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IS THE price of second-hand computers about to plunge in Iran? Those in its nuclear facilities have been infected by the Stuxnet worm, an ingenious cyber-weapon seemingly designed specifically to sabotage uranium-refining by disrupting centrifuges’ industrial-control systems. On November 29th President Mahmoud Ahmadinejad admitted Stuxnet had hit “a limited number” of the centrifuges. He had previously said that only administrative machines at nuclear facilities had been infected. The International Atomic Energy Agency reported a few days earlier that engineers at Iran’s Natanz plant had stopped feeding uranium into its centrifuges, but Iran said it restarted the process six days later. IAEA figures also showed the refining was less productive.

This is just what a Stuxnet attack would look like. According to Symantec, a computer-security company, the worm performs an inventory of the systems it is running on, looking specifically for “frequency converter drives” made by two firms, one Iranian and the other Finnish, running at speeds between 807 Hz and 1210 Hz. (These high frequencies correspond to the rotation speeds of centrifuges; America tightly controls the export of frequency converter drives able to operate at frequencies above 600 Hz.)

If it finds the right configuration, Stuxnet sabotages it by making subtle changes to the speeds of the centrifuges over several weeks, while displaying normal readings to cover its tracks.