Half a million online accounts have been compromised by a virus described as "one of the most advanced pieces of crimeware ever created".
The "Sinowal" trojan has been tracked by RSA, an online security company that has found the virus has infected computers all over the plant.
"The effect has been really global with over 2000 domains compromised", Sean Brady of RSA's security division told the BBC.
"This is a serious incident on a very noticeable scale and we have seen an increase in the number of trojans and their variants, particularly in the States and Canada."
Brady said since 2006, more than 270,000 banking accounts and 240,000 credit and debit cards have been compromised from financial institutions in the US, UK, Australia and Poland.
The team at RSA have not discovered who is behind the malicious code: "There is a lot of talk about where it comes from and anecdotal evidence points to Russia and Eastern Europe".
"Historically there have been connections with an online gang connected to the Russian Business Network but in reality no one knows for sure."
The RSA has said it has passed information onto financial institutes, and law enforcement agencies, around the world.