ip-address: Address6.isLinkLocal() recognizes fe80::/64 rather than fe80::/10, allowing SSRF and trust-boundary bypass to on-link hosts

Summary

Address6.isLinkLocal() recognizes fe80::/64 rather than fe80::/10. Link-local unicast is the whole /10 under RFC 4291 §2.4 and the IANA IPv6 Special-Purpose Address Registry, so the method returns false for every link-local address outside the one /64 that stateless address autoconfiguration happens to use. new Address6('fe81::1').isLinkLocal() is false.

The library contradicts itself on the same object: for fe81::1, getType() returns 'Link-local unicast', getScope() returns 'Link local', and isHostInSubnet(new Address6('fe80::/10')) returns true, while isLinkLocal() returns false.

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 a link-local target as unremarkable 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.

Details

isLinkLocal() in src/ipv6.ts compares the first 64 bits of the address against a literal string:

// Zeroes are required, i.e. we can't check isHostInSubnet with 'fe80::/10'
if (
  this.getBitsBase2(0, 64) ===
  '1111111010000000000000000000000000000000000000000000000000000000'
) {
  return true;
}

The comparison requires the first 64 bits to be exactly fe80:0000:0000:0000, so it accepts 2⁶⁴ of the 2¹¹⁸ addresses in fe80::/10. The comment states a premise the library disproves: getType() classifies the same range with isHostInSubnet against the 'fe80::/10': 'Link-local unicast' entry in src/v6/constants.ts, and Address4.isLinkLocal() is a plain isHostInSubnet test against 169.254.0.0/16. RFC 4291 §2.5.6 constrains the format of an autoconfigured link-local address; it does not define the range, and reading it as the definition is the likeliest origin of the /64 comparison.

The IPv4-mapped and NAT64 well-known paths of isLinkLocal() are unaffected: ::ffff:169.254.169.254 and 64:ff9b::a9fe:a9fe are classified by their embedded IPv4 address and report true.

Affected versions

<= 10.5.0. The comparison has had this shape since Address6.isLinkLocal() was introduced, so every release exposing the method is affected.

Impact

Every well-formed address in fe80::/10 outside fe80::/64 is parsed successfully, isValid() is true, and the classifier reports something untrue about it. No other classifier catches these addresses: isPrivate() covers ULA (fc00::/7), not link-local.

Python's ipaddress module, the IN6_IS_ADDR_LINKLOCAL macro in netinet6/in6.h, and Linux's ipv6_addr_type() all apply a ten-bit prefix test and classify every row above as link-local.

A request admitted through a guard built on isLinkLocal() reaches a link-local host on the server's own segment: a neighboring machine or the on-link router. An IPv6 link-local destination generally needs a zone index and a neighbor on the same link, so the reach is the server's own segment rather than the internet or a universal metadata endpoint, and the severity reflects that.

Proof of concept

npm i ip-address@10.5.0, 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 ['fe80::1', 'fe81::1', 'febf::1', 'fe80:0:0:1::1']) {
  const a = new Address6(h);
  console.log(isBlocked(h) ? 'BLOCK' : 'ALLOW', h, '-> getType()', a.getType());
}

On affected versions:

BLOCK fe80::1 -> getType() Link-local unicast
ALLOW fe81::1 -> getType() Link-local unicast
ALLOW febf::1 -> getType() Link-local unicast
ALLOW fe80:0:0:1::1 -> getType() Link-local unicast

Remediation

Upgrade to the patched release. In the fix, isLinkLocal() tests the address against fe80::/10 with the same isHostInSubnet predicate getType() and Address4.isLinkLocal() use. The same release adds 2001::/32 to the type table so getType() reports 'Teredo' for the addresses isTeredo() returns true for; that is a consistency correction with no security effect.

If you cannot upgrade immediately, test the range directly:

const LINK_LOCAL = new Address6('fe80::/10');
const linkLocal = new Address6(host).isHostInSubnet(LINK_LOCAL);

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.