Fastify Middie Middleware Path Bypass

Summary

A security vulnerability exists in @fastify/middie where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., /%61dmin instead of /admin). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints.

Details

The vulnerability is caused by how middie matches requests against registered middleware paths.

1. Regex Generation: When fastify.use('/admin', ...) is called, middie uses path-to-regexp to generate a regular expression for the path /admin.
2. Request Matching: For every request, middie executes this regular expression against req.url (or req.originalUrl).
3. The Flaw: req.url in Fastify contains the raw, undecoded path string.
   • The generated regex expects a decoded path (e.g., /admin).
   • If a request is sent to /%61dmin, the regex comparison fails (/^\/admin/ does not match /%61dmin).
   • middie assumes the middleware does not apply and calls next().
4. Route Execution: The request proceeds to Fastify's internal router, which performs URL decoding. It correctly identifies /%61dmin as /admin and executes the corresponding route handler.

Incriminated Source Code: In the provided middie source:

// ... inside Holder function
if (regexp) {
  const result = regexp.exec(url) // <--- 'url' is undecoded.
  if (result) {
    // ... executes middleware ...
  } else {
    that.done() // <--- Middleware skipped on mismatch
  }
}

PoC

Step 1: Run the following Fastify application (save as app.js):

const fastify = require('fastify')({ logger: true });

async function start() {
  // Register middie for Express-style middleware support
  await fastify.register(require('@fastify/middie'));

  // Middleware to block /admin route
  fastify.use('/admin', (req, res, next) => {
    res.statusCode = 403;
    res.end('Forbidden: Access to /admin is blocked');
  });

  // Sample routes
  fastify.get('/', async (request, reply) => {
    return { message: 'Welcome to the homepage' };
  });

  fastify.get('/admin', async (request, reply) => {
    return { message: 'Admin panel' };
  });

  // Start server
  try {
    await fastify.listen({ port: 3008 });
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
}

start();