Leading antivirus software companies issued alerts for MyDoom.O, which was first detected Monday and arrives in e-mail message attachments that, when open, install the virus and open a back door that remote attackers can use to access infected machines. While similar to other versions of MyDoom, the O-variant is testing a new approach: using major search engines to harvest e-mail addresses on Web domains that it discovers, slowing those sites, according to Johannes Ullrich, chief technology officer at The SANS Institute's Internet Storm Center.

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