CVE-2025-64166

A Cross-Site Request Forgery (CSRF) vulnerability was identified in Mercurius versions 16. The issue arises from incorrect parsing of the Content-Type header in requests. Specifically, requests with Content-Type values such as application/x-www-form-urlencoded, multipart/form-data, or text/plain could be misinterpreted as application/json. This misinterpretation bypasses the preflight checks performed by the fetch() API, potentially allowing unauthorized actions to be performed on behalf of an authenticated user.

Proof of Concept

// Server-side Fastify setup
const Fastify = require('fastify');
const mercurius = require('mercurius');

const app = Fastify();
const schema = `
  type Query {
    hello(name: String): String
  }
`;

const resolvers = {
  Query: {
    hello: (_, { name }) => `Hello ${name || 'World'}!`
  }
};

app.register(mercurius, { schema, resolvers });

app.listen(3000, () => {
  console.log('Server listening on http://localhost:3000');
});

// Malicious client-side code
fetch('http://localhost:3000/graphql', {
  method: 'POST',
  body: JSON.stringify({ query: '{ hello(name: "attacker") }' }),
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded'
  },
  credentials: 'include'
});

In the above example, the malicious request is crafted to exploit the CSRF vulnerability by using a Content-Type that Fastify incorrectly parses as application/json.

Mitigation

To address this vulnerability, CSRF protection has been implemented.

References

• https://github.com/mercurius-js/mercurius/pull/1187

An attacker could exploit this vulnerability by crafting a malicious request with a Content-Type that Fastify incorrectly parses as application/json. When such a request is made from a different origin, it bypasses the Cross-Origin Resource Sharing (CORS) protections, leading to a potential CSRF attack. This could result in unauthorized actions being performed on behalf of an authenticated user without their consent.

A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.

CVE-2025-64166 has a CVSS score of 5.4 (Medium). The vector is network-reachable, no privileges required, and user interaction required. 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 (16.4.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-64166? CVE-2025-64166 is a medium-severity cross-site request forgery (CSRF) vulnerability in mercurius (npm), affecting versions <= 16.3.0. It is fixed in 16.4.0. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
2. How severe is CVE-2025-64166? CVE-2025-64166 has a CVSS score of 5.4 (Medium). 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 mercurius are affected by CVE-2025-64166? mercurius (npm) versions <= 16.3.0 is affected.
4. Is there a fix for CVE-2025-64166? Yes. CVE-2025-64166 is fixed in 16.4.0. Upgrade to this version or later.
5. Is CVE-2025-64166 exploitable, and should I be worried? Whether CVE-2025-64166 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-2025-64166 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-2025-64166? Upgrade mercurius to 16.4.0 or later.