---
id: "CVE-2026-55368"
kind: "vulnerability"
aliases: ["CVE-2026-55368"]
title: "H2O HTTP server: access-log formatter overflow can crash the server"
severity: "high"
cvss: 8.1
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["h2o", "runtime", "self-hosted"]
not_affected_if: ["default-access-log", "no-custom-log-format"]
published_at: "2026-11-04T22:18:06+00:00"
first_seen_at: "2026-11-04T23:08:49+00:00"
updated_at: "2026-11-04T23:09:48+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-55368", "first_seen_at": "2026-11-04T23:08:49+00:00"}]
exploit_sources: []
---

# H2O HTTP server: access-log formatter overflow can crash the server

H2O's access-log formatter can write past its log-line buffer when a custom access-log format uses %{error}x, %q with an empty query string, or numeric elements. A request that matches the affected format can crash the server, and the sources do not establish any further effect from the one-byte NUL overwrite.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check whether the H2O config sets a custom `access-log` format containing `%{error}x`, `%q`, or numeric elements.
- Check the installed H2O version against the fixed commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`.

## Fix

- Move to a build that includes commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`; no fixed release version is stated in the sources.

## Timeline

- 2026-11-04T23:08:49+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-55368

## Documents

- [advisory] Vendor advisory: github.com https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf
- [fix] Patch: github.com https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_