Hoards of zombie computers connected to botnets are bought, sold, traded and rented out in underground hacking forums. There are thousands of botnets of various sizes with new ones being created every day. Cybercriminals use botnets to send out spam and spread malware and phishing campaigns. Botnets can also be used to increase the effectiveness of DDoS campaigns.
The notorious Cutwail botnet, which is made up of about 1.5 million infected computers, sends out billions of spam messages every day. Dell estimates that the cybercriminals behind Cutwail made between $1.7 and $4.2 million over a two-year period by providing spam services. A botnet consisting of 1,000 infected systems sells for $20 and can go as high as $250 for 15,000 infected machines, Dell said.