This indicates an attack attempt to exploit a Denial of Services Vulnerability NTP Daemon
The vulnerability is due to an improper validation when vulnerable application parsing a crafted request. A remote attacker could exploit this to cause denial of services conditions within the affected service on the target machine, via a crafted request.
ntp.org NTP prior to 4.2.8p4
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor