---
id: CVE-2026-55368
title: 'h2o: buffer overflow when using the %{error}x log directive'
summary: >-
  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 c…
severity: high
cvss: 8.1
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
cwe:
  - CWE-787
vendor: h2o
product: h2o
affected:
  - h2o < 48b7ae12107fd2b118e4641da49599e9aac8edd8
published: '2026-11-04'
updated: '2026-11-04'
sourceUpdated: '2026-11-04T21:38:04.613Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-55368'
references:
  - url: 'https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf'
    label: 'https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf'
  - url: 'https://github.com/h2o/h2o/pull/3757'
    label: 'https://github.com/h2o/h2o/pull/3757'
  - url: 'https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8'
    label: 'https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8'
tags:
  - cve.org
ingestedAt: '2026-11-05T21:36:00.621Z'
epss: 0.00321
epssPercentile: 0.25234
---

## Overview

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

- `h2o < 48b7ae12107fd2b118e4641da49599e9aac8edd8`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.