NTP 4.x before 4.2.8p6 and 4.3.x before 4.3.90 do not verify peer associations of symmetric keys when authenticating packets, which might allow remote attackers to conduct impersonation attacks via an arbitrary trusted key, aka a "skeleton key."
http://rhn.redhat.com/errata/RHSA-2016-2583.html
https://cert-portal.siemens.com/productcert/pdf/ssa-497656.pdf
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbhf03750en_us
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbhf03766en_us
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:09.ntp.asc
https://security.gentoo.org/glsa/201607-15
https://security.netapp.com/advisory/ntap-20171031-0001/
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-11