The FBI is warning of a so-called ransomware Trojan that extorts money from unwary internet users by masquerading as the law enforcement agency and claiming to have detected crimes committed using their PC.
Reveton is a drive-by Trojan that can install itself without the need for the user to open a corrupted file or attachment, instead being able to launch when users visit a compromised website.
The malware locks the user's computer and displays a bogus message pretending to be from the FBI and demanding payment of fines.
The US law enforcement agency reported detecting the Reveton ransomware on Thursday after being inundated with complaints regarding it from the general public.
"While browsing the internet a window popped up with no way to close it," read one Reveton victim's complaint.
"The window was labelled FBI and said I was in violation of one of the following: illegal use of downloaded media, under-age porn viewing, or computer-use negligence. It listed fines and penalties for each and directed me to pay $200 via a MoneyPak order.
"Instructions were given on how to load the card and make the payment. The page said if the demands were not met, criminal charges would be filed and my computer would remain locked on that screen."
The FBI has confirmed that the malware has already successfully stolen money from a number of innocent victims, but is yet to give an exact number.
While Reveton has mainly targeted the US, the FBI warned it has spread and is now operating on an international scale.
The agency has also reported finding a number of tailored variants of the malware, which add snooping capabilities such as the ability to turn on the computer's webcam remotely.
The malware's discovery follows on from warnings from security vendor Panda that cyber criminals use of ransomware is on the rise.