It was found that when NTP is configured in broadcast mode, a man-in-the-middle attacker or a malicious client could replay packets received from the broadcast server to all (other) clients. This could cause the time on affected clients to become out of sync over a longer period of time.
Created ntp tracking bugs for this issue:
Affects: fedora-all [bug 1300277]
The issue described here is an inherent problem with securing packet transfer in broadcast mode. The man-in-the-middle attacker also must have access to the same network in which the packets are being broadcasted.
Do not use NTP's broadcast mode by not configuring the "broadcast" directive in the ntp.conf file.
This issue affects the versions of ntp as shipped with Red Hat Enterprise Linux 5, 6, and 7. Red Hat Product Security has rated this issue as having Moderate security impact. A future update may address this issue in Red Hat Enterprise Linux 6 and 7. For additional information, refer to the Issue Severity Classification: https://access.redhat.com/security/updates/classification/.