NTP before 4.2.8p10 and 4.3.x before 4.3.94 allows remote attackers to cause a denial of service (ntpd crash) via a malformed mode configuration directive.
https://access.redhat.com/errata/RHSA-2017:3071
https://access.redhat.com/errata/RHSA-2018:0855
https://security.FreeBSD.org/advisories/FreeBSD-SA-17:03.ntp.asc
https://support.apple.com/HT208144
https://support.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbux03962en_us