Apple wrote:A recent phishing scam has targeted Mac users by redirecting them from legitimate websites to fake websites which tell them that their computer is infected with a virus. The user is then offered Mac Defender "anti-virus" software to solve the issue.
This “anti-virus” software is malware (i.e. malicious software). Its ultimate goal is to get the user's credit card information which may be used for fraudulent purposes.
The most common names for this malware are MacDefender, MacProtector and MacSecurity.
In the coming days, Apple will deliver a Mac OS X software update that will automatically find and remove Mac Defender malware and its known variants. The update will also help protect users by providing an explicit warning if they download this malware.
If you have inadvertently installed those malware, Apple provided a step-by-step instruction at this page.
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