"The vulnerability exists because Windows incorrectly parses shortcuts in such a way that malicious code may be executed when the operating system displays the icon of a malicious shortcut file," it says in the security bulletin.
To patch the vulnerability, the security update corrects the validation of shortcut icon references. Those Windows users who have enabled automatic updating don't have to do anything, but those who have disabled the option have to download the update and install it themselves.
Since the vulnerability is currently being exploited by various malware families and since Microsoft has issued this update before the monthly Patch Tuesday, it is obvious that you should patch your Windows as soon as possible - just visit the Microsoft Download Center, insert the code (MS10-046) of the security bulletin regarding the vulnerability in the search box, choose your Windows version from the list and download the patch.
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