBUNTU-CVE-2026-55368

Affected products (as reported)

Vendor	Product	Versions
h2o	h2o	< 48b7ae12107fd2b118e4641da49599e9aac8edd8

Weakness: CWE-787 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

References

https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf x_refsource_CONFIRM
https://github.com/h2o/h2o/pull/3757 x_refsource_MISC
https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8 x_refsource_MISC