GHSA-j4rj-2jr5-m439

Summary

ssrfcheck v1.3.0 (latest) fails to block Server-Side Request Forgery attacks when the target private IP address is encoded as an IPv4-mapped IPv6 address (e.g. http://[::ffff:127.0.0.1]/). The WHATWG URL parser built into Node.js silently normalizes the IPv4 notation inside the brackets to compressed hex form ([::ffff:7f00:1]) before the library's private-IP regex ever runs. The regex was written to match dot-notation only and therefore never matches any real input — all seven IANA private IPv4 ranges, including the AWS/GCP/Azure metadata address 169.254.169.254, are bypassed. Any application using isSSRFSafeURL() to guard HTTP requests made with user-supplied URLs is fully exposed to SSRF.

Details

Vulnerable file: src/is-private-ip.js

The library detects IPv6 private addresses using the privIp6() function. The relevant portion:

// src/is-private-ip.js  (lines ~40-60 of the published source)
function privIp6 (ip) {
  return /^::$/.test(ip) ||
    /^::1$/.test(ip) ||
    /^::f{4}:([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$/.test(ip) ||
    /^::f{4}:0.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$/.test(ip) ||
    /^64:ff9b::([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})\.([0-9]{1,3})$/.test(ip) ||
    // ... more patterns, all expect dot-notation ...
}

The third line is the IPv4-mapped IPv6 check. It expects input in the form ::ffff:127.0.0.1 (dots). However, the IP is extracted from the URL using url.hostname, which goes through the WHATWG URL parser first.

How WHATWG URL normalizes the address (src/parse-url.js):

const url = new URL(normalizeURLStr(input));   // WHATWG URL parser runs here
const ipcheck = trimBrackets(url.hostname);    // e.g. '::ffff:7f00:1'  ← hex, no dots
const ipVersion = isIP(ipcheck);               // returns 6

The WHATWG URL spec (§5.3 IPv6 serializer) converts all embedded IPv4 notation to two 16-bit hex groups during parsing:

127.0.0.1...