The previously unknown W32/Sober-A worm has caused more infections than any other virus in November 2003, according to antivirus firm Sophos' monthly ranking.

The firm said that although Mimail variants made five appearances in this month's chart - totalling over a quarter of all reports - the multi-lingual Sober worm had caused the most problems.

"Sober-A cunningly disguises itself using a multitude of subject titles and messages, making it difficult to spot," said Chris Belthoff, senior security analyst at firm Sophos.

"It can even present itself in German if it thinks it is being examined on a German user's computer."

The Mimail worms attempt a number of different tricks, including trying to steal credit card information from Paypal users.

"Virus writers and hackers are becoming more determined to steal confidential information, which could leave a deep hole in a victim's pocket," warned Belthoff.

The top 10 viruses in November 2003, according to Sophos, were:

1 W32/Sober-A 32.6% (New entry)
2= W32/Mimail-C 9.5%
2= W32/Mimail-F 9.5%
4 W32/Dumaru-A 8.0%
5 W32/Mimail-A 5.1%
6 W32/Gibe-F 4.5%
7 W32/Nachi-A 2.6%
8 W32/Mimail-J 2.4%
9 W32/Klez-H 2.2% (22 months in chart)
10 W32/Mimail-E 1.5%
Others 22.1%

The security company's monthly research also revealed that a revamped version of the Hotmail hoax, promising users extra storage space if they forward an email to their friends, helped keep the hoax at the top of the list.

Belthoff said in a statement: "Creeping into the chart is the 'Do not push 90#' chain letter, which is based on an actual telephone scam from the early 1990s and has caused people to be unnecessarily alarmed.

"If you receive a virus hoax, chain letter or email scam, you should delete it immediately and resist any temptation to forward it on."

The top 10 hoaxes reported to Sophos during November are as follows:

1 Hotmail hoax 31.3%
2 Meninas da Playboy 12.7%
3 Bonsai Kitten 6.2%
4 Budweiser frogs screensaver 4.7%
5 JDBGMGR 4.0%
6 A virtual card for you 3.8%
7 Bill Gates Fortune 3.0%
8 Frog in a blender/Fish in a bowl 2.9%
9 WTC Survivor 2.3%
10 Do not push 90# 1.8%
Others 27.3%