The symmetric-key feature in the receive function in ntp_proto.c in ntpd in NTP 4.x before 4.2.8p2 requires a correct MAC only if the MAC field has a nonzero length, which makes it easier for man-in-the-middle attackers to spoof packets by omitting the MAC.
This entry has been deprecated. It was originally used for organizing the Development View (CWE-699) and some other views, but it introduced unnecessary complexity and depth to the resulting tree.