The most widespread botnet in history, ZeuS is a Trojan horse that infiltrates computers in order to steal data by logging keystrokes and spread copies of itself to other devices via instant messaging and e-mail messages. Computers infected by a ZeuS variant can be controlled by the attacker and monitored for keystrokes in order to gain access to online accounts and other sensitive data.
More than 50,000 variants of the ZeuS Trojan have been recognized since ZeuS made its first known appearance in 2007. The most prolific period for ZeuS came in 2009 and 2010, with some security reports estimating as many as 3.6 million ZeuS-infected computers in the United States alone during that time.
Several mobile botnet variants of the ZeuS Trojan have surfaced more recently, with some targeting Google Android phones while others attack Blackberry devices, Symbian phones, or Windows Mobile phones. These variants are considered part of the Zitmo family of mobile malware and are designed to steal the mobile transaction authentication numbers that banks use to strengthen security for logging in to online accounts.