CVE-2026-6556: fastify fastify\/express Improper Authorization

Vulnerability summary

CVE-2026-6556 is a critical (CVSS 9.1) Improper Authorization vulnerability in fastify fastify\/express. EPSS estimates a 0.52% chance of exploitation in the next 30 days, higher than 43.1% of scored vulnerabilities.

Timeline

1. 30-06-2026
   PublishedVulnerability first disclosed.
2. 30-06-2026
   Last modifiedVulnerability information updated.

Description

@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.

• CWE-285Improper AuthorizationThe product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action.