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CVE-2026-54340

7.5 · HIGH

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3.

Published Jul 17, 2026CWE-400EPSS 0.49%

Overview

CVE-2026-54340 is a high-severity vulnerability. It was published on July 17, 2026 and has a CVSS 3.1 base score of 7.5 (HIGH).

This vulnerability has a CVSS 3.1 base score of 7.5, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

GitHubgithub.com/h2o/h2o/commit/9265bdd9a996ed992681055e3996baf3e09d2063GitHubgithub.com/h2o/h2o/security/advisories/GHSA-qcrr-wrhc-pgq9

How CyberStrike helps

Frequently asked questions

What is CVE-2026-54340?

CVE-2026-54340 is a high-severity vulnerability. It was published on July 17, 2026 and has a CVSS 3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-54340?

This vulnerability has a CVSS 3.1 base score of 7.5, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-54340?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-54340?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-54340 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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