Axios: NO_PROXY bypass for 0.0.0.0 local addresses in axios

Summary

Axios versions containing lib/helpers/shouldBypassProxy.js do not treat 0.0.0.0 as a local address when evaluating NO_PROXY rules. In Node.js applications that use HTTP_PROXY or HTTPS_PROXY together with NO_PROXY=localhost,127.0.0.1,::1 or similar, a request to http://0.0.0.0:<port>/ can be routed through the configured proxy instead of bypassing it.

The issue is exploitable when an attacker can influence the axios request URL or a followed redirect target, and when the proxy can reach or relay 0.0.0.0 to local services. This is a Node.js runtime proxy-routing issue, not a browser, install-time, or development-tooling issue.

Impact

Applications are affected when all of the following are true:

• The application runs axios in Node.js with the HTTP adapter.
• The process uses environment proxy variables such as HTTP_PROXY or HTTPS_PROXY.
• The process uses NO_PROXY entries such as localhost, 127.0.0.1, or ::1 to keep local traffic out of the proxy path.
• Attacker-controlled input can influence the request URL or redirect target.
• The configured proxy does not reject 0.0.0.0 and can reach the local destination.

For plain HTTP targets, the proxy can receive the full request URL, headers, and body, and may be able to observe the local service response. HTTPS targets are less exposed because axios uses CONNECT tunneling in current versions.

Affected Functionality

Affected functionality is limited to environment-derived proxy selection in the Node HTTP adapter:

• lib/adapters/http.js calls getProxyForUrl(location) and then shouldBypassProxy(location) before applying the proxy.
• lib/helpers/shouldBypassProxy.js normalizes and compares NO_PROXY entries.
• Explicit caller-provided config.proxy remains trusted caller configuration.
• Browser, React Native, XHR, and fetch adapter behavior are not affected.

Technical Details

lib/helpers/shouldBypassProxy.js defines local loopback equivalence through isLoopback(). The current implementation recognizes localhost, IPv4 127.0.0.0/8, IPv6 ::1, and IPv4-mapped loopback forms, but it does not include 0.0.0.0.

At lib/helpers/shouldBypassProxy.js:176, axios treats two hosts as matching when both are considered loopback:

return hostname === entryHost || (isLoopback(hostname) && isLoopback(entryHost));

Because isLoopback('0.0.0.0') returns false, NO_PROXY=localhost,127.0.0.1,::1 does not match http://0.0.0.0:<port>/. lib/adapters/http.js:185-193 then applies the environment proxy.

Proof of Concept of Attack

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

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

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

const origin = await listen((req, res) => res.end('origin'), '0.0.0.0');

let proxyRequests = 0;
const proxy = await listen((req, res) => {
  proxyRequests += 1;
  res.end('proxied');
});

process.env.http_proxy = `http://127.0.0.1:${proxy.address().port}`;
process.env.HTTP_PROXY = process.env.http_proxy;
process.env.no_proxy = 'localhost,127.0.0.1,::1';
process.env.NO_PROXY = process.env.no_proxy;

try {
  const direct = await axios.get(`http://127.0.0.1:${origin.address().port}/`);
  const zero = await axios.get(`http://0.0.0.0:${origin.address().port}/`);

  console.log({ direct: direct.data, zero: zero.data, proxyRequests });
} finally {
  await close(origin);
  await close(proxy);
}

Expected safe behavior: both 127.0.0.1 and 0.0.0.0 bypass the proxy when the NO_PROXY policy is intended to cover local destinations.

Observed behavior: 127.0.0.1 bypasses the proxy, while 0.0.0.0 is sent through the proxy.

Workarounds

• Add 0.0.0.0 explicitly to NO_PROXY where local addresses must bypass proxies.
• Reject or normalize 0.0.0.0 in application URL validation before calling axios.
• Set proxy: false on axios requests that must never use environment proxies.
• Configure the proxy itself to reject 0.0.0.0, loopback, link-local, and internal address ranges.

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

Summary

axios versions 1.15.0–1.16.1 contain an incomplete loopback-address check in lib/helpers/shouldBypassProxy.js. The isLoopback() function correctly identifies 127.0.0.0/8 and ::1 as loopback addresses but does not recognise 0.0.0.0 — the IPv4 unspecified address, which routes to the local machine on Linux and macOS.

