fastify express up to 4.0.6 improper authorization

Summaryinfo

A vulnerability identified as critical has been detected in fastify express up to 4.0.6. Affected by this issue is some unknown functionality. Performing a manipulation results in improper authorization. This vulnerability is cataloged as CVE-2026-6556. It is possible to initiate the attack remotely. There is no exploit available. You should upgrade the affected component.

Detailsinfo

A vulnerability classified as critical has been found in fastify express up to 4.0.6. This affects some unknown functionality. The manipulation with an unknown input leads to a improper authorization vulnerability. CWE is classifying the issue as CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

@fastify/express versions 4.0.6 and earlier only rewrite the plugin prefix for middleware mount paths when the path argument is a string. Non-string mount paths (arrays of paths and regular expressions) are left unprefixed inside prefixed plugin scopes, so middleware registered with those forms does not match the actual prefixed request path. Applications that use path-scoped middleware for authentication, authorization, rate limiting, or auditing on routes inside a prefixed scope can be bypassed by sending a request to the prefixed route, because Fastify still matches the route but the middleware is skipped. Patches: upgrade to @fastify/express 4.0.7. Workarounds: use string mount paths instead of arrays or regular expressions in prefixed plugins, or register one use call per path.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-6556 since 04/18/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available. The attack technique deployed by this issue is T1548.002 according to MITRE ATT&CK.

Upgrading to version 4.0.7 eliminates this vulnerability.

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Productinfo

Vendor

• fastify

Name

• express

Version

• 4.0.0
• 4.0.1
• 4.0.2
• 4.0.3
• 4.0.4
• 4.0.5
• 4.0.6

CPE 2.3info

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CPE 2.2info

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