@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application registers authentication middleware in a parent scope and then registers child plugins with @fastify/middie, the child scope does not inherit the parent middleware. This allows unauthenticated requests to reach routes defined in child plugin scopes, bypassing authentication and authorization checks. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds.

Affected: npm: @fastify/middie

### Impact

`@fastify/middie` v9.3.1 and earlier incorrectly re-prefixes middleware paths when propagating them to child plugin scopes. When a child plugin is registered with a prefix that overlaps with a parent-scoped middleware path, the middleware path is modified during inheritance and silently fails to match incoming requests.

This results in complete bypass of middleware security controls for all routes defined within affected child plugin scopes, including nested (grandchild) scopes. Authentication, authorization, rate limiting, and any other middleware-based security mechanisms are skipped. No special request crafting or configuration is required.

This is the same vulnerability class as [GHSA-hrwm-hgmj-7p9c](https://github.com/fastify/fastify-express/security/advisories/GHSA-hrwm-hgmj-7p9c) (CVE-2026-33807) in `@fastify/express`.

### Patches

Upgrade to `@fastify/middie` v9.3.2 or later.

### Workarounds

None. Upgrade to the patched version.