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
• Only affects the default keyGenerator (custom key generators are not affected)

Impact

• Denial of Service: A single client can block all IPv4 traffic by exhausting the shared rate limit
• Affects default configuration: No special options needed to trigger this

Affected Versions

All versions of express-rate-limit between v8.0.0 and v8.2.1.

Fix

This issue was fixed in commit 14e53888cdfd1b9798faf5b634c4206409e27fc4. This fix has been included in release v8.3.0, and backported to all affected minor versions in the form of releases v8.2.2, v8.1.1, and v8.0.2.