Well, to not answer your question like the person above me, I can say this: Generally, if an application runs from someone's comp via a certain port, that port is only vulnerable if the said application is vulnerable. In other words, if you want to get into his comp, you'll have to exploit minecraft itself. I don't play minecraft, so I have no idea if there's a vulnerability in it, but seeing as it is wildly popular now, I doubt it. If there is one, it's either been fixed now, or has yet to be found. So either find it yourself or google and hope that there's a hole that hasn't been patched. That's about all you can do regarding that.