An off-path attacker can cause a preemptible client association to be demobilized in NTP 4.2.8p4 and earlier and NTPSec a5fb34b9cc89b92a8fef2f459004865c93bb7f92 by sending a crypto NAK packet to a victim client with a spoofed source address of an existing associated peer. This is true even if authentication is enabled.
http://rhn.redhat.com/errata/RHSA-2016-1552.html
https://access.redhat.com/errata/RHSA-2016:1141
https://cert-portal.siemens.com/productcert/pdf/ssa-211752.pdf
https://cert-portal.siemens.com/productcert/pdf/ssa-497656.pdf
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:16.ntp.asc
https://security.gentoo.org/glsa/201607-15
https://security.netapp.com/advisory/ntap-20171004-0002/
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-11
http://www.debian.org/security/2016/dsa-3629
http://www.oracle.com/technetwork/topics/security/bulletinapr2016-2952098.html
http://www.oracle.com/technetwork/topics/security/linuxbulletinapr2016-2952096.html