NTP can be abused to amplify denial-of-service attack traffic
UDP protocols such as NTP can be abused to amplify denial-of-service attack traffic. Servers running the network time protocol (NTP) based on implementations of ntpd prior to version 4.2.7p26 that use the default unrestricted query configuration are susceptible to a reflected denial-of-service (DRDoS) attack. Other proprietary NTP implementations may also be affected.
Read more about this vulnerability here - http://www.kb.cert.org/vuls/id/348126