---
id: CVE-2026-55367
title: 'h2o: heap exhaustion using capsules'
summary: >-
  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…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-770
vendor: h2o
product: h2o
affected:
  - h2o < 4212ce114506a073505fd389db16ad2ba4e582a0
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-11-05T19:59:16.316316Z'
published: '2026-11-04'
updated: '2026-11-05'
sourceUpdated: '2026-11-05T20:01:20.480Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-55367'
references:
  - url: 'https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c'
    label: 'https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c'
  - url: 'https://github.com/h2o/h2o/pull/3754'
    label: 'https://github.com/h2o/h2o/pull/3754'
  - url: 'https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0'
    label: 'https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0'
tags:
  - cve.org
epss: 0.00339
epssPercentile: 0.27484
ingestedAt: '2026-11-05T22:38:31.690Z'
---

## Overview

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.

## Affected

- `h2o < 4212ce114506a073505fd389db16ad2ba4e582a0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.