### Summary
`isInSubnet()` and `isHostInSubnet()` accept an address of either family and compare masked binary strings without checking that both operands are the same family. `Address4` pads to 32 bits and `Address6` to 128, so whenever the leading bits agree the strings are equal: `new Address6('a00::1').isInSubnet(new Address4('10.0.0.0/8'))` is `true`, and `new Address4('32.0.0.1').isInSubnet(new Address6('2000::/3'))` is `true`. No IPv4 address is inside an IPv6 network, so both answers are untrue.
An application that parses untrusted input as whichever family accepts it and then tests the result against a fixed-family allowlist can admit an address outside the list.
### Details
Both methods are in `src/common.ts`:
```ts
export function isInSubnet(this: Address4 | Address6, address: Address4 | Address6) {
if (this.subnetMask < address.subnetMask) {
return false;
}
return isHostInSubnet.call(this, address);
}
export function isHostInSubnet(this: Address4 | Address6, address: Address4 | Address6) {
return this.mask(address.subnetMask) === address.mask();
}
```
`mask(n)` returns the first `n` bits of the address as a string of `0` and `1`, taken from a representation padded to the family's width. `new Address6('a00::1').mask(8)` is `'00001010'`, and so is `new Address4('10.0.0.0/8').mask()`, so string equality reports containment. The signature admits either family on either side, so TypeScript raises nothing, and the `v4` property on `Address6` marks IPv4 notation (`::ffff:10.0.0.1`) rather than family, so it does not discriminate either.
Which cross-family pairs coincide depends on the network's prefix length. `mask(n)` on an `Address4` returns at most 32 bits, so an IPv6 network longer than `/32` never matches an IPv4 address, and an IPv6 network of `/32` or shorter matches exactly the IPv4 addresses whose leading bits equal its prefix. An IPv4 network is at most 32 bits and matches every IPv6 address whose leading bits equal its prefix.
### Affected versions
`<= 10.7.0`. The comparison has had this shape since the methods were written, so every release is affected.
### Impact
| Expression | Result | Reason |
|---|---|---|
| `new Address6('a00::1').isInSubnet(new Address4('10.0.0.0/8'))` | `true` | both mask to `00001010` |
| `new Address4('10.0.0.1').isInSubnet(new Address6('a00::/8'))` | `true` | the same bits, reversed |
| `new Address4('32.0.0.1').isInSubnet(new Address6('2000::/3'))` | `true` | both begin `001` |
| `new Address4('32.1.13.184').isInSubnet(new Address6('2001:db8::/32'))` | `true` | the `/32` spells an IPv4 address |
| `new Address6('cb00:7100::1').isInSubnet(new Address4('203.0.113.0/24'))` | `true` | the `/24` spells an IPv6 prefix |
| `new Address4('10.0.0.1').isInSubnet(new Address6('::ffff:10.0.0.0/104'))` | `false` | `/104` is longer than 32 bits |
| `new Address4('8.8.8.8').isInSubnet(new Address4('10.0.0.0/8'))` | `false` | same-family control |
In the allowlist direction the check admits an address outside the list; in the denylist direction it blocks an address outside the list. What the coincidence can admit is narrow. An IPv6 allowlist of `/32` or shorter admits the IPv4 addresses its prefix spells: `2001:db8::/32` admits exactly `32.1.13.184`, and `2000::/3` admits `32.0.0.0/3`. Those are public IPv4 addresses; the private, loopback, and link-local ranges begin with bit patterns no allocated IPv6 prefix shares. An IPv4 allowlist admits the IPv6 addresses its prefix spells, and those all sit in blocks IANA has not allocated. So a request admitted through this defect reaches an address outside the intended list, not an internal host, and the severity reflects that.
### Proof of concept
`npm i
[email protected]`, then:
```js
const { Address4, Address6 } = require('ip-address');
// An allowlist of the application's own IPv6 range, tested against whichever
// family the input parses as.
const allowed = new Address6('2001:db8::/32');
function parse(host) {
return Address4.isValid(host) ? new Address4(host) : new Address6(host);
}
for (const h of ['2001:db8::1', '2001:db9::1', '32.1.13.184', '32.1.13.185']) {
console.log(parse(h).isInSubnet(allowed) ? 'ALLOW' : 'BLOCK', h);
}
```
On affected versions:
```
ALLOW 2001:db8::1
BLOCK 2001:db9::1
ALLOW 32.1.13.184
BLOCK 32.1.13.185
```
The IPv6 rows are right. The IPv4 address whose 32 bits equal the allowlist's prefix is admitted; the one next to it is not.
### Remediation
Upgrade to the patched release. In the fix, `isHostInSubnet()` returns `false` when the two addresses are of different families, and `isInSubnet()` inherits the answer. The methods keep accepting either family so existing call sites compile. A caller that means to compare across families converts first, with `Address6.fromAddress4()`, `to4()`, or `toAddress4Nat64()`: `new Address6('::ffff:10.0.0.1').to4().isInSubnet(new Address4('10.0.0.0/8'))` is `true`, as before.
If you cannot upgrade immediately, compare the classes before you compare the addresses:
```js
const contained = host.constructor === network.constructor && host.isInSubnet(network);
```
### 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.
