But security threats come from both the Internet and from internal sources. In fact, the internal threat is considered the biggest problem for a number of reasons. First, internal users know more about your systems and where valuable data is located. Internal users are also more likely to hijack some other user's account or access some system because improper security measures are in place. You probably trust many of your coworkers, but in an open internetworked environment, you need to reevaluate how much trust you want to extend.
People also have more knowledge about computers and networks and know how to access information.
Often, a person gets curious and starts peaking into places where they don't belong. Before long, they discover all the areas of your network where security is weak. If you connect your internal network to it, anyone anywhere in the world can access your systems. But only if you leave the door wide-open. The Windows NT Security Handbook is about tightening the security on your Windows NT systems and networks to prevent malicious users from attacking your systems and valuable data.