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.
Source: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-1798
Reference (s):
- APPLE:APPLE-SA-2015-06-30-2
- URL: http://lists.apple.com/archives/security-announce/2015/Jun/msg00002.html
- BID:73951
- URL: http://www.securityfocus.com/bid/73951
- CERT-VN:VU#374268