# CVE-2026-55367: h2o: heap exhaustion using capsules

Status: published. Published: 2026-11-04. Updated: 2026-11-04.

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to keep buffering received bytes without a per-Capsule limit while waiting for the remainder. Deployments that enabled proxy.connect-udp and accepted CONNECT-UDP requests could exhaust server memory and lose availability. This vulnerability is fixed with commit 4212ce114506a073505fd389db16ad2ba4e582a0.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- h2o / h2o: < 4212ce114506a073505fd389db16ad2ba4e582a0 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-770

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/h2o/h2o/pull/3754 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-55367; retrieved 2026-11-04; source updated 2026-11-04

Revision: 2055af16d3c73160f61cebe69e95bb120019e81a2c1942c2e6b9ebfb29fffd64

Canonical: https://y2.dev/cve/CVE-2026-55367

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