Researchers have uncovered new clues that the Stuxnet worm may have been created to sabotage Iranian attempts to turn uranium into atomic bomb-grade fuel.
According to Eric Chien, one of three Symantec researchers who have dug into Stuxnet, the worm targets industrial systems that control very high speed electrical motors, such as those used to spin gas centrifuges, one of the ways uranium can be enriched into fissionable material.
One expert called Symantec's discovery "very interesting indeed."
Chien reported Symantec's new findings in a blog post last Friday and in a revised paper first published in September.
Stuