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."
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-11
https://security.netapp.com/advisory/ntap-20171031-0001/
https://security.gentoo.org/glsa/201607-15
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:09.ntp.asc
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbhf03766en_us
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbhf03750en_us
https://cert-portal.siemens.com/productcert/pdf/ssa-497656.pdf
http://www.securitytracker.com/id/1034782
http://www.securityfocus.com/bid/81960
http://www.debian.org/security/2016/dsa-3629
http://support.ntp.org/bin/view/Main/NtpBug2936