ntpd can be vulnerable to Sybil attacks. If a system is set up to use a trustedkey and if one is not using the feature introduced in ntp-4.2.8p6 allowing an optional 4th field in the ntp.keys file to specify which IPs can serve time, a malicious authenticated peer -- i.e. one where the attacker knows the private symmetric key -- can create arbitrarily-many ephemeral associations in order to win the clock selection of ntpd and modify a victim's clock. Three additional protections are offered in ntp-4.2.8p11. One is the new noepeer directive, which disables symmetric passive ephemeral peering. Another is the new ippeerlimit directive, which limits the number of peers that can be created from an IP. The third extends the functionality of the 4th field in the ntp.keys file to include specifying a subnet range. https://nvd.nist.gov/vuln/detail/CVE-2018-7170
