# CVE-2026-55367: h2o HTTP server denial of service in CONNECT-UDP parsing

Source: https://junglewise.ai/threats/cve-2026-55367-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-2026-55367 · Severity: high · CVSS 7.5 · Published 2026-11-04

h2o is an open-source HTTP server supporting HTTP/1.x, HTTP/2, and HTTP/3 protocols. When the CONNECT-UDP proxy feature is enabled, an attacker can send a specially crafted incomplete CONNECT-UDP message with an inflated declared size, causing the server to buffer incoming data indefinitely and exhaust memory, leading to service outages.

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

## Quick answers

**Is CVE-2026-55367 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55367 being exploited in the wild.

**How severe is CVE-2026-55367?** High severity, with a CVSS score of 7.5, published 4 November 2026.

**Which products and versions are affected?** H2o prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0.

**Is there a fix for CVE-2026-55367?** Yes: fixed in commit 4212ce114506a073505fd389db16ad2ba4e582a0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55367 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-55367?** 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 4212ce114506a073505fd389db16ad2ba4e582a0

## Technical details

The vulnerability exists in h2o's CONNECT-UDP capsule parsing logic, where the server lacks per-capsule buffering limits when processing incomplete CONNECT-UDP Capsules. An attacker can send a capsule with an excessively large declared length value; the server will continue buffering received bytes without enforcing a size limit, waiting for the remainder to arrive. This unbounded buffering exhausts server memory and causes denial of service on deployments with proxy.connect-udp enabled.

## Affected products

- h2o h2o prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0

## Timeline

- 2026-11-04: disclosed

## References

- [https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0](https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0)
- [https://github.com/h2o/h2o/pull/3754](https://github.com/h2o/h2o/pull/3754)

## Related threats

- [H2O HTTP server buffer overflow in access-log formatter](https://junglewise.ai/threats/cve-2026-55368-h2o-is-an-http-server-with-support-for-http-1-x-http-2-and-http-3)