GHSA-6hxq-p678-4hr2: Npm/@simplewebauthn/server vulnerability

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):

ts 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):

ts 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.