CVE-2026-101910

No classifier on Address6 recognizes the NAT64 local-use range 64:ff9b:1::/48 (RFC 8215). isPrivate(), isLoopback(), isLinkLocal() and their siblings all return false for every address in it, so an internal IPv4 destination written through a local-use NAT64 prefix (64:ff9b:1:7f00:0:100:: for 127.0.0.1, 64:ff9b:1:a9fe:a9:fe00:: for 169.254.169.254) reads as an ordinary global address. getType() names the range 'NAT64 (local-use)', so the library knows what the address is and classifies it as nothing.

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) will classify 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

The fix for GHSA-22jq-vg5j-6vgg (10.2.1) classifies IPv4-mapped (::ffff:0:0/96) and NAT64 well-known (64:ff9b::/96) addresses by their embedded IPv4 address: embeddedIPv4() in src/ipv6.ts decodes the trailing 32 bits and every special-use classifier delegates to the resulting Address4. Those are the only two ranges embeddedIPv4() handles.

The local-use range differs in kind from the well-known prefix, which is why extending the decoder does not fix it. RFC 8215 reserves 64:ff9b:1::/48 so an operator can carve their own NAT64 prefix out of it, of any length RFC 6052 allows (/48, /56, /64, or /96). Where the IPv4 address sits depends on that choice: 127.0.0.1 is 64:ff9b:1:7f00:0:100:: under a /48 prefix and 64:ff9b:1::7f00:1 under a /96 prefix, and each of those decodes to a different address under the other prefix length. There is no single embedded IPv4 address for the library to classify by.

What is well-defined is the range as a whole. The IANA IPv6 Special-Purpose Address Registry lists 64:ff9b:1::/48 as not globally reachable, and Python's ipaddress module reports is_private as True and is_global as False for every address in it. isPrivate() covered ULA (fc00::/7) plus the decoded mapped and well-known cases, and nothing in the local-use range.

Affected versions

>= 10.2.0, <= 10.5.0. The is* classification API was extended to Address6 in 10.2.0; releases before it do not expose the method and are not affected through this vector.

Reachability

Reaching an internal host through one of these addresses requires that the server's network run a NAT64 translator on a prefix inside 64:ff9b:1::/48 and that the attacker guess or know the prefix length. That is the same precondition the well-known prefix carries (a translator on 64:ff9b::/96), with the additional constraint that the prefix is operator-chosen rather than fixed. The severity reflects that precondition.

Proof of concept

npm i [email protected], then:

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

// A guard of the shape the library documents.
function isBlocked(host) {
  const a = new Address6(host);
  return a.isLoopback() || a.isLinkLocal() || a.isPrivate() || a.isMulticast() || a.isUnspecified();
}

for (const h of ['64:ff9b::7f00:1', '64:ff9b:1:7f00:0:100::', '64:ff9b:1::7f00:1']) {
  console.log(isBlocked(h) ? 'BLOCK' : 'ALLOW', h, '-> getType()', new Address6(h).getType());
}

On affected versions:

BLOCK 64:ff9b::7f00:1 -> getType() NAT64 (well-known)
ALLOW 64:ff9b:1:7f00:0:100:: -> getType() NAT64 (local-use)
ALLOW 64:ff9b:1::7f00:1 -> getType() NAT64 (local-use)

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.