CVE-2026-54340 | h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and … | CVSS 7.5 HIGH

Description

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 9265bdd, there is an HTTP/2 state amplification issue that combines HPACK decompression amplification with Slowloris-style stream stalling. Amplified decoded header state can be retained by stalled HTTP/2 streams, and depending on the configuration, additional limits are needed to bound decoded header state and prevent attack. This issue has been fixed by commit 9265bdd.

Metadata

CVE ID: CVE-2026-54340
State: PUBLISHED
Assigner: GitHub_M
Reserved: 2026-06-12 19:23 UTC
Published: 2026-07-16 23:29 UTC
Last updated: 2026-07-16 23:29 UTC
Primary CWE: CWE-400
CWE-400: Uncontrolled Resource Consumption
Vendor / Product: h2o / h2o
Sources: cve.org · NVD

Severity & Metrics

7.5 HIGH CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products (1)

Vendor	Product	Platform	Versions
h2o	h2o	—	< 9265bdd

Weakness (CWE)

CWE	Source	Description
CWE-400	cna	CWE-400: Uncontrolled Resource Consumption
CWE-770	cna	CWE-770: Allocation of Resources Without Limits or Throttling

CVSS scores (1)

Score	Severity	Version	Source	Vector
7.5	HIGH	3.1	cna	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References (2)

https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9 https://github.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9
https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063 https://github.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063