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It looks like the days of "security by obscurity" protection for Mac users may be coming to a close. As described over on Brian Krebs' blog, socially-engineered malware authors are now going after Mac OS X users with targeted exploits that attack both Windows and Mac users, pivoting the exploit based on the visitor's OS. IE8 users on Windows benefit from SmartScreen Filter's anti-malware feature, which can help identify and block malicious downloads.
Users looking for free updates to the latest MacOS are getting virus-changing trojans instead: