ip-address: misclassification of IPv4-mapped/NAT64 IPv6 addresses can bypass SSRF and trust-boundary checks

Summary

Address6's special-property checks misclassify IPv4-mapped (::ffff:0:0/96) and NAT64 well-known (64:ff9b::/96) IPv6 addresses. These checks classify an address by its IPv6 wrapper rather than by the IPv4 address it embeds, so isLoopback(), isLinkLocal(), isMulticast(), and isUnspecified() all return false for literals such as ::ffff:127.0.0.1 or ::ffff:169.254.169.254 that actually route to loopback, RFC 1918, or link-local (cloud-metadata) destinations. Address6 also had no isPrivate() method, so a mapped RFC 1918 address could not be detected at all.

An application that builds a network trust-boundary decision on these checks (for example, a filter intended to block Server-Side Request Forgery, or SSRF) may therefore treat an internal target as external and allow the request. SSRF is an attack in which a user-supplied address coaxes the server into making a request to an internal destination the user could not otherwise reach, such as a loopback service or a cloud metadata endpoint.

Details

Address6.getType() classifies an address by matching it against a table of known IPv6 special-use prefixes, returning Global unicast when nothing matches. That table had no entry for the IPv4-mapped range (::ffff:0:0/96), so every mapped address fell through to Global unicast; NAT64 addresses matched their own NAT64 … labels. The boolean checks isLoopback, isUnspecified, and isMulticast compared getType() against a fixed label and so returned false, while isLinkLocal and isULA checked only the native IPv6 ranges.

The library already exposed isMapped4() and to4(), but did not apply them inside these checks, so a mapped or NAT64 address was never normalized to its embedded IPv4 address before classification. The underlying CIDR matching is correct; the defect is that the special-use table omitted the IPv4-mapped range and the checks performed no embedded-IPv4 normalization.

Affected versions

>= 10.1.1, <= 10.2.0. The is* classification API was introduced for Address4 in 10.1.1 and extended to Address6 in 10.2.0. Releases before 10.1.1 do not expose this API and are not affected through this vector.

Impact

The misclassification covers the entire ::ffff:0:0/96 range, in both dotted and hex notation and case-insensitively, plus the 64:ff9b::/96 NAT64 well-known prefix:

For IPv4-mapped addresses the host OS routes to the IPv4 stack, so the misclassification is reachable on any dual-stack host. For NAT64, the classification bypass is unconditional but end-to-end reachability additionally requires a NAT64/DNS64 gateway in the deployment network.

Proof of concept

A guard assembled from these checks lets internal hosts through:

const { Address4, Address6 } = require('ip-address');

// true => block as internal, false => allow outbound
function isBlocked(host) {
  try {
    const a = new Address4(host);
    return a.isPrivate() || a.isLoopback() || a.isLinkLocal() || a.isCGNAT()
        || a.isMulticast() || a.isUnspecified() || a.isBroadcast();
  } catch {}
  try {
    const a = new Address6(host);
    return a.isLoopback() || a.isLinkLocal() || a.isULA()
        || a.isMulticast() || a.isUnspecified();
  } catch {}
  return false;
}

for (const h of ['127.0.0.1', '::1', '10.0.0.1', '8.8.8.8',
                 '::ffff:127.0.0.1', '::ffff:10.0.0.1',
                 '::ffff:169.254.169.254', '64:ff9b::7f00:1']) {
  console.log(isBlocked(h) ? 'BLOCK ' : 'ALLOW ', h);
}

On affected versions this prints (note that every ::ffff:… and 64:ff9b::… internal target is allowed):

BLOCK  127.0.0.1
BLOCK  ::1
BLOCK  10.0.0.1
ALLOW  8.8.8.8
ALLOW  ::ffff:127.0.0.1
ALLOW  ::ffff:10.0.0.1
ALLOW  ::ffff:169.254.169.254
ALLOW  64:ff9b::7f00:1

The first three lines (native loopback, native IPv6 loopback, and a literal RFC 1918 address) are blocked as expected; the IPv4-mapped and NAT64 forms of the same internal destinations are allowed through.

Remediation

Upgrade to the patched release. In the fix, Address6 normalizes IPv4-mapped and NAT64 well-known addresses to their embedded IPv4 address before classifying, via a new embeddedIPv4() helper that isLoopback, isLinkLocal, isMulticast, and isUnspecified consult first. Address6 also gains isPrivate(), isCGNAT(), and isBroadcast() for parity with Address4, and getType() now labels the ::ffff:0:0/96 range as IPv4-mapped. After upgrading, new Address6('::ffff:127.0.0.1').isLoopback() returns true and new Address6('::ffff:10.0.0.1').isPrivate() returns true.

If you cannot upgrade immediately, normalize embedded IPv4 addresses yourself before classifying: call to4() on any address where isMapped4() (or membership in 64:ff9b::/96) is true, and run your IPv4 checks against the result.

A note on SSRF defense

These methods are address classifiers, not a complete SSRF defense. Regardless of this fix, a robust SSRF guard must resolve the hostname and validate the resolved IP against the socket it connects to, and account for DNS rebinding and redirects. Treat these checks as one layer, not the only one.

Credit

Reported by @OV-0-VO.