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Viruses, worms, Trojan horses, and network intrusions are among the threats that security administrators worry about on a regular basis. However, there is a less familiar threat that many experts say could be just as dangerous: malicious bot software. A bot is a program that operates automatically as an agent for a user or another program. Hackers forward bots to victims by a number of means, and the software automatically infects vulnerable computers. The bots then wait for commands from a hacker, who can manipulate them and the infected systems without the user's knowledge. A hacker can install bots on multiple computers to set up botnets that they can use for massive distributed-denial-of-service (DDoS) attacks that overwhelm victimized systems' defenses. Botnets can also be used for mass spam mailings, installing key-logging software that can steal victims' passwords and data, and compromising computers to prepare them for infection by future viruses.