express-rate-limit: IPv4-mapped IPv6 addresses bypass per-client rate limiting on servers with dual-stack network

Summary

The default keyGenerator in express-rate-limit applies IPv6 subnet masking (/56 by default) to all addresses that net.isIPv6() returns true for. This includes IPv4-mapped IPv6 addresses (::ffff:x.x.x.x), which Node.js returns as request.ip on dual-stack servers.

Because the first 80 bits of all IPv4-mapped addresses are zero, a /56 (or any /32 to /80) subnet mask produces the same network key (::/56) for every IPv4 client. This collapses all IPv4 traffic into a single rate-limit bucket: one client exhausting the limit causes HTTP 429 for all other IPv4 clients.

Details

Root Cause

In source/ip-key-generator.ts:

export function ipKeyGenerator(ip: string, ipv6Subnet: number | false = 56) {
  if (ipv6Subnet && isIPv6(ip)) {
    return `${new Address6(`${ip}/${ipv6Subnet}`).startAddress().correctForm()}/${ipv6Subnet}`
  }
  return ip
}

net.isIPv6('::ffff:192.168.1.1') returns true, so IPv4-mapped addresses enter the subnet masking path. With a /56 prefix, the start address for any ::ffff:x.x.x.x is ::, producing the key ::/56.

Proof of Concept

const { isIPv6 } = require('net');
const { Address6 } = require('ip-address');

function ipKeyGenerator(ip, ipv6Subnet = 56) {
  if (ipv6Subnet && isIPv6(ip)) {
    return `${new Address6(`${ip}/${ipv6Subnet}`).startAddress().correctForm()}/${ipv6Subnet}`;
  }
  return ip;
}

console.log(ipKeyGenerator('::ffff:192.168.1.1', 56)); // ::/56
console.log(ipKeyGenerator('::ffff:10.0.0.1', 56));    // ::/56
console.log(ipKeyGenerator('::ffff:8.8.8.8', 56));     // ::/56
// ALL produce '::/56' — same bucket

End-to-End Validation

On a dual-stack Express server (app.listen(port, '::')), tested with Express 5.2.1:

• request.ip for IPv4 clients is ::ffff:127.0.0.1
• Rate limit key resolves to ::/56
• After limit requests from any IPv4 client, all other IPv4 clients receive 429

When This Occurs

• Node.js dual-stack servers (default on Linux when listening on ::)
• Any environment where request.ip contains IPv4-mapped IPv6 addresses