Now posing as messages from the UK's National Hi-Tech Crime Unit, the Sober worm has now spoofed the likes of the FBI, the CIA and the German Bundeskriminalamt. In this latest scam, the mail claims to recipients that their internet use has been monitored and that they have accessed illegal web sites. Recipients are then directed to download an infected attachment, which will further propagate the outbreak, as well as leaving machines open to additional attacks.

"These emails did not come from the NHTCU. Anybody who receives such an email should delete it without opening it," the police agency warns.