The monlist command against NTP servers is being used to launch DDOS attacks.

ESXi 4.1 contains a vulnerability that can be used to create an amplification attack using the molist command in NTP.

If you’re using that version, you can mitigate the issue by editing /etc/ntp.conf and adding noquery and nopeer to the restrict line 

restrict default kod nomodify notrap noquery nopeer restrict driftfile /etc/ntp.drift

Once done, restart NTP with /etc/init.d/ntpd restart