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CVE-2026-55367h2o: heap exhaustion using capsules
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.
7.5 HIGHEPSS 0.33%Not in KEVNVD: Received
Overview
Severity
| Score | Severity | Version | Vector String |
|---|---|---|---|
| 7.5 | HIGH | 3.1GitHub_M (CNA) | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
Details
- CVE ID
- CVE-2026-55367
- Status
- Received
- Assigner (CNA)
- GitHub_M
- Published
- Nov 4, 2026
- Last modified
- 2026-11-05 20:17 UTC
- Weaknesses
- CWE-770 · Allocation of Resources Without Limits or Throttling
Affected Products
As published by the CNA in the CVE record.
h2oh2o1 version range
- < 4212ce114506a073505fd389db16ad2ba4e582a0