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.

• CWE-787

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

Published

Nov 4, 2026

Last Modified

Nov 4, 2026

Status

Received

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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