Security firm Sunbelt Software said it stumbled across a US-based server storing megabytes of data stolen from compromised computers while researching spyware infections.
The server held passwords for online accounts from 50 banks, Ebay and Paypal logins, hundreds of credit card numbers and reams of personal data.
The FBI has reportedly now started investigating the ring of ID thieves.
What made this bug so effective was its ability to grab text stored on the clipboard and by Internet Explorer, said Eric Sites, vice president of research and development at Sunbelt Software.
Microsoft's browser has a feature, called AutoComplete, that automatically populates boxes on web forms where people typically fill in names, addresses, e-mail addresses, credit card numbers and other biographical details.
The feature is supposed to make filling in forms on websites less of a chore. In this case, said Mr Sites, it helped the ID thieves get hold of enormously valuable data.
Typically a keylogger produces a file containing an unbroken string of characters, said Mr Sites.
"It's usually very hard to take that and do anything with it," he told the BBC News website.
By contrast, AutoComplete data is already labelled and sorted because the browser has to know where to put each item.
"The way the data is laid out, the quality of it, it's very easy to go through and use it for nefarious purposes," he said. "This is about getting money and stealing."
Megabytes of data
The BBC News website was shown the server and some of the files containing personal data that it was storing. Each file was full of login names, e-mail addresses, credit card details and everything needed to steal someone's identity or simply empty their bank account.
Analysis of information in the files revealed login details for online services at 50 banks as well as user details for many Ebay and Paypal accounts. One bank account had more than $380,000 in it.
Sunbelt has contacted some of the people identified in the files to warn them that they have fallen victim to the bug. Banks, credit card firms, Ebay and Paypal have been told about compromised accounts.
The server at the centre of the ID theft ring had many multi-megabyte sized files on it, said Mr Sites. The server, which was based in the US, was regularly cleaned out by the thieves who created the trojan.
Infected machines sent files back hourly or when the logs of data they were collecting had reached a certain size.
Mr Sites said that, so far, the trojan had been found on porn sites and websites offering cracks for pirated software. But, he said, the trojan was likely to be on many other websites as it had managed to infect so many users.
Sunbelt believes the trojan has been circulating for about three weeks and in that time has probably infected thousands of victims.
The vulnerability it exploits means that all a user has to do to fall victim is to visit the wrong site.
"Type in a web link and your machine is infected," said Mr Sites. "You do not have to click on anything, the website forces the installation."
Many victims may have no idea that they have been infected.
"This version of the trojan was very successful," he said. "It was very small, hard to detect, the file had a very innocuous name and did not cause any problems to the machine.
The size and sophistication of the ID theft ring led anti-virus and security companies to quickly produce tools that can spot if a machine has been compromised by the server and clean up infected machines.
The trojan was tricky to spot because the files being sent back to the server were disguised as data traffic generated by a user's browser.
The US-based security company has alerted the FBI to the online scam and it is reportedly investigating the matter.