The URLzone Trojan, identified by researchers at web filtering vendor Finjan Software earlier this month, represents "the next generation of bank Trojans", according to Yuval Ben-Itzhak, Finjan's chief technology officer.
After it infected about 6,400 computer users last month, the Trojan was clearing about €12,000 (£11,000) per day. That puts it on track to rake in as much as €7.3 million annually.
Criminals installed the Trojan by luring visitors to infected websites and using a variety of PC software flaws. They managed to infect about 7.5 percent of the 90,000 computers they attacked before Finjan got access to their command-and-control server, the company said.
More widespread Trojans such as Zeus and Clampi have been siphoning millions of dollars per day out of banks by stealing victim's online credentials and then moving money to unsuspecting 'money mules' who then transfer the cash offshore.
These mules are often recruited from job sites such as Monster.com and they typically believe they're doing legitimate payroll work for overseas companies, and not organised criminal enterprises. Once they send the stolen money offshore, they can be the ones who are held accountable for the loss.
But URLzone is even more sophisticated than its predecessors, Ben-Itzhak said.
Its sophisticated user interface lets the thieves set some controls that help keep fraud detection systems at bay.
From a central server, they can, for example, set the system to ensure that the account's balance never drops below zero; they can pre-set the system to make a series of small withdrawals that will appear unsuspicious; and the software will change the way the victim's banking page is displayed so the true transactions don't get displayed.
"Basically they say, 'I will steal from you €5,000, but I want to make sure at least 5 percent will remain in your balance,'" Ben-Itzhak said.
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