CVE-2026-6270

Versions @fastify/middie 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization, allowing requests with duplicate slashes to bypass authentication and authorization checks.

The @fastify/middie plugin prior to version 9.1.0 has a vulnerability that allows bypassing registered middleware using URL-encoded characters. Attackers can access protected endpoints without the constraints imposed by the middleware.

In @fastify/middie versions >= 9.1.0 and before 9.3.4, the decision to run path-scoped middleware is made by matching against the raw request target, while the Fastify router resolves an absolute-form request target to its path before dispatching. Because the two layers evaluate different strings, a request using an absolute-form target reaches the route handler while the path-scoped middleware, such as authentication or authorization, is skipped. An unauthenticated network attacker can use this to bypass path-based access controls in a Fastify application that relies on middie for those controls.

A vulnerability in @fastify/middie versions 9.1.0 through 9.3.2 causes a mismatch in handling encoded slashes (%2F) between the middleware and Fastify's router. An attacker can bypass middleware used for authentication, authorization, rate limiting, or auditing by sending a crafted request with an encoded slash in the parameter position.

A vulnerability in @fastify/middie versions < 9.2.0 can result in authentication/authorization bypass when using path-scoped middleware. When Fastify router normalization options are enabled, crafted request paths may bypass middleware checks.

Vulnerability in @fastify/middie versions 9.1.0 through 9.3.2 causes Node.js process termination when processing malformed percent-encoded sequences in request paths. The issue only affects the standalone engine API (middie.run), not the Fastify plugin path.