A new variant of the MyDoom worm that exploits an unpatched flaw in Microsoft's Internet Explorer browser is in the wild and posing particular risk to home and small business users, security experts warned this week.

The worm, which security firms dubbed MyDoom.AF, MyDoom.AH and MyDoom.AG, spreads by e-mail and exploits a recently discovered buffer overflow vulnerability in IE.

Internet users should avoid opening suspicious e-mail with the subject headers "funny photos :)," "hello," "hey!" and blank headers, according to security firm iDefense. Users who open the infected e-mail and click on links in the message body will be directed to destinations from which an attack may be launched.

Microsoft issued a statement saying that it was aware of the new variant and an investigation is underway. According to early reports the vulnerability does not exist on Windows XP Service Pack 2 (SP2) so customers running the security update are at a reduced risk of the threat, Microsoft said. The software maker and security experts advised users to install SP2 if they have not already.

Users less likely to have a perimeter defense, such as those in homes and small offices, are more vulnerable to attack, security experts warned. The variant exploits a flaw in the way IE handles "iframe" and "frame" HTML tags. Code that takes advantage of the flaw began circulating last week and researchers warned that several new codes exploiting the vulnerability may appear in the next few weeks since it is still unpatched.

Microsoft said however that it believes the current threat is low and that it is not aware of any significant customer impact. The company added that it will decide what further action to take upon completing its investigation and said it could issue a patch ahead of its regular monthly security update.

The IDG News Service is a Network World affiliate.

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