The bank says that its customers opened an email attachment that resulted in a virus being executed on their machines. This virus changed their browsers' behaviour so when they went to open the real ABN Amro online banking site, they were instead re-directed to a spoof site.

The customers then typed in their passwords, which the attacker in turn used to access the bank's real Web site. The customer's own transactions were passed along to the real site, so they didn't notice anything wrong right away, while the attacker simultaneously made their own fraudelent transactions using the bank's urgent payment feature.

ABN Amro has issued its customers with two-factor authentication tokens for several years. But the man-in-the middle attack gets around this security measure by passing the ever-changing part of the password from the token to the bank along with the never-changing part - essentially piggybacking on a legitimate log-in.

Johan van Hall, executive board member, ABN Amro Netherlands, says, "We take this incident very seriously and plan to take further action to educate our customers. If the user sticks to the rules, Internet banking is a very safe, fast and easy way to bank."

ABN Amro's five rules are:
1- Check the lock symbol in the browser and the ABN AMRO certificate
2- Always check your payments instructions
3- Never open e-mails from someone you don't know
4- Only install software from trusted sources
5- Protect your PC with a virus-scanner and a firewall.