This indicates an attack attempt to exploit a Security Bypass 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 bypass authentication mechanism of the target application, via a crafted request.
ntp.org NTP prior to 4.2.8p4
Security Bypass: Remote attackers can bypass security checks of vulnerable systems
Apply the most recent upgrade or patch from the vendor