CVE-2026-55367

h2o: heap exhaustion using capsules

7.5 High

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.

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None

What an attacker can affect

Confidentiality: None
Integrity: None
Availability: High

Exploit likelihood

0.34%Scored on 2026-11-05. The trend appears after a second daily score.

What each source reports

CVE List Score used 7.5

h2o: heap exhaustion using capsules

Record: CVE-2026-55367
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness: CWE-770
Updated: 2026-11-05

EUVDEUVD-2026-117179 7.5

Record: EUVD-2026-117179
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-05

NVD 7.5

Record: CVE-2026-55367
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness: CWE-770
Updated: 2026-11-05

OSV 7.5

h2o: heap exhaustion using capsules

Record: CVE-2026-55367
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness: CWE-770
Updated: 2026-11-05

OSVDEBIAN-CVE-2026-55367 7.5

Record: DEBIAN-CVE-2026-55367
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-05

OSVUBUNTU-CVE-2026-55367 7.5

Record: UBUNTU-CVE-2026-55367
CVSS 3.1: 7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated: 2026-11-06

Affected products

Affected packages

Affected packages
Package	Affected	Fixed
`h2o` Debian:12	All versions, plus listed versions 2.2.5+dfsg2-102.2.5+dfsg2-112.2.5+dfsg2-72.2.5+dfsg2-82.2.5+dfsg2-8.12.2.5+dfsg2-8.1~exp12.2.5+dfsg2-9	None known
`h2o` Ubuntu:Pro:18.04:LTS	All versions, plus listed versions 2.2.3+dfsg-22.2.4+dfsg-12.2.4+dfsg-1build12.2.4+dfsg-1ubuntu0.1~esm2	None known
`h2o` Ubuntu:Pro:20.04:LTS	All versions, plus listed versions 2.2.5+dfsg2-32.2.5+dfsg2-3build12.2.5+dfsg2-3ubuntu0.1~esm1	None known
`h2o` Ubuntu:Pro:22.04:LTS	All versions, plus listed versions 2.2.5+dfsg2-62.2.5+dfsg2-6.12.2.5+dfsg2-6.1ubuntu12.2.5+dfsg2-6.1ubuntu22.2.5+dfsg2-6.1ubuntu2+esm1	None known
`h2o` Ubuntu:24.04:LTS	All versions, plus listed versions 2.2.5+dfsg2-72.2.5+dfsg2-82.2.5+dfsg2-8.1ubuntu12.2.5+dfsg2-8.1ubuntu22.2.5+dfsg2-8.1ubuntu3	None known
`dnsdist` Ubuntu:Pro:24.04:LTS	All versions, plus listed versions 1.8.0-11.8.2-11.8.2-21.8.2-31.8.3-1 Show 4 moreShow less 1.8.3-21.8.3-2build11.8.3-2build21.8.3-2ubuntu0.1~esm1	None known

History

History
Recorded	Changed	Sources
Nov 6, 2026	affected, aliases, references	CVE List, EUVD, NVD, OSV
Nov 5, 2026	affected, aliases, references	OSV
Nov 4, 2026	aliases, published	EUVD, NVD
Nov 4, 2026	first indexed	CVE List

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-55367 is a high vulnerability (CVSS 7.5) in H2o, published November 4, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.34%, the 27th percentile. No source names a fixed version.