Affected: npm: axios

# Denial of Service via **proto** Key in mergeConfig

### Summary

The `mergeConfig` function in axios crashes with a TypeError when processing configuration objects containing `__proto__` as an own property. An attacker can trigger this by providing a malicious configuration object created via `JSON.parse()`, causing complete denial of service.

### Details

The vulnerability exists in `lib/core/mergeConfig.js` at lines 98-101:

```javascript utils.forEach(Object.keys({ ...config1, ...config2 }), function computeConfigValue(prop) { const merge = mergeMap[prop] || mergeDeepProperties; const configValue = merge(config1[prop], config2[prop], prop); (utils.isUndefined(configValue) && merge !== mergeDirectKeys) || (config[prop] = configValue); }); ```

When `prop` is `'__proto__'`:

1. `JSON.parse('{"__proto__": {...}}')` creates an object with `__proto__` as an own enumerable property 2. `Object.keys()` includes `'__proto__'` in the iteration 3. `mergeMap['__proto__']` performs prototype chain lookup, returning `Object.prototype` (truthy object) 4. The expression `mergeMap[prop] || mergeDeepProperties` evaluates to `Object.prototype` 5. `Object.prototype(...)` throws `TypeError: merge is not a function`

The `mergeConfig` function is called by:

- `Axios._request()` at `lib/core/Axios.js:75` - `Axios.getUri()` at `lib/core/Axios.js:201` - All HTTP method shortcuts (`get`, `post`, etc.) at `lib/core/Axios.js:211,224`

### PoC

```javascript import axios from "axios";

cons…