Ok, here we go.
First of all, there are several different pieces to the hacking puzzle. There's software, obviously: code, backdoors, and exploits. There's hardware: knowing what hardware the server's running can give you insight into what can be done to break it. And there's social engineering: nowadays, hackers need to be able to talk to Real People, either in person or over the phone.
So. Primary stat would be intelligence, but charisma and wisdom are both useful as well. Hackers need to be able to multitask, and need to be thinking several steps ahead of their current position.
You might do something like this: Give the character the goal... say it's hacking into a bank, for instance. There would be several ways to do this. Hang around the building for a few days, maybe go inside and "apply for a loan", just to make notes of names and job descriptions. Keep an eye out for any security company names - they'll almost always be contracted, so you can drop names and be fairly confident that no one at the bank will catch onto the ruse. The software is easy to see - just poke at their website and see if you can't get an error page... they'll usually tell what software is running the server. From there, you can usually deduce what exploits are needed.
Of course, if you want to abstract it a bit, then you could just say, "Roll a spot check, roll an Int check, roll a Cha check," etc.
brb, someone's knocking at the door-