The worm has been spreading via an email pretending to be from law enforcement agencies, including the FBI and CIA, and informing the user that they have logged onto illegal websites and need to answer an attached 'questionnaire'. Once activated, the attachment puts the Sober worms on to compromised PCs.
"These emails did not come from the NHTCU," said the organisation in a statement.
"Anybody who receives such an email should delete it without opening it."
The social engineering tactic by the virus writers has proved very successful, with Sober becoming the biggest virus outbreak of the year.
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