Why Do Rootkits Exist?
Rootkits are a relatively recent invention, but spies are as old as war. Rootkits exist for the same reasons that audio bugs exist. People want to see or control what other people are doing. With the huge and growing reliance on data processing, computers are natural targets.
Rootkits are useful only if you want to maintain access to a system. If all you want to do is steal something and leave, there is no reason to leave a rootkit behind. In fact, leaving a rootkit behind always opens you to the risk of detection. If you steal something and clean up the system, you may leave no trace of your operation.
Rootkits provide two primary functions: remote command and control, and software eavesdropping.
Remote Command and Control
Remote command and control (or simply "remote control") can include control over files, causing reboots or "Blue Screens of Death," and accessing the command shell (that is, cmd.exe or /bin/sh). Figure 1-1 shows an example of a rootkit command menu. This command menu will give you an idea of the kinds of features a rootkit might include.
Example 1-1. Menu for a kernel rootkit.
Win2K Rootkit by the team rootkit.com Version 0.4 alpha ----------------------------------------- command description ps show process list help this data buffertest debug output hidedir hide prefixed file or directory hideproc hide prefixed processes debugint (BSOD)fire int3 sniffkeys toggle keyboard sniffer echo <string> echo the given string *"(BSOD)" means Blue Screen of Death if a kernel debugger is not present! *"prefixed" means the process or filename starts with the letters '_root_'. *"sniffer" means listening or monitoring software.
Software eavesdropping is all about watching what people do. This means sniffing packets, intercepting keystrokes, and reading e-mail. An attacker can use these techniques to capture passwords and decrypted files, or even cryptographic keys.
Legitimate Uses of Rootkits
As we alluded to already, rootkits can be used for legitimate purposes. For instance, they can be used by law-enforcement agencies to collect evidence, in an advanced bugging operation. This would apply to any crime in which a computer is used, such as computer trespass, creating or distributing child pornography, software or music piracy, and DMCA  violations.
Rootkits can also be used to fight wars. Nations and their militaries rely heavily on computing machinery. If these computers fail, the enemy's decision cycle and operations can be affected. The benefits of using a computer (versus conventional) attack include that it costs less, it keeps soldiers out of danger, it causes little collateral damage, and in most cases it does not cause permanent damage. For instance, if a nation bombs all the power plants in a country, then those power plants will need to be rebuilt at great expense. But if a software worm infects the power control network and disables it, the target country still loses use of the power plants' output, but the damage is neither permanent nor as expensive.