"Most likely this is just internet bravado from someone who is hoping that the other person doesn't know any better. Even if they did hack into the forum software, there still are mountains of other barricades against hacking directly into your computer -- for example, if you use a router this makes it so your IP address isn't even externally available.
The way they could do damage is by getting you to run something malicious. Either by sending you a file that contains some sort of malware or trojan or by getting you to visit a site that loads the software on your computer."
