Even with a private server box on a private server farm, if some one wants in they'll find a chink in the armour and get in. Server security can be taken only so far and the rest is how secure you make your site. Use a top name program or have one custom written with security in mind.
is great software and out of the box is very secure. It's when mods are made or mods installed you open up some holes that tend to get hammered.
Bottom line a server and the site are only as safe as you'll make them.