GHSA-2vr4-cq9g-pvrc

### Summary

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.

### Impact

Every address in `64:ff9b:1::/48` parses successfully, `isValid()` is `true`, and no classifier catches it. Which internal target is reached depends on the NAT64 prefix deployed on the server's network; the well-known prefix column is the patched control from GHSA-22jq-vg5j-6vgg.

| Internal target | Well-known form | Classified | Local-use form (`/48` prefix) | Classified | |---|---|---|---|---| | `127.0.0.1` | `64:ff9b::7f00:1` | loopback | `64:ff9b:1:7f00:0:100::` | nothing | | `10.0.0.1` | `64:ff9b::a00:1` | private | `64:ff9b:1:a00:0:100::` | nothing | | `169.254.169.254` | `64:ff9b::a9fe:a9fe` | link-local | `64:ff9b:1:a9fe:a9:fe00::` | nothing | | `192.168.1.1` | `64:ff9b::c0a8:101` | private | `64:ff9b:1:c0a8:1:100::` | nothing |

### 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 ip-address@10.5.0`, then:

```js 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) ```

### Remediation

Upgrade to the patched release. In the fix, `isPrivate()` returns `true` for every address in `64:ff9b:1::/48`, alongside ULA and the decoded mapped and well-known cases. The range is reported private as a whole rather than by a decoded IPv4 address, for the reason given above; `toAddress4Nat64(prefix)` remains the way to decode an address under a known deployment prefix. `isLoopback()` and `isLinkLocal()` are unchanged for this range, since without the prefix length the library cannot know which IPv4 address is embedded.

If you cannot upgrade immediately, test the range directly:

```js const NAT64_LOCAL_USE = new Address6('64:ff9b:1::/48'); const localUse = new Address6(host).isHostInSubnet(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.