axios's shouldBypassProxy does not recognize IPv4-mapped IPv6 addresses, allowing NO_PROXY bypass (incomplete fix for CVE-2025-62718)

Summary

shouldBypassProxy, introduced in v1.15.0 to fix CVE-2025-62718, does not normalise IPv4-mapped IPv6 addresses. When NO_PROXY lists an IPv4 address such as 127.0.0.1 or 169.254.169.254, a request URL using the IPv4-mapped IPv6 form (::ffff:7f00:1, ::ffff:a9fe:a9fe) still routes through the configured proxy. Node.js resolves these addresses to the underlying IPv4 host, so the request reaches the internal service via the proxy rather than being blocked.

Details

lib/helpers/shouldBypassProxy.js (v1.15.0):

  const LOOPBACK_ADDRESSES = new Set(['localhost', '127.0.0.1', '::1']);                                                                                                      
  const isLoopback = (host) => LOOPBACK_ADDRESSES.has(host);                                                                                                                    
                                                                                                                                                                                
  // normalizeNoProxyHost strips brackets and trailing dots, but not ::ffff: prefix                                                                                             
  return hostname === entryHost || (isLoopback(hostname) && isLoopback(entryHost));                                                                                             

The WHATWG URL parser canonicalises http://[::ffff:127.0.0.1]/ to hostname [::ffff:7f00:1]. After bracket-stripping: ::ffff:7f00:1. This string does not match 127.0.0.1 in NO_PROXY and is not in LOOPBACK_ADDRESSES, so shouldBypassProxy returns false and the proxy is used. proxy-from-env (called before shouldBypassProxy) has the same gap - it does not equate ::ffff:7f00:1 with 127.0.0.1 - so neither layer catches the bypass.

PoC

// NO_PROXY=127.0.0.1,localhost,::1  HTTP_PROXY=http://attacker:8080
import shouldBypassProxy from 'axios/lib/helpers/shouldBypassProxy.js';                                                                                                       
                                                                                                                                                                              
// All three should return true (bypass proxy). Only the first two do.                                                                                                        
console.log(shouldBypassProxy('http://127.0.0.1/'));          // true  [OK]                                                                                                     
console.log(shouldBypassProxy('http://[::1]/'));               // true  [OK]                                                                                                     
console.log(shouldBypassProxy('http://[::ffff:127.0.0.1]/')); // false <- bypass                                                                                             
console.log(shouldBypassProxy('http://[::ffff:7f00:1]/'));     // false <- bypass

Node.js routes ::ffff:7f00:1 to 127.0.0.1:

// net.connect({ host: '::ffff:7f00:1', port: 80 }) reaches a service                                                                                                       
// bound to 127.0.0.1:80 — confirmed on Node.js v24, Linux and macOS.                                                                                                         

Cloud metadata SSRF: ::ffff:a9fe:a9fe = ::ffff:169.254.169.254. If NO_PROXY=169.254.169.254 is set to block IMDS access, a request to http://[::ffff:a9fe:a9fe]/latest/meta-data/ bypasses it.

Fix

Canonicalise IPv4-mapped IPv6 in normalizeNoProxyHost before any comparison:

const ipv4MappedDotted = /^::ffff:(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})$/i;                                                                                                    
const ipv4MappedHex    = /^::ffff:([0-9a-f]{1,4}):([0-9a-f]{1,4})$/i;                                                                                                         
                                                                                                                                                                             
function hexToIPv4(a, b) {                                                                                                                                                    
 const hi = parseInt(a, 16), lo = parseInt(b, 16);                                                                                                                           
 return `${hi >> 8}.${hi & 0xff}.${lo >> 8}.${lo & 0xff}`;                                                                                                                   
}                                                                                                                                                                             
                                                                                                                                                                             
const normalizeNoProxyHost = (hostname) => {                                                                                                                                  
 if (!hostname) return hostname;                                                                                                                                           
 if (hostname[0] === '[' && hostname.at(-1) === ']')
   hostname = hostname.slice(1, -1);                                                                                                                                         
 hostname = hostname.replace(/\.+$/, '').toLowerCase();
                                                                                                                                                                             
 let m;                                                                                                                                                                    
 if ((m = hostname.match(ipv4MappedDotted))) return m[1];                                                                                                                    
 if ((m = hostname.match(ipv4MappedHex)))    return hexToIPv4(m[1], m[2]);                                                                                                   
 return hostname;                                                                                                                                                            
};

Impact

Any application that sets NO_PROXY to exclude internal or metadata endpoints and uses an HTTP/HTTPS proxy can have those exclusions bypassed by a URL using IPv4-mapped IPv6 notation. The attacker must control the request URL. In cloud environments with instance metadata services, this can lead to credential exfiltration.