The NTP Protocol allows for both non-authenticated and authenticated associations, in client/server, symmetric (peer), and several broadcast modes. In addition to the basic NTP operational modes, symmetric mode and broadcast servers can support an interleaved mode of operation. In ntp-4.2.8p4 a bug was inadvertently introduced into the protocol engine that allows a non-authenticated zero-origin (reset) packet to reset an authenticated interleaved peer association. If an attacker can send a packet with a zero-origin timestamp and the source IP address of the "other side" of an interleaved association, the 'victim' ntpd will reset its association. The attacker must continue sending these packets in order to maintain the disruption of the association. In ntp-4.0.0 thru ntp-4.2.8p6, interleave mode could be entered dynamically. As of ntp-4.2.8p7, interleaved mode must be explicitly configured/enabled.
References:
http://support.ntp.org/bin/view/Main/NtpBug3454