NTP before 4.2.8p9 changes the peer structure to the interface it receives the response from a source, which allows remote attackers to cause a denial of service (prevent communication with a source) by sending a response for a source to an interface the source does not use.
https://www.kb.cert.org/vuls/id/633847
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbux03706en_us
https://bto.bluecoat.com/security-advisory/sa139
http://www.securitytracker.com/id/1037354
http://www.securityfocus.com/bid/94453
http://www.oracle.com/technetwork/security-advisory/cpuoct2017-3236626.html
http://support.ntp.org/bin/view/Main/SecurityNotice#Recent_Vulnerabilities
http://support.ntp.org/bin/view/Main/NtpBug3072