---
id: GHSA-hcpx-6fm6-wx23
title: 'Axios form serializer maxDepth bypass via {} metatoken'
summary: 'Axios form serializer maxDepth bypass via {} metatoken'
severity: medium
cwe:
  - CWE-674
vendor: axios
product: axios
ecosystem: npm
affected:
  - 'axios >= 0.31.1, < 0.33.0'
  - 'axios >= 1.15.1, < 1.18.0'
patched:
  - axios 0.33.0
  - axios 1.18.0
published: '2026-07-20'
updated: '2026-07-20'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-hcpx-6fm6-wx23'
references:
  - url: 'https://github.com/axios/axios/security/advisories/GHSA-hcpx-6fm6-wx23'
  - url: 'https://github.com/axios/axios/pull/11000'
  - url: 'https://github.com/axios/axios/pull/11001'
  - url: >-
      https://github.com/axios/axios/commit/1417285c69344bbcc6420a021f67dee0c6fedb2d
  - url: >-
      https://github.com/axios/axios/commit/32fc489632377d214db55bfa4e2c48486a7d7ce2
  - url: 'https://github.com/axios/axios/releases/tag/v0.33.0'
  - url: 'https://github.com/axios/axios/releases/tag/v1.18.0'
  - url: 'https://github.com/advisories/GHSA-hcpx-6fm6-wx23'
tags:
  - ghsa
  - npm
ingestedAt: '2026-07-20T22:43:34.942Z'
---

## Overview

## 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 `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>

## Affected packages

- `axios >= 0.31.1, < 0.33.0`
- `axios >= 1.15.1, < 1.18.0`

## Remediation

Upgrade to a patched release:

- `axios 0.33.0`
- `axios 1.18.0`