An attacker can spoof a packet from a legitimate ntpd server with an origin
timestamp that matches the peer->dst timestamp recorded for that server.
After making this switch, the client in NTP 4.2.8p4 and earlier and NTPSec
aa48d001683e5b791a743ec9c575aaf7d867a2b0c will reject all future legitimate
server responses. It is possible to force the victim client to move time
after the mode has been changed. ntpq gives no indication that the mode has
been switched.