A botnet is a collection of compromised (infected) computers under the collective control of remote attackers. The malware on the infected computer is known as a bot, a type of backdoor or remote access trojan (RAT). Bots communicate with botnet command and control (c&c) servers, enabling the remote attacker to update existing infections, push new malware, or instruct the infected computer to carry out specific tasks. In general, the presence of the bot gives the remote attacker the same abilities as the legitimate logged in user.
Botnets are used for everything from delivering spam, phishing attacks, data theft, to distributed denial of service attacks. Most botnets sell 'space' or 'services' on the botnet to bidders who may then deliver additional malware or use it for additional malicious purposes. These botnets-for-hire make it difficult to define what any specific botnet is intended to do, as that intent may change depending on the bidder.
Common botnets include the following:
