SimpleWebAuthn: Registration verification does not sufficiently ensure that attestation certificates chain to a trust anchor

Summary

validateCertificatePath() does not verify that an attestation's certificate chain actually terminates at a configured trust anchor. When walking the chain it stops at the first self-signed certificate it finds (which could be user-supplied), and exits early.

This happens before the configured Apple/Google/etc trust anchor (which is concatenated to the end of the chain) is reached.

A user can therefore register a credential and have the server accept it as if it were backed by a genuine Apple / Android SafetyNet / Yubikey / etc.

Details

packages/server/src/helpers/validateCertificatePath.ts:

The configured trust anchor is appended to the end of the untrusted chain (line 83):

const x5cWithTrustAnchor = x5cCertsParsed.concat([anchor]);

The walk then verifies each cert was signed by the next, but breaks on the first self-signed cert (lines 104–116):

if (issuer.subject === issuer.issuer) {
  // Root cert detected, make sure it signed itself
  const issuerSignedIssuer = await issuer.verify(
    { publicKey: issuer.publicKey, signatureOnly: true },
    WebCrypto,
  );
  if (!issuerSignedIssuer) {
    throw new InvalidSubjectAndIssuer();
  }
  break;   // <-- exits before the appended trust anchor is ever checked if user supplied self-signed cert comes first
}

The success condition is therefore "the certs form an internally-consistent chain ending in some self-signed cert" Rather than "the chain terminates at one of the configured trust anchors."

Exploit shape

attacker sends:  x5c = [ forgedLeaf (signed by attacker root),
                         attackerSelfSignedRoot ]

library builds:  [ forgedLeaf, attackerSelfSignedRoot, <configured Apple/Google/etc root> ]

walk:  forgedLeaf -> attackerSelfSignedRoot        (verifies, attacker controls both)
       attackerSelfSignedRoot is self-signed        -> break
       attackerSelfSignedRoot -> configured root    (NEVER CHECKED)

return true. The configured anchor never gets checked.

As far as observed, all attestation enforcement uses validateCertificatePath when using MDS etc.