The latest fake anti-virus attack tricks users into installing malicious software by posing as a Microsoft security update, warns security firm Sophos.
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A bogus web page that is an almost exact replica of the real Microsoft Update page appears while surfing using a Firefox browser.
The use of high quality graphics and professional interfaces means more users are likely to fall for the scam, says Sophos.
The attack is designed to take advantage of the monthly Patch Tuesday security updates that Microsoft issues.
"Users need to be more vigilant than ever before, as bogus security alerts pop-up in their browsers," says Graham Cluley, senior technology consultant at Sophos.
Fake anti-virus attacks are big business for cybercriminals and they are investing time and effort into making them as convincing as possible, Graham Cluley says.