diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 917e669cd..fd4325b3a 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -39,6 +39,13 @@ the key handle be supplied for each signature operation. U2F tokens
 primarily use ECDSA signatures in the NIST-P256 field, though the FIDO2
 standard specifies additional key types, including one based on Ed25519.
 
+Use of U2F security keys does not automatically imply multi-factor
+authentication. From sshd’s perspective, a security key constitutes a
+single factor of authentication, even if protected by a PIN or biometric
+authentication.  To enable multi-factor authentication in ssh, please
+refer to the AuthenticationMethods option in sshd_config(5).
+
+
 SSH U2F Key formats
 -------------------