The ntpq protocol is vulnerable to replay attacks. The sequence number being included under the signature fails to prevent replay attacks for two reasons. Commands that don't require authentication can be used to move the sequence number forward, and NTP doesn't actually care what sequence number is used so a packet can be replayed at any time. If, for example, an attacker can intercept authenticated reconfiguration commands that would. for example, tell ntpd to connect with a server that turns out to be malicious and a subsequent reconfiguration directive removed that malicious server, the attacker could replay the configuration command to re-establish an association to malicious server.
Upstream bug report:
Upstream has not released a fix for this issue and has opted for publishing a mitigation instead.
This issue can be mitigated by one of the following methods: disabling ntpq in ntp.conf, configuring ntpd to get time from multiple sources, or using a restriction list in your ntp.conf to limit who is allowed to issue ntpq queries.