This year, botnets were used to inject an array of legitimate Websites with an IFrames malicious code that reroutes visitors to websites that download computer viruses into their machines, according to Cisco.

"The botnet is, in many cases, ground-zero for online criminal threats," Peterson said.

"Using malware to infect someone's computers is an incredibly common mechanism and harnessing them all together is a way they do their click fraud, spam emails, and data stealing."

As computer security vendors such as Cisco get better at protecting machines from hackers and users grow wary of clicking on unsolicited Web links or email attachments, online criminals are turning botnets on Web-based email accounts.

Hackers are "reputation hijacking" by using botnets to figure out weak passwords protecting Web-based email accounts, according to Peterson.

Weak passwords consist of family names, birthdays, home addresses, or other terms considered relatively easy to deduce.