A security firm issued a warning of an internet Trojan that set itself as a middleman between computer users and their banking websites, potentially enabling online thieves to drain bank accounts worldwide.

The malicious software, dubbed Trojan.Silentbanker, targets financial transactions at over 400 banks worldwide, including those in Canada.

Symantec researcher Liam OMurchu, writing on the company's security blog, said the software's ability to set itself in between banks and consumers is particularly worrisome.

"The Trojan can intercept transactions that require two-factor authentication," he wrote in a posting on Monday. "It can then silently change the user-entered destination bank account details to the attacker's account details instead."

The Trojan hides its presence by offering the individual a similar-looking authentication screen to provide the results the user expects to see, he wrote.

Symantec said it had yet to reproduce such a transaction in the lab but said an analysis of the Trojan's code shows that this feature is available to attackers.

The company said this method would only be used if an easier route is not available.

What makes the new Trojan unique is that it uses the bank's genuine website as part of the attack. Traditionally hackers seeking to access banking passwords set up fake websites designed to resemble the banking website.

The company suggested computer users should update their security software and web browsers to fix flaws that might allow Trojans to nest in their computer.

The Trojan only affects systems with Microsoft operating systems, including Windows 2000, NT, XP and Vista.

Trojans are the name given to any piece of malicious software that comes aboard a computer disguised as a useful or desirable program. They take their name from the wooden horse used by the ancient Greeks to sneak into Troy and capture the city, as described in Homer's The Odyssey.