1. ddos
2. Sql Injection
3. Brute Force attack @ passwords.
4. Vulnerability inside the source.
Protection
1. Get an good Dedicated server
2. Fix Sql injections on your website and source.
3. If its on your website then you can set an timer that if an person has submitted 3x on wrong password it will block it for some time.
Flyff already has build in brute force. Is it on your server then the server hoster needs to have an timer to.
4. Fix the source.
have you been try this before ? , i mean about the security of flyff website and so on , since you already mentioned about fixing source so can you show us what we need to change on the source
Not sure if that was a tongue-in-cheek comment making fun of zealous Linux users, but an inexperienced configuration of Linux is generally less secure than an inexperienced configuration of Windows.
Also generally there aren't any hackers left in the Flyff scene that will even target your operating system.
The one thing that many forget is to change the Administrator account name for your server. This is the first thing you must do because they can brute force attempt to crack the password with ease.