NTP before 4.2.8p9 rate limits responses received from the configured sources when rate limiting for all associations is enabled, which allows remote attackers to cause a denial of service (prevent responses from the sources) by sending responses with a spoofed source address.
http://rhn.redhat.com/errata/RHSA-2017-0252.html
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbux03706en_us
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:39.ntp.asc
https://usn.ubuntu.com/3707-2/