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22 Aug 2003

Sobig-F worm has twist in tail - Sophos warns of possible "Trojan horse" download


Sophos has today warned that the mass-mailing W32/Sobig-F worm, which has flooded computer users this week, could attempt to download a Trojan horse between 8pm and 11pm BST today.

At 19:00-22:00 GMT (which is 8-11pm in the UK) on Fridays and Sundays, the worm has been programmed to automatically direct infected PCs to a server controlled by the virus writer from which a malicious program could be downloaded. At the moment, it is not known what the download material will do, but possibilities include launching another virus or spam attack, collecting sensitive information, a denial of service attack, or deleting files stored on an infected computer or network.

"The main effect of Sobig-F to date has been to slow down the internet with the sheer quantity of emails it has generated," said Graham Cluley, senior technology consultant at Sophos Anti-Virus. "At 8pm tonight, most British companies will have left the office for the bank holiday weekend, but any infected computers that are left on have the potential to become zombies, doing whatever the virus writer wants. If the writer of Sobig succeeds in installing a Trojan on infected PCs, users could be in for a nasty shock when they return to work next week. The message is simple: ensure your anti-virus is up-to-date, run your anti-virus to check for infection, disinfect if necessary and ensure your computer's firewall is properly configured."

"What the worm downloads will not be known until this evening - it could display an offensive but largely harmless message or launch a malicious attack. But the download is timed to coincide with the regular business afternoon in the United States, so users should be concerned about unauthorised code running on their computers. On Monday morning businesses in the Far East and Australia will be beginning their working day when the worm tries a second time to download unknown code from the net," continued Cluley.

Sophos advises that the download can be avoided by configuring firewalls to block outgoing connection attempts to UDP port 8998. In addition, anti-virus software should be updated, and any infected PCs disinfected.

Sophos has published information about how to disinfect computers and prevent the Trojan download.

About Sophos

More than 100 million users in 150 countries rely on Sophos as the best protection against complex threats and data loss. Sophos is committed to providing complete security solutions that are simple to deploy, manage, and use and that deliver the industry's lowest total cost of ownership. Sophos offers award-winning encryption, endpoint security, web, email, mobile and network security solutions backed by SophosLabs - a global network of threat intelligence centers.

Sophos is headquartered in Boston, US and Oxford, UK. More information is available at