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Back to active CVEs
High · CVSS 7.5CVE-2026-55367

h2o: heap exhaustion using capsulesCVE-2026-55367

Answer in brief

CVE-2026-55367 records a High severity (CVSS 7.5) vulnerability in h2o: heap exhaustion using capsules. The current sources do not mark it as known exploited. The current feed maps h2o/h2o (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Nov 4, 2026Updated Nov 4, 2026Source checked Nov 4, 2026First seen by HOL Nov 4, 2026Material review Nov 4, 2026
Upstream Advisory

Key facts

Risk
High · CVSS 7.5
Exploitation
Not marked as known exploited
Affected software
1 mapped package or product
Fix availability
Available

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps h2o/h2o (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
PackageAffected rangeFixed version
h2o/h2ogeneric<4212ce114506a073505fd389db16ad2ba4e582a04212ce114506a073505fd389db16ad2ba4e582a0

Recommended response

  1. 1Check inventory. Check lockfiles and deployed manifests for h2o/h2o.
  2. 2Review the reported fix. Update h2o/h2o to 4212ce114506a073505fd389db16ad2ba4e582a0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

  1. Published upstream

    Nov 4, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  2. Source modified

    Nov 4, 2026

    Evidence: source:cvelist:source_dates:source-dates:record

  3. First seen by HOL

    Nov 4, 2026

Sources and claim methodology

  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
  • GitHub security advisorygithub.com
Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.

Related CVEs

  • h2o: buffer overflow when using the %{error}x log directiveSame h2o/h2o package

Record context

Vulnerability class
Vulnerability
EPSS
Not reported
CWE IDs
CWE-770
Source
CVE List V5
Source checked
Nov 4, 2026
References
3 linked sources
Open source record