Axios: CIDR-form NO_PROXY entries are ignored, causing proxy exclusion bypass for internal IP ranges

Not affected:

• Exact host or exact IP NO_PROXY entries where axios matching succeeds.
• Requests configured with proxy: false.
• Browser adapters.

Technical Details

lib/helpers/shouldBypassProxy.js parses each NO_PROXY entry into a host and optional port, normalizes hostnames, and then compares exact hostnames, suffix entries, wildcard-prefix entries, and loopback equivalents. It does not parse CIDR notation.

Local verification on axios 1.18.1:

process.env.NO_PROXY = '127.0.0.0/8';
shouldBypassProxy('http://127.0.0.1:1234/'); // false

The expected result for CIDR-aware bypass policy is true.

Proof of Concept of Attack

Constrained local demonstration:

1. Set HTTP_PROXY=http://127.0.0.1:<proxy-port>.
2. Set NO_PROXY=127.0.0.0/8.
3. Request http://127.0.0.1:<internal-port>/metadata.
4. Observe that axios sends the request through the proxy instead of directly to the internal listener.

Workarounds

Use exact host or IP entries in NO_PROXY for sensitive destinations until CIDR matching is fixed, for example 127.0.0.1,localhost,169.254.169.254. For individual requests that must not use a proxy, set proxy: false.

Summary

Axios 1.17.0 honors HTTP_PROXY / HTTPS_PROXY and supports NO_PROXY host exclusions, but CIDR-form NO_PROXY entries such as 127.0.0.0/8 are not treated as network ranges. As a result, requests to IPs covered by a configured CIDR exclusion may still be sent through the configured proxy.

In the attached PoC, a request to 127.0.0.1 is sent through HTTP_PROXY despite NO_PROXY=127.0.0.0/8.

This can cause proxy exclusion bypass in environments where operators use CIDR notation to exclude loopback, private, internal, Kubernetes, CI, or cloud metadata address ranges from proxying.

Details

Axios supports proxy environment variables, including HTTP_PROXY / HTTPS_PROXY and NO_PROXY-style exclusions. Axios’s threat model treats environment proxy handling as security-relevant and lists NO_PROXY as a mitigation for proxy environment variable hijack, including hardening for CIDR ranges, IPv6 literals, and wildcard patterns. See: https://github.com/axios/axios/blob/a8e4f13aeecc45a3b8fab3ecfd9ddb5d70fb772b/THREATMODEL.md#t-r9-proxy-environment-variable-hijack

The issue is that CIDR-form NO_PROXY entries are not interpreted as network ranges. For example:

NO_PROXY=127.0.0.0/8
HTTP_PROXY=http://127.0.0.1:<proxy-port>
Target URL=http://127.0.0.1:<internal-port>/metadata

Since 127.0.0.1 is inside 127.0.0.0/8, an operator may reasonably expect Axios to bypass the proxy for this request. Instead, Axios sends the request through HTTP_PROXY.

This appears to affect the proxy bypass decision path used for NO_PROXY / no_proxy handling. The relevant behavior is in Axios's Node proxy handling and NO_PROXY evaluation logic, including the shouldBypassProxy helper introduced for no_proxy hostname normalization and bypass checks.

The issue is not that Axios ignores NO_PROXY entirely. Exact host exclusions work. The issue is specifically that CIDR-form exclusions are silently treated as non-matching host/domain tokens rather than as network ranges, causing the request to be proxied.

This is security-relevant because CIDR notation is commonly used in container, CI, enterprise proxy, and cloud environments for ranges such as:

127.0.0.0/8
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
169.254.169.254/32

If operators rely on those entries to prevent internal or metadata-style requests from traversing a proxy, Axios may violate that expectation.

PoC

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

function listen(server, host) {
  return new Promise((resolve, reject) => {
    server.once('error', reject);
    server.listen(0, host, () => resolve(server.address().port));
  });
}

function close(server) {
  return new Promise((resolve) => server.close(resolve));
}

let proxyHits = 0;
let internalHits = 0;

const internal = http.createServer((req, res) => {
  internalHits += 1;
  res.writeHead(200, { 'content-type': 'text/plain' });
  res.end(`internal service saw ${req.url}`);
});

const proxy = http.createServer((req, res) => {
  proxyHits += 1;
  res.writeHead(200, { 'content-type': 'text/plain' });
  res.end(`proxy saw request for ${req.url}`);
});

const internalHost = process.env.POC_INTERNAL_HOST || '127.0.0.2';
const proxyHost = process.env.POC_PROXY_HOST || '127.0.0.1';

let internalPort;
let proxyPort;

try {
  internalPort = await listen(internal, internalHost);
  proxyPort = await listen(proxy, proxyHost);
} catch (error) {
  console.error('Failed to bind local PoC servers.');
  console.error('On some systems 127.0.0.2 is unavailable; try:');
  console.error('  POC_INTERNAL_HOST=127.0.0.1 node poc-no-proxy-cidr-axios.mjs');
  console.error('');
  throw error;
}

const targetUrl = `http://${internalHost}:${internalPort}/metadata`;
const proxyUrl = `http://${proxyHost}:${proxyPort}`;
const noProxy = process.env.POC_NO_PROXY || '127.0.0.0/8';

process.env.http_proxy = proxyUrl;
process.env.HTTP_PROXY = proxyUrl;
process.env.no_proxy = noProxy;
process.env.NO_PROXY = noProxy;

console.log('Axios NO_PROXY CIDR full axios network PoC');
console.log(`axios VERSION=${axios.VERSION || 'unknown'}`);
console.log(`NO_PROXY=${process.env.no_proxy}`);
console.log(`HTTP_PROXY=${process.env.http_proxy}`);
console.log(`Target URL=${targetUrl}`);
console.log('');

try {
  const response = await axios.get(targetUrl, {
    timeout: 2000,
  });

  console.log(`Response=${response.data}`);
  console.log(`Proxy hits=${proxyHits}`);
  console.log(`Internal direct hits=${internalHits}`);
  console.log('');

  if (proxyHits > 0 && internalHits === 0) {
    console.log(`POC RESULT: axios sent the target through the proxy with NO_PROXY=${noProxy}.`);
  } else if (proxyHits === 0 && internalHits > 0) {
    console.log(`POC RESULT: axios bypassed the proxy with NO_PROXY=${noProxy}.`);
  } else {
    console.log('POC RESULT: mixed/ambiguous routing; inspect counts above.');
  }
} finally {
  delete process.env.http_proxy;
  delete process.env.HTTP_PROXY;
  delete process.env.no_proxy;
  delete process.env.NO_PROXY;
  await close(proxy);
  await close(internal);
}

Run the failing CIDR case:

POC_INTERNAL_HOST=127.0.0.1 node poc-no-proxy-cidr-axios.mjs

Observed:

Axios NO_PROXY CIDR full axios network PoC
axios VERSION=1.17.0
NO_PROXY=127.0.0.0/8
HTTP_PROXY=http://127.0.0.1:34315
Target URL=http://127.0.0.1:43993/metadata

Response=proxy saw request for http