@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)
Date: 2026-01
Affected: npm/@fastify/express
Attack vector: other
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-22037
CWE: CWE-177
CWE-288
CVSS: 8.4 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L)

### 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);


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