This indicates an attack attempt to exploit a Denial of Service Vulnerability in NTP Daemon
The vulnerability is due to an improper validation when a vulnerable application parses a crafted mrulist query packet. A remote attacker could exploit this to crash vulnerable systems, via a crafted request.
ntp.org NTP prior to 4.2.8p9
Denial of Service: Remote attackers can crash vulnerable systems.
Refer to the vendor's web site for suggested workaround.