Researchers at security firm Finjan have discovered details of a new type of banking Trojan horse that doesn't just steal your bank log-in credentials but actually steals money from your account while you are logged in and displays a fake balance.

The bank Trojan, dubbed URLZone, has features designed to thwart fraud detection systems which are triggered by unusual transactions, Yuval Ben-Itzhak, chief technology officer at Finjan, said in an interview Tuesday. For instance, the software is programmed to calculate on-the-fly how much money to steal from an account based on how much money is available.

It exploits a hole in Firefox, Internet Explorer 6, IE7, IE8, and Opera, and it is different from previously reported banking Trojans, said Ben-Itzhak. The Trojan runs an executable only on Windows systems, he said. The executable can come via a number of avenues, including malicious JavaScript or an Adobe PDF, he added.

The specific Trojan Finjan researchers analyzed targeted customers of unnamed German banks, according to the latest Finjan report. It was linked back to a command-and-control server in Ukraine that was used to send instructions to the Trojan software sitting inside infected PCs. Finjan has notified German law enforcement, Ben-Itzhak said.

"It's a next generation bank Trojan," he said. "This is part of a new trend of more sophisticated Trojans designed to evade antifraud systems."

Finjan researchers were able to trace the communications from the code on an infected machine back to the command-and-control server, which was left unsecured, according to Ben-Itzhak. On that server, they saw the LuckySploit administration console and were able to see exactly what types of rules the Trojan was written to follow and statistics on victims.

About 90,000 computers visited the sites housing the malware and 6,400 of them were infected, a 7.5 percent success rate, he said. Of those whose computers installed the Trojan, a few hundred had money stolen from their bank accounts, he said.

During the span of 22 days in mid-August, the criminals behind the Trojan stole the euro equivalent of nearly $438,000.

The Trojan code includes detailed instructions on how the Trojan should calculate the amount to steal from a victim's bank account.

(Credit: Finjan)

Here's how the Trojan works:

Potential victims get their computers infected either by opening an e-mail and clicking on a link to a Web site created to distribute malware or by visiting a site that has been compromised and malware hidden on it.

In this case the malware, a toolkit called LuckySploit, exploits a known security hole in the browser, and installs the Trojan on the computer. When the Trojan notices the computer user visiting the site of a targeted bank it springs into action.

While the computer user goes about his or her business on the site, the Trojan looks at the available balance and figures out how much money to steal. The Trojan is given a minimum and a maximum range that is below the amount that triggers antifraud systems and to leave a certain percentage in the account, Ben-Itzhak said.

After performing the calculation, the Trojan then makes the transaction, communicating with the bank site through the browser without the computer user knowing.

"The Trojan is sending requests to the bank and getting replies that your browser doesn't display," Ben-Itzhak said. "You are looking at your account and you don't see any of it."

A Finjan blog post describes it like this:

URLZone is a Trojan Kit that allows the attacker with the use of the 'URLZone Builder' to create a configuration file. This file contains precise orders to the bot, enabling the attacker to target any bank he wants...The URLZone successfully managed to bypass the German banks' protection using 'One Time Password.' This is a technique used to enable the user to get a new password every time he logs into his account. Its goal is to make the theft of usernames and passwords worthless. In order to be successful, the malware must execute itself on the browser to change the parameters and fool the the user to approve a fraudulent money transaction from his account...So far the malware behavior is similar to many other Trojans. However, URLZone uses the delivered configuration file to manipulate the user.

The Trojan has the money sent to the bank account of a money mule, someone who has an account set up to receive the funds. Money mules are typically people recruited online as "independent contractors" or "financial managers" whose sole purpose is to wire the money placed into their account to someone else, typically out of the country, in exchange for a commission. Because their accounts are used only once or twice, they often do not realize the ruse immediately, Ben-Itzhak said.

Meanwhile, the Trojan hides the theft by erasing it from the report of account activity displayed to the computer user and shows a fake balance--what the amount would be if not for the theft. The victim will not notice something is wrong until a different, uncompromised computer is used to access the account, an ATM is used, or a transaction is denied because of insufficient funds.

The Trojan also keeps a log of the victim's bank account log in credentials, takes screenshots, and snoops on the user's other Web accounts, such as PayPal, Facebook, and Gmail, according to the Finjan report.

This is the first Trojan Finjan has come across that hijacks a victim's browser session, steals the money while the victim is doing online banking, and then covers its tracks by modifying information displayed to the victim, all in real time, Ben-Itzhak said.

People should keep their antivirus, operating system, browser and other software up to date to protect against this type of attack, he said.

Updated 5:30 a.m. PDT to specify that the Trojan targets Firefox, Internet Explorer 6, IE7, IE8, and Opera, that is different from previous Trojans, and that it affects Windows only. Also, more technical details were added, as well as links to the report and blog post from Finjan.