A site can be compromised for no other reason than to provide a staging post for attacks on other sites, and to the attacker, it means little more than another IP address. Simplicity Software is complex. As the size of a piece of software grows it becomes increasingly difficult to test all eventualities. Complex code will probably have unknown loopholes that an attacker can exploit. These loopholes may be convoluted but that will not prevent an attacker from trying to exploit them, some of the exploit attacks against sendmail have been extremely intricate.
Simplicity is an important factor in sound network defences.
Application level gateway network Security systems should have all extraneous functionality removed and should be kept as small and simple as possible. Host Based Security Host based security is probably the most common computer security model in current use. The major problem with the host based security model is that it does not scale well. Even if all hosts are identical, the sheer number of them at some sites makes securing each of them difficult. Effectively implementing and maintaining host security takes a significant amount of time and effort, and is a complex task.