MUCH OF THE POWER OF A GNU/LINUX SYSTEM COMES FROM its support for multiple users and for networking. Many people can use the system at once, and they can connect to the system from remote locations. Unfortunately, with this power comes risk, especially for systems connected to the Internet. Under some circumstances, a remote “hacker” can connect to the system and read, modify, or remove files that are stored on the machine. Or, two users on the same machine can read, modify, or remove each other’s files when they should not be allowed to do so. When this happens, the system’s security is said to have been compromised.