Botnets such as Zeus target those using online banking
Zeus, a virus that steals online banking details from infected computer users, is more powerful than ever, warns a web security company.
Trusteer says it has spotted the Trojan virus in one of every 3,000 of the 5.5m computers it monitors in the US and UK.
Zeus 1.6 can infect Windows machines using Firefox and Internet Explorer web browsers, the company claims.
The malware steals login information by recording keystrokes when the infected user is on a list of target websites.
These websites are usually banks and other financial institutions.
The user's data is then sent to a remote server to be used and sold on by cyber-criminals.
"We expect this new version of Zeus to significantly increase fraud losses, since nearly 30% of internet users bank online with Firefox and the infection is growing faster than we have ever seen before," said Amit Klein, chief technology officer at Trusteer.
In March 2010, many parts of the command and control (C&C) system for the Zeus botnet were destroyed when the Kazakhstani ISP that was being used to administer it was cut off.
However, it does not take long for malware controllers to spring up elsewhere, and toolkits for assembling botnets are readily available on the black market.
"There are plenty of opportunities for people to purchase access to these systems through underground chat rooms," said Dr JD Marsters, from the department of electronics and computer science at the University of Southampton.
"It's a game of cat and mouse between anti-virus vendors and botnet developers."
Computer users should ensure that their anti-virus software and operating systems are kept up to date, he advised.