Multiple vulnerabilities has been found and corrected in ntp:

The symmetric-key feature in the receive function in ntp_proto.c
in ntpd in NTP before 4.2.8p2 requires a correct MAC only if the MAC
field has a nonzero length, which makes it easier for man-in-the-middle
attackers to spoof packets by omitting the MAC (CVE-2015-1798).

The symmetric-key feature in the receive function in ntp_proto.c
in ntpd in NTP before 4.2.8p2 performs state-variable updates
upon receiving certain invalid packets, which makes it easier
for man-in-the-middle attackers to cause a denial of service
(synchronization loss) by spoofing the source IP address of a peer
(CVE-2015-1799).

The updated packages provides a solution for these security issues.