To prevent the intrusion, we need to know why the intruder is intruding.
Today's hackers accomplish many things. They create jobs in the information security market. They scare people out of their minds. They get companies to spend a lot of money on network security. Their attacks provide the police and FBI with somebody to chase. They give back to the people with community service and fund the governments by paying fines. They test hacking tools provided by people who know what they are doing. Many other things like this are accomplished by hackers around the world today. However, the one thing they do that really gets my goad and smacks of stupidity is ruin the internet, not only for everybody else, but for themselves also. For a lot of hackers, the internet is a source of entertainment, a job, or a hobby. Here is a simple point of logic that these intruders seem to be lacking. If, for example, there is something you do for enjoyment, or is the means of your livelihood, do you ruin it? A child can appreciate this logic. This inability to look beyond immediate benefits to oneself is a major problem in this society, and has given birth to hackers, criminals, PETA. This may seem to go wide of the scope for this paper, but think of it this way. If the "me first" attitude of current society were to be somehow eradicated, and replaced with a different philosophy, say the "Golden Rule". Would there be any malicious intrusions? On a broader scope, this would solve most of the problems in the world, not all. What I am getting at is this. In order to understand what we need to secure against, we need to understand the motivation, as well as the methods, that malicious intruders work under. Some of these motives can include, but are not limited to, greed, fame, revenge, misguided over righteousness, to prove something, to be a nuisance, and I suppose some form of insanity. .
As an IS professional, you would generally be more concerned with attacks made by individuals motivated by greed rather than fame.