The Stuxnet worm repeatedly attacked five industrial plants inside Iran over a 10-month period, according to new data collected by researchers from antivirus firm Symantec.
Three of the undisclosed organizations were targeted once, one was hit twice and one was targeted three times, members of Symantec's Security Response Team wrote in the report (PDF), which updates findings first released in September. The attacks took place in 12,000 separate infections in 2009 and 2010 and weren't discovered until July.
The new information was collected by researchers who monitored data recorded by Stuxnet itself. Attackers programmed the worm to store the location and type of each computer infected, most likely so the programmers would know when they successfully reached their intended target. Researchers have speculated that the malware was designed by Israel, the US, or another enemy of Iran that wanted to disrupt that country's nuclear ambitions.
The Symantec researchers said Stuxnet used two different techniques to sabotage centrifuge arrays, but that one, known as the 417 code, had been disabled. The worm first infected Windows-based industrial-control systems while it searched for software made by Siemens Corporation that monitors critical factory operations.
The researchers discovered three or possibly four versions of Stuxnet. The first was completed just 12 hours before the first successful infection in June 2009. They guess that it was the result of an malware-tainted email that was opened, or a booby-trapped USB device that was connected to a computer.