Nest has a Fastify URL Encoding Middleware Bypass (TOCTOU)

A NestJS application is vulnerable if it meets all of the following criteria:

1. Platform: Uses @nestjs/platform-fastify.
2. Security Mechanism: Relies on NestMiddleware (via MiddlewareConsumer) for security checks (authentication, authorization, etc.), or through app.use()
3. Routing: Applies middleware to specific routes using string paths or controllers (e.g., .forRoutes('admin')). Example Vulnerable Config:

// app.module.ts
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(AuthMiddleware) // Security check
      .forRoutes('admin');   // Vulnerable: Path-based restriction
  }
}

Attack Vector:

• Target Route: /admin
• Middleware Path: admin
• Attack Request: GET /%61dmin
• Result: Middleware is skipped (no match on %61dmin), but controller for /admin is executed.

Consequences:

• Authentication Bypass: Unauthenticated users can access protected routes.
• Authorization Bypass: Restricted administrative endpoints become accessible to lower-privileged users.
• Input Validation Bypass: Middleware performing sanitization or validation can be skipped.

Patches

Patched in @nestjs/platform-fastify@11.1.11

Resources

Credit goes to Hacktron AI for reporting this issue.