Chapter 2 starts with a short section on understanding Linux security and even has an example of how a cracker might try to break into your system so you can start to understand how different pieces of security fit together. A bit later in this chapter, dangerous services such as NFS and finger are discussed. Many distributions enable these by default even though most users do not need them. They allow crackers to break in quite easily. Simply by spending a few minutes to turn them off, a system can be made much more secure than it was before.
Many networks are so vulnerable that anyone can do a lot of damage.
In organizations with at least some level of security consciousness, from what I've seen, it's employees who consistently cause the most harm. While the FBI and others claim that about half of security breaches are by employees, my experience is that most of them are by outsiders. With most systems connected to the Internet the external threat is much higher. The probabilities of attacks by recreational crackers, ideological extremists, and people with grudges vary greatly by site in a generally obvious way. More important than an intruder's philosophy is their capability.