More viruses are being written with an explicitly criminal motive

The first seven months of the year produced no big-hitting viruses but all that changed in August when three malicious programs struck within days of each other.

Home users and companies were sent reeling by the Blaster and Nachi worms as well as the Sobig-F virus.

August ended up being the worst month ever for virus outbreaks and infections.

Big hitter

The three viruses that struck that month dominated the top 10 malicious programs of the year, according to security firm Sophos.

The biggest virus of the year, Sobig-F, was also one of the biggest ever.

"Sobig-F was huge because of the sheer amount of e-mail it generated," said Graham Cluley, senior technology consultant at Sophos.

When Sobig-F was at its most virulent Sophos received more than 400,000 copies of it every day said Mr Cluley.

TOP VIRUSES OF 2003 1) Sobig-F 2) Blaster-A 3) Nachi-A 4) Gibe-F 5) Dumaru-A 6) Sober-A 7) Mimail-A 8) Bugbear-B 9) Sobig-E 10) Klez-H

The top two viruses of the year, Sobig-F and Blaster-A, accounted for more than one-third of all the malicious programs seen during 2003.

All the viruses in the top 10 target Windows computers and most try to exploit security lapses in Microsoft's Outlook e-mail program.

The two that did not target Outlook, the Blaster and Nachi worms, instead got through vulnerabilities in the way that Windows NT, 2000, XP and Server 2003 swap files across networks.

Mr Cluley said 2003 also saw changes in the way that viruses tricked users into helping them spread.

Viruses such as Palyh claimed to be from Microsoft technical support and the Gibe-F and Dumaru viruses posed as security patches from the software giant.

Online con

This year has also seen a rise in malicious programs written to steal information that criminals could use to plunder net accounts or to carry out identity theft.

"Viruses used to be essentially pointless, like electronic graffiti," said Mr Cluley. "Now we are seeing worms that try to extract financial information from users."

For instance, Mimail-J poses as a message from net payments firm PayPal and asks for credit card information and Bugbear-B attempts to steal keystrokes relating to credit cards in use online.

This change in motive meant that organisations such as the National Hi-Tech Crime Unit were taking more interest in virus writers, said Mr Cluley.

Despite the fact that there were now 86,000 known viruses and about 700 new ones were being created every week, Mr Cluley said the fight against the pernicious programs was not hopeless.

"Businesses are pretty much coping," he said, "generally we are winning."