The symmetric-key feature in the receive function in ntp_proto.c in ntpd in NTP 4.x 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
)
Impact
For BIG-IP systems using a default network time protocol (NTP) configuration, there is no impact. However, BIG-IP systems with an NTP configuration that is customized in line with the requirements of the advisory may be vulnerable.