CVE-2026-6556
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk23th percentile — higher than 23% of all known CVEs
Summary
In @fastify/express versions 4.0.6 and earlier, plugin prefixes are not applied to middleware mount paths when they are arrays or regular expressions. Consequently, middleware registered this way is not executed for requests to prefixed routes, allowing security mechanisms to be bypassed.
Risk Assessment
The organization risks bypassing authentication, authorization, rate limiting, or auditing on routes within prefixed plugin scopes, potentially leading to unauthorized access to protected resources.
Recommendation
Upgrade @fastify/express to version 4.0.7 or later immediately. As a temporary workaround, use only string mount paths for middleware in prefixed plugins, avoiding arrays and regular expressions.
Original NVD description (English source)
@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.