DEFINITION of 'Botnet'
An combination of "robot" and "network," botnet is a group of programs that work together by communicating over the Internet. The word has negative connotations: although botnets can be used perform innocuous tasks, they are often used to send spam email, steal money from bank accounts or perform denial-of-service attacks.
BREAKING DOWN 'Botnet'
Illegal botnets that do the most damage are made up of hundreds of thousands of computers whose legitimate users don't even realize that malware (malicious software) had been installed on their machines, turning them into "bots" that can be controlled by the botnet's operator. Botnets exploit weaknesses in browser security, but often, computer users inadvertently download malware themselves when they click on deceptive pop-ups while browsing. Malware can also arrive via a "Trojan horse," such as an email attachment. Users should always keep their browser security up to date, avoid clicking on suspicious links and pop-ups, and avoid opening attachments that they suspect may contain malware. Botnets are able to make it look as if the attachment came from one of the user's contacts. Check with the sender before opening an unexpected attachment. The sender is probably unaware that their computer has become recruited into a botnet and is sending spam. (For related reading, see: Does Your Portfolio Need A Does Of Cybersecurity?)