Important configuration changes for NTP servers
If you are using the standard ntpd daemon to serve time to the public internet, it’s important that you make sure it is configured to not reply to “monlist” queries. Many routers and other equipment are included in this.
The configuration recommendations include the appropriate “restrict” lines to disallow any management queries to ntpd. Most Linux distributions will have an updated version by now that just disables the “monlist” queries, that will also solve the primary problem.
The NTP Support wiki has more information.
If you operate a network you can use the Open NTP Project to see if you have vulnerable devices on your network.