When working with contractors/interns how protective should one be of your codebase?

Question

We're considering hiring a contractor or intern to work with us on our iOS project but this causes me to get really paranoid that we're giving somebody who doesn't work for the company access to our precious codebase. I mean, I could just give them access to the classes I'd like them to work on but that seems rude and it would make it more difficult for them to develop what we need them to.

How paranoid should one be about people running off with their entire application?