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.
That is not all. Ralph Langner, a German researcher, says Stuxnet has a "second warhead". It targets a different industrial-control system that just happens to be used at Bushehr, Iran's much-delayed nuclear-power station, replaying previously recorded normal readings as it causes havoc. Mr Langner likens its complexity to "the arrival of an F-35 fighter jet on a World War I battlefield."