One of the most common one is Sqli (Sql injection), most people use this to get into databases etc that use Mysql, their is other ways that people can hack into forums, but i'm not sure on all the methods, Take a look at "Hack Forums" It shows a lot of things about hacking websites/forums so you can use that to learn how to secure your website/blog/forum better, Hope i helped! Good luck! 🙂