Two-factor authentication -- used to protect online bank accounts with both a password and a computer-generated one-time passcode -- is supposed to be more secure than relying on a single password.

But Gartner Research VP Avivah Litan warns that cyber criminals have had success defeating two-factor authentication systems in Web browsing sessions using Trojan-based man-in-the-middle attacks.

A Gartner Research note written by Litan explains that in the past few months, Gartner has heard from many banks around the world that rely on one-time-password authentication systems. Accounts at these banks have been compromised by man-in-the-middle attacks -- the report uses the term "man-in-the-browser" -- despite the use of two-factor security.

One technique that the fraudsters have been using to bypass security controls is call forwarding.

"[B]anks that rely on voice telephony for user transaction verification have seen those systems and processes compromised by thieves who persuade telecom carriers to forward legitimate user phone calls to the thief's cell phone," the report says. "These targeted attacks have resulted in theft of money and/or information, if the bank has no other defenses sufficient to prevent unauthorized access to their applications and customer accounts."

A man-in-the-middle attack involves using software or hardware to intercept network traffic then send it to its destination so that the information can be used without the knowledge of the sender or the intended recipient.

In an e-mail, Litan said that the attacks have involved the Zeus Trojan and other customized malware.

The malware sometimes uses anti-forensic capabilities that re-write account balances in the user's browser, so that the user believes his or her bank account has the funds it should, even through it is empty.

The Gartner report recommends addition defenses to monitor user behavior and/or transaction values, as well as out-of-band transaction verification.

According to the Internet Crime Complaint Center, which issued a warning about attacks on commercial bank accounts in November, total losses as of October amounted to about $100 million so far this year.