Axios form serializer maxDepth bypass via {} metatoken

## Summary Axios versions in the fixed lines for GHSA-62hf-57xw-28j9 still contain an incomplete depth-limit bypass in `lib/helpers/toFormData.js`. When serializing an object with a top-level key ending in `{}`, axios calls `JSON.stringify()` on that value before the `formSerializer.maxDepth` guard can inspect the nested structure. An attacker who can control object keys and nested values passed by an application into axios form or parameter serialization can trigger a raw `RangeError: Maximum call stack size exceeded`, causing a denial of service in the affected request path. ## Impact The impact is availability only. No confidentiality or integrity impact was confirmed. Server-side applications are the primary concern when they accept user-controlled input and pass it into axios as `data` or `params` for `multipart/form-data`, `application/x-www-form-urlencoded`, or default parameter serialization. Browser impact is limited to the page or request context unless the application builds a broader failure mode around the thrown exception. The attack requires control over a top-level object key ending in `{}` and a deeply nested object value. The option `formSerializer.metaTokens: false` is not a workaround because it only changes the emitted key name; the value is still stringified. ## Affected Functionality Affected paths include: - `lib/helpers/toFormData.js` when a top-level key ends with `{}`. - `lib/helpers/toURLEncodedForm.js`, which delegates to `helpers.defaultVisitor`. - `lib/helpers/AxiosURLSearchParams.js`, used by default params serialization. - Request transforms in `lib/defaults/index.js` when object data is serialized as `multipart/form-data` or `application/x-www-form-urlencoded`. Unaffected paths include: - Already-created `FormData` or `URLSearchParams` values that axios does not walk with `toFormData`. - Custom `paramsSerializer.serialize` implementations that do not call axios `toFormData`. - Non-`{}` deeply nested values in `toFormData`, which hit `ERR_FORM_DATA_DEPTH_EXCEEDED` as intended. ## Technical Details In `lib/helpers/toFormData.js`, `defaultVisitor()` handles top-level keys ending in `{}` before recursive traversal: ```js if (value && !path && typeof value === 'object') { if (utils.endsWith(key, '{}')) { key = metaTokens ? key : key.slice(0, -2); value = JSON.stringify(value); } } ``` The depth guard is in `build()`: ```js if (depth > maxDepth) { throw new AxiosError( 'Object is too deeply nested (' + depth + ' levels). Max depth: ' + maxDepth, AxiosError.ERR_FORM_DATA_DEPTH_EXCEEDED ); } ``` For `{}` metatoken values, `build()` only sees the top-level property. The nested value is handed directly to native `JSON.stringify()`, which recurses internally and can throw `RangeError` before axios emits the intended `AxiosError`. ## Proof of Concept of Attack Safe local PoC with no network I/O: ```js import toFormData from './lib/helpers/toFormData.js'; function buildDeep(depth) { const head = {}; let cur = head; for (let i = 0; i < depth; i += 1) { cur.x = {}; cur = cur.x; } return head; } try { toFormData({ 'evil{}': buildDeep(10000) }); } catch (err) { console.log(err.name, err.code || '', err.message); } // Expected affected result: // RangeError Maximum call stack size exceeded ``` Expected fixed behavior is an `AxiosError` with code `ERR_FORM_DATA_DEPTH_EXCEEDED`. ## Workarounds Reject or depth-limit untrusted objects before passing them to axios serialization. Strip or reject top-level keys ending in `{}` from untrusted objects when using axios form serialization. For query parameters, use a custom `paramsSerializer.serialize` that enforces a depth limit. For form bodies, construct `FormData` or `URLSearchParams` manually after validating input depth. <details> <summary>Original Report</summary> ## Summary The `maxDepth=100` guard added in axios 1.15.0 to fix GHSA-62hf-57xw-28j9 lives inside the `build()` recursion in `lib/helpers/toFormData.js`. The default visitor at `lib/helpers/toFormData.js:166-170` still has a top-level shortcut that calls `JSON.stringify(value)` whenever a key ends in `'{}'`, before `build()` ever sees the nested value. JSON.stringify on a deeply nested object stack-overflows with `RangeError: Maximum call stack size exceeded`, which propagates synchronously out of the axios call. The exact attacker-data flow that the original advisory described (proxy-style code that forwards client JSON into `axios({ data, params })`) still crashes the process at depth ~3000 on a default Node.js stack, despite v1.16.0 being patched. ## Details Affected: axios 1.15.0 - 1.16.0 (every released version that carries the GHSA-62hf-57xw-28j9 fix). The bug is reachable from any code path that hits `toFormData`, which includes: - `axios.post(url, data, { headers: { 'content-type': 'application/x-www-form-urlencoded' } })` -> `defaults.transformRequest` -> `toURLEncodedForm(data)` -> `toFormData` - `axios.post(url, data, { headers: { 'content-type': 'multipart/form-data' } })` -> same path via `toFormData` - `axios.get(url, { params })` -> `buildURL` -> `new AxiosURLSearchParams(params)` -> `toFormData` Vulnerable code, `lib/helpers/toFormData.js`: ```javascript // 156 function defaultVisitor(value, key, path) { // 165 if (value && !path && typeof value === 'object') { // 166 if (utils.endsWith(key, '{}')) { // 167 // eslint-disable-next-line no-param-reassign // 168 key = metaTokens ? key : key.slice(0, -2); // 169 // eslint-disable-next-line no-param-reassign // 170 value = JSON.stringify(value); // <-- V8 native, NOT depth-checked // 171 } else if (... ``` `build()` later does enforce `maxDepth`: ```javascript // 211 function build(value, path, depth = 0) { // 212 if (utils.isUndefined(value)) return; // 213 // 214 if (depth > maxDepth) { // 215 throw new AxiosError( // 216 'Object is too deeply nested (' + depth + ' levels). Max depth: ' + maxDepth, // 217 AxiosError.ERR_FORM_DATA_DEPTH_EXCEEDED // 218 ); ``` The `'{}'` shortcut runs in `defaultVisitor`, which is invoked from inside `build()` for top-level keys (the `!path` clause at line 165 means the shortcut only triggers at top level, where `path` is `undefined`). At that point `depth === 0` and the maxDepth check has already passed; the recursion-aware guard never sees the nested value because `defaultVisitor` reassigns `value = JSON.stringify(value)` and returns the rendered string straight to `formData.append`. JSON.stringify itself is recursive in V8 and stack-overflows on deeply nested objects, throwing `RangeError` synchronously. The behaviour is independent of the `metaTokens` option: line 168 only changes whether `'{}'` stays on the key name, line 170 stringifies regardless. `toURLEncodedForm`'s wrapper visitor in `lib/helpers/toURLEncodedForm.js:11-14` falls through to the same `defaultVisitor`, so the form-encoded path is also affected. The attacker payload is a single top-level key ending in `'{}'` whose value is a nested object. The keys themselves do not have to be deep, so the payload is small to send (a few KB of `{"x":{"x":...}}` produces enough nesting to overflow). The original advisory's threat model -- a server that forwards `req.body` or `req.query` into axios -- is unchanged: ```javascript app.post('/forward', async (req, res) => { await axios.post('https://upstream/api', req.body); // req.body attacker-controlled res.send('ok'); }); // attacker POST /forward with content-type: application/x-www-form-urlencoded // body: {"evil{}": <8000-deep object>} // -> JSON.stringify recurses inside defaultVisitor -> RangeError -> handler crashes ``` The error is not an `AxiosError`; it is a raw `RangeError` thrown from the stringifier, so handlers that look for `err.code === 'ERR_FORM_DATA_DEPTH_EXCEEDED'` (the documented signal that the maxDepth guard fired) do not see it. Synchronous startup paths or worker threads still take the whole process down. The fix is to also depth-limit (or pre-walk) the value before calling `JSON.stringify` on line 170, or to remove the top-level `'{}'` shortcut and rely on the depth-checked `build()` recursion to handle it. A minimal patch that preserves observable behaviour for legal payloads: ```diff if (utils.endsWith(key, '{}')) { // eslint-disable-next-line no-param-reassign key = metaTokens ? key : key.slice(0, -2); + // Reject objects that would exceed maxDepth before handing to JSON.stringify, + // which is recursive in V8 and stack-overflows on deeply nested input. + (function checkDepth(v, d) { + if (d > maxDepth) { + throw new AxiosError( + 'Object is too deeply nested (' + d + ' levels). Max depth: ' + maxDepth, + AxiosError.ERR_FORM_DATA_DEPTH_EXCEEDED + ); + } + if (v && typeof v === 'object') { + for (const k in v) checkDepth(v[k], d + 1); + } + })(value, 0); // eslint-disable-next-line no-param-reassign value = JSON.stringify(value); } ``` (The recursion in `checkDepth` itself is bounded by `maxDepth`, so it cannot itself overflow.) ## PoC Reproduces against a clean clone of `axios/axios` at v1.16.0 with `npm install` already run. `targets/axios/poc_jsonstringify_dos.mjs` is the script: ```javascript import axios from './source/index.js'; function buildDeep(depth) { let head = {}; let cur = head; for (let i = 0; i < depth; i++) { cur.x = {}; cur = cur.x; } return head; } const malicious = buildDeep(5000); const safeAdapter = () => Promise.resolve({ data: 'never reached', status: 200, statusText: 'OK', headers: {}, config: {} }); // 1. POST x-www-form-urlencoded try { await axios.post('http://example.test/x', { 'evil{}': malicious }, { headers: { 'content-type': 'application/x-www-form-urlencoded' }, adapter: safeAdapter }); } catch (e) { console.log('POST form-encoded:', e.name, '-', e.message); } // 2. GET with params try { await axios.get('http://example.test/x', { params: { 'evil{}': malicious }, adapter: safeAdapter }); } catch (e) { console.log('GET params:', e.name, '-', e.message); } ``` 3/3 runs reproduce the same `RangeError` on `[email protected]` with Node.js 24: ``` $ node poc_jsonstringify_dos.mjs POST form-encoded: RangeError - Maximum call stack size exceeded GET params: RangeError - Maximum call stack size exceeded ``` `safeAdapter` is a stub that returns a fake response, so the crash is provably inside axios's serialization layer, not in HTTP I/O. Removing the `'{}'` suffix from the key and re-running gives the expected `AxiosError: Object is too deeply nested ... ERR_FORM_DATA_DEPTH_EXCEEDED` from the maxDepth guard, confirming the fix is wired correctly elsewhere -- it just does not cover this branch. Crash threshold on a default-stack Node.js process is roughly depth 2500-3000; 8000 is comfortably above that, and the payload is a few KB. ## Impact A remote, unauthenticated attacker who can influence an object that the application passes to axios as request `data` or `params` triggers an uncaught `RangeError` from inside the synchronous `JSON.stringify` call in `defaultVisitor`. In server-side applications that proxy or re-forward client JSON through axios -- the same threat model that motivated GHSA-62hf-57xw-28j9 -- this crashes the request handler and, in worker/cluster setups, the whole process. The previously shipped `maxDepth` guard does not stop it because the `'{}'` suffix path bypasses `build()` entirely. Same severity class as the original advisory (CWE-674 Uncontrolled Recursion, network-reachable DoS); the only difference is the attacker has to suffix one of their object keys with `'{}'` to land on the unguarded code path. </details> ---

Quoted source text, attributed separately from HOL analysis.