The process_packet function in ntp_proto.c in ntpd in NTP 4.x before 4.2.8p8 allows remote attackers to cause a denial of service (peer-variable modification) by sending spoofed packets from many source IP addresses in a certain scenario, as demonstrated by triggering an incorrect leap indication.
https://www.kb.cert.org/vuls/id/321640
https://us-cert.cisa.gov/ics/advisories/icsa-21-159-11
https://us-cert.cisa.gov/ics/advisories/icsa-21-103-11
https://security.gentoo.org/glsa/201607-15
https://security.FreeBSD.org/advisories/FreeBSD-SA-16:24.ntp.asc
https://h20566.www2.hpe.com/hpsc/doc/public/display?docLocale=en_US&docId=emr_na-hpesbhf03757en_us
https://cert-portal.siemens.com/productcert/pdf/ssa-497656.pdf
https://cert-portal.siemens.com/productcert/pdf/ssa-211752.pdf
http://www.ubuntu.com/usn/USN-3096-1
http://www.securitytracker.com/id/1036037
http://www.securityfocus.com/archive/1/archive/1/540683/100/0/threaded
http://www.securityfocus.com/archive/1/archive/1/538600/100/0/threaded
http://www.securityfocus.com/archive/1/archive/1/538599/100/0/threaded
http://www.securityfocus.com/archive/1/540683/100/0/threaded
http://www.securityfocus.com/archive/1/538600/100/0/threaded
http://www.securityfocus.com/archive/1/538599/100/0/threaded
http://www.oracle.com/technetwork/topics/security/bulletinapr2016-2952098.html
http://www.kb.cert.org/vuls/id/321640
http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20160603-ntpd
http://support.ntp.org/bin/view/Main/SecurityNotice
http://support.ntp.org/bin/view/Main/NtpBug3044
http://packetstormsecurity.com/files/137322/FreeBSD-Security-Advisory-FreeBSD-SA-16-24.ntp.html
http://packetstormsecurity.com/files/137321/Slackware-Security-Advisory-ntp-Updates.html
http://lists.opensuse.org/opensuse-security-announce/2016-08/msg00042.html
http://lists.opensuse.org/opensuse-security-announce/2016-07/msg00026.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00040.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00028.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00024.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00023.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00020.html
http://lists.opensuse.org/opensuse-security-announce/2016-06/msg00018.html