Verify passkey algorithms from COSE keys

authwebauthn/service.go

@@ -375,7 +375,7 @@ func (service *Service) finishRegistrationCeremony(ctx context.Context, ceremony
 	if err != nil {
 		return Credential{}, fmt.Errorf("authwebauthn: verify registration: %w", err)
 	}
-	if verified.Attestation.PublicKeyAlgorithm != int64(webauthncose.AlgES256) {
+	if err = enforceCredentialAlgorithm(verified); err != nil {
 		return Credential{}, ErrUnsupportedCredential
 	}
 	encoded, err := json.Marshal(verified)
@@ -407,6 +407,26 @@ func (service *Service) finishRegistrationCeremony(ctx context.Context, ceremony
 	return record, nil
 }
 
+// enforceCredentialAlgorithm derives the algorithm from the verified COSE key
+// carried inside authenticator data. AuthenticatorAttestationResponse's
+// publicKeyAlgorithm member is an optional browser convenience value: clients
+// that serialize the mandatory attestation object directly may omit it, and it
+// is not the cryptographically authoritative representation.
+func enforceCredentialAlgorithm(credential *wa.Credential) error {
+	if credential == nil {
+		return ErrUnsupportedCredential
+	}
+	parsed, err := webauthncose.ParsePublicKey(credential.PublicKey)
+	if err != nil {
+		return ErrUnsupportedCredential
+	}
+	key, ok := parsed.(webauthncose.EC2PublicKeyData)
+	if !ok || key.Algorithm != int64(webauthncose.AlgES256) {
+		return ErrUnsupportedCredential
+	}
+	return nil
+}
+
 func (service *Service) BeginLogin(ctx context.Context) (BeginResult, error) {
 	challenge, err := service.randomBytes(32)
 	if err != nil {