ntpd in ntp before 4.2.8p14 and 4.3.x before 4.3.100 allows remote attackers to cause a denial of service by predicting transmit timestamps for use in spoofed packets. The victim must be relying on unauthenticated IPv4 time sources. There must be an off-path attacker who can query time from the victim"s ntpd instance. ntpd in ntp before 4.2.8p14 and 4.3.x before 4.3.100 allows an off-path attacker to block unauthenticated synchronization via a server mode packet with a spoofed source IP address, because transmissions are rescheduled even when a packet lacks a valid origin timestamp