OSX/Flashback is a series of Trojan downloaders. Early variants imitated a Flash Player plug-in installer and required user interaction. Later variants relied upon known, unpatched, Java vulnerabilities and compromised computers using redirected websites . At least one variant also attempted to disable XProtect and its signature updates once installed . According to a Dr. Web malware analyst, over 600,000 Mac computers had been infected with OSX/Flashback as of April, 2012 .