The most pervasive banking Trojan evades detection by antivirus software most of the time, according to new research.

Zeus, the Trojan that steals financial credentials and data and is spread via the Zbot botnet, is detected only 23 percent of the time by up-to-date antivirus applications, researchers at Trusteer discovered. Trusteer sampled 10,000 machines that were infected by Zbot, and of these Zeus-infected machines, 55 percent were running-up-to date AV software.

The massive Zbot botnet -- made up of 3.6 million PCs in the U.S., or 1 percent of all PCs in the country, according to Damballa data -- spreads Zeus, which is the No. 1 financial Trojan, representing 44 percent of all financial malware infections today, according to Trusteer. The malware steals users' online financial credentials and moves them to a remote server, where it can inject HTML onto pages rendered by the victim's browser to display its own content mimicking, for instance, a bank's Web page.

"Zeus' infection rate is higher than that of any other financial Trojan. We are seeing actual fraud linked to Zeus -- accounts being compromised, [and] money transferred from accounts of customers infected with Zeus," says Mickey Boodaei, founder and CEO of Trusteer, which sells online banking security tools. "When we investigate some of our banking customers' [machines infected by it], we find evidence of abuse on the computer, so we know this crime ring is very active and dangerous."

It's unclear exactly why Zeus is so wily, but Boodaei says there are multiple variants of the malware, which could make it more difficult to pinpoint.

"One thing we didn't do is check the same thing for other Trojans. It could be that the infection rates are like this for all Trojans," Boodaei says. "But we know Zeus is very effective at hiding in the operating system, and it's hard to remove it."

Of the Zeus-infected bots, 31 percent weren't running any AV program, while 14 percent were running AV that wasn't up-to-date. The rest were running up-to-date AV.

Trusteer estimates that among all Windows users, 71 percent run up-to-date AV, 6 percent run AV that's out of date, and about 23 percent don't run AV at all.

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