http://www.snopes.com/computer/virus/klez.asp

Klez

Virus name:   Klez   (also known as W32/Klez.gen@MM or W32.Klez.E).

Status:   Real.

Origins:   The Klez virus arrives as an attachment to e-mails bearing spoofed return addresses and subject lines selected randomly from a large pool of choices. The subject line might be any of the following: Where [random phrase] is one or two words selected from the following list (e.g., "W32.Elkern removal tools," "a special powful tool"): Klez exploits a bug in Microsoft's Internet Explorer (version 5) to infect a user's system, and once installed it sends out e-mail messages to addresses found in local files, Microsoft Outlook address books, and ICQ address books. It will also overwrite any txt, .htm, .html, .wab, .doc, .xls, .jpg, .cpp, .c, .pas, .mpg, .mpeg, .bak, or .mp3 file on the 6th of every odd numbered month (January, March, May, July, September, and November).

See the links below for more information on how to detect and remove Klez.

Additional Information:
    W32.Klez.H@mm W32.Klez.H@mm
(Symantec Security Response)
    W32/Klez.e@MM W32/Klez.e@MM
(McAfee Virus Information Library)
    How to save your PC from virus attacks How to Save Your PC from Virus Attacks
(CNN.com)
Last updated:   28 January 2008

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