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The worm is not destructive and has not yet infected many systems, but as with any mass-mailer worm, it could become a nuisance.
Like other mass-mailing worms such as Anna Kournikova or Badtrans, Gokar spreads through Microsoft's Outlook and Outlook Express e-mail clients when a user clicks on an attachment sent with the infected message, according to antivirus software vendors Symantec, F-Secure and Trend Micro.
Infected e-mail arrives in user in-boxes with dozens of combinations of different subject lines, body messages and file names, although each attachment will end with the .pif, .scr, .exe, .com or .bat extensions, the companies said.
When a user double-clicks on the attachment, the worm installs a file called Karen.exe on the infected system and mails itself to all addresses listed in the computer's address book. The worm then runs every time the infected computer is booted up. Users can determine whether a system is infected or not by searching for the Karen.exe file.
The worm also uses the chat program mIRC (Internet Relay Chat), the companies said. Gokar searches the infected PC for the mIRC application, and if it finds it, attempts to infect IRC users in the same discussion, or channel, as the infected system whenever the application is started, said Trend Micro.
If an infected system is running Microsoft's Internet Information Services (IIS) Web server software, the worm will modify the default Web page on the system and offer users visiting the site a chance to download the worm, according to F-Secure.
An infected Web site will be changed to display the text "We are Forever" and point users to a link to download a file called Web.exe, which contains the Gokar worm, said Symantec.
Security specialists believe the impact of the Gokar worm will be exacerbated by employees sending electronic Christmas cards.
"We recommend that organisations establish and enforce reasonable policies for e-mail and Web usage, without creating inappropriate barriers," said Alyn Hockey, VP of Research and Development, Baltimore MIMEsweeper Group. "Simply implementing antivirus software is not enough".
The Nimda worm also defaced Web sites and downloaded files to the computers of users viewing the site. Unlike Nimda, which downloaded a file automatically through the browser, Gokar requires that the user click a link to download the worm. Both Nimda and Code Red exploited IIS.
Users should check with their antivirus companies for software updates. Organisations are urged to block attachments, particularly .exe, .scr and .pif files, at their mail gateways to avoid infection.