CVE-2026-55368
h2o: buffer overflow when using the %{error}x log directive
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.
Fixes and workarounds
No remediation reported in collected sources.
Missing remediation information is not evidence that no patch exists.
Affected products and versions
| Product | Versions | Authority |
|---|---|---|
| h2o h2o |
< 48b7ae12107fd2b118e4641da49599e9aac8edd8 (affected) | CNA: GitHub_M |
Version ranges are source assertions. Check the vendor advisory and your installed configuration before determining exposure.
CVSS assessments
| Version / score | Vector | Authority |
|---|---|---|
| 3.1 / 8.1 high |
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H | CNA: GitHub_M |
Exploit evidence
No exploit evidence in collected sources. This is not proof of no exploitation.
References
- https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjfCNA: GitHub_M · x_refsource_CONFIRM
- https://github.com/h2o/h2o/pull/3757CNA: GitHub_M · x_refsource_MISC
- https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8CNA: GitHub_M · x_refsource_MISC
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