Regardless of installing zpanel or not. If your machine is connected to the internet, you always should install and configure a firewall. Here is a helpfull tutorial: https://www.digitalocean.com/community/articles/how-to-set-up-a-firewall-using-ip-tables-on-ubuntu-12-04

Also make sure you open up these ports (preferably to your IP only):

http://docs.zpanelcp.com/?node=40