Not sure what version of Nuke your running over there but I did run a number of exploits that I know of and the site seems pretty secure from sql injections that would allow a hacker to gain administrator access. That aside your host may have globals turned on in the php.ini folder this can be a big problem. Best thing to do is ask your host about that. 

Now if you want my opinion the most secure nuke that I have used is called Raven Nuke 7.6(google search it) it has a lot of built in security protection. You could try to update your current site with Raven Nuke but this will involve a lot of database work and if your not to good with that this may not be the option for you. Another option would be to install Nuke Sentinel it will prolly require you to edit a number of files in your current installation of nuke. It's not !00% but it will stop about 95% of the known hacks for phpNuke. Other than that the only other thing I can think to do is find out what version of Nuke your running and make sure you have the latest patches for it.