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CVE-2026-33807

9.1 · CRITICAL

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins.

Published Apr 15, 2026CWE-436EPSS 0.53%

Overview

CVE-2026-33807 is a critical-severity vulnerability. It was published on April 15, 2026 and has a CVSS 3.1 base score of 9.1 (CRITICAL).

This vulnerability has a CVSS 3.1 base score of 9.1, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical description

@fastify/express v4.0.4 and earlier contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. When a child plugin is registered with a prefix that matches a middleware path, the middleware path is prefixed a second time, causing it to never match incoming requests. This results in complete bypass of Express middleware security controls, including authentication, authorization, and rate limiting, for all routes defined within affected child plugin scopes. No special configuration or request crafting is required.

Upgrade to @fastify/express v4.0.5 or later.

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

Refcna.openjsf.org/security-advisories.htmlGitHubgithub.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9cGitHubgithub.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c

How CyberStrike helps

Frequently asked questions

What is CVE-2026-33807?

CVE-2026-33807 is a critical-severity vulnerability. It was published on April 15, 2026 and has a CVSS 3.1 base score of 9.1 (CRITICAL).

How severe is CVE-2026-33807?

This vulnerability has a CVSS 3.1 base score of 9.1, rated CRITICAL. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-33807?

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-2026-33807?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-33807 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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