# CVE-2026-55368: H2O HTTP server buffer overflow in access-log formatter

Source: https://junglewise.ai/threats/cve-2026-55368-h2o-is-an-http-server-with-support-for-http-1-x-http-2-and-http-3  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-04. Updated: 2026-11-04  
License: CC BY 4.0 with a link back

CVE-2026-55368 · Severity: high · CVSS 8.1 · Published 2026-11-04

H2O is an HTTP server that handles web traffic using multiple protocol versions. When certain custom access-log formats are configured, a specially crafted request can cause the server to crash by writing beyond the log buffer's boundaries. An attacker could exploit this to disrupt web service availability.

Vendors: [H2o](https://junglewise.ai/threats/vendors/h2o).

## Quick answers

**Is CVE-2026-55368 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55368 being exploited in the wild.

**How severe is CVE-2026-55368?** High severity, with a CVSS score of 8.1, published 4 November 2026.

**Which products and versions are affected?** H2o prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.

**Is there a fix for CVE-2026-55368?** Yes: fixed in commit 48b7ae12107fd2b118e4641da49599e9aac8edd8. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55368 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-55368?** Check whether you run H2o at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in commit 48b7ae12107fd2b118e4641da49599e9aac8edd8

## Technical details

The access-log formatter in H2O's logconf.c failed to reserve sufficient buffer capacity when formatting log lines with specific format specifiers (%{error}x, %q with empty query string, or numeric elements). This allows a single-byte NUL overwrite past the buffer, causing an out-of-bounds write that crashes the server. A network attacker can trigger this with a matching request requiring no authentication.

## Affected products

- h2o H2O prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8

## Timeline

- 2026-11-04: disclosed
- 2026-10-23: patched: commit 48b7ae1 merged

## References

- [https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8](https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8)
- [https://github.com/h2o/h2o/pull/3757](https://github.com/h2o/h2o/pull/3757)

## Related threats

- [h2o HTTP server denial of service in CONNECT-UDP parsing](https://junglewise.ai/threats/cve-2026-55367-h2o-is-an-http-server-with-support-for-http-1-x-http-2-and-http-3)