An attacker who controls a URL passed to axios can use http://0.0.0.0/<path> to bypass proxy-based SSRF filtering that the application relies upon.

Details

Affected versions

>= 1.15.0, <= 1.16.1

The vulnerability was introduced in v1.15.0 when the shouldBypassProxy helper was added as a security improvement (PR #10661).

Root cause

File: lib/helpers/shouldBypassProxy.js

// Line 1 — static allowlist (incomplete)
const LOOPBACK_HOSTNAMES = new Set(['localhost']);   // ← 0.0.0.0 missing

const isIPv4Loopback = (host) => {
  const parts = host.split('.');
  if (parts.length !== 4) return false;
  if (parts[0] !== '127') return false;   // ← 0.0.0.0: parts[0] = '0' → false
  return parts.every((p) => /^\d+$/.test(p) && Number(p) >= 0 && Number(p) <= 255);
};

const isLoopback = (host) => {
  if (!host) return false;
  if (LOOPBACK_HOSTNAMES.has(host)) return true;   // ← '0.0.0.0' not in set
  if (isIPv4Loopback(host)) return true;           // ← returns false for 0.0.0.0
  return isIPv6Loopback(host);
};

isLoopback('0.0.0.0') returns false.

Node's WHATWG URL parser does not normalise 0.0.0.0 to 127.0.0.1. Other bypass forms are safe: new URL('http://0177.0.0.1/').hostname → '127.0.0.1' (octal), new URL('http://2130706433/').hostname → '127.0.0.1' (decimal), new URL('http://0x7f000001/').hostname → '127.0.0.1' (hex). Only 0.0.0.0 escapes normalisation.


### PoC
'use strict';

// Verbatim copy of relevant logic from axios v1.16.1 shouldBypassProxy.js

const LOOPBACK_HOSTNAMES = new Set(['localhost']);

const isIPv4Loopback = (host) => {
  const parts = host.split('.');
  if (parts.length !== 4) return false;
  if (parts[0] !== '127') return false;
  return parts.every((p) => /^\d+$/.test(p) && Number(p) >= 0 && Number(p) <= 255);
};

const isLoopback = (host) => {
  if (!host) return false;
  if (LOOPBACK_HOSTNAMES.has(host)) return true;
  return isIPv4Loopback(host);
};

// 1. Show URL parser does NOT normalise 0.0.0.0
console.log(new URL('http://0.0.0.0/').hostname);    // → '0.0.0.0'   ← NOT normalised
console.log(new URL('http://0177.0.0.1/').hostname); // → '127.0.0.1' ← normalised (safe)
console.log(new URL('http://2130706433/').hostname);  // → '127.0.0.1' ← normalised (safe)

// 2. Show isLoopback fails for 0.0.0.0
console.log(isLoopback('0.0.0.0'));   // → false  ← BUG: should be true
console.log(isLoopback('127.0.0.1')); // → true   ← correct

Verified output on Node.js v22 / axios v1.16.1:
0.0.0.0     ← NOT normalised by URL parser
127.0.0.1   ← octal normalised correctly
127.0.0.1   ← decimal normalised correctly
false       ← 0.0.0.0 not detected as loopback  ⚠
true        ← 127.0.0.1 correctly detected

### Impact
Applications that:

Accept user-supplied URLs and pass them to axios
Use a proxy with NO_PROXY=localhost (or similar) for SSRF filtering
…can be bypassed by supplying http://0.0.0.0/<path>. Axios routes the request through the proxy (shouldBypassProxy returns false). If the proxy itself does not filter 0.0.0.0, the connection reaches the local machine — exposing internal services such as cloud IMDS endpoints, internal admin panels, or microservice APIs.

Fix
Minimal (one line):

- const LOOPBACK_HOSTNAMES = new Set(['localhost']);
+ const LOOPBACK_HOSTNAMES = new Set(['localhost', '0.0.0.0']);

Comprehensive:

const isIPv4Unspecified = (host) => host === '0.0.0.0';

const isLoopback = (host) => {
  if (!host) return false;
  if (LOOPBACK_HOSTNAMES.has(host)) return true;
  if (isIPv4Loopback(host)) return true;
  if (isIPv4Unspecified(host)) return true;   // add this line
  return isIPv6Loopback(host);
};
</details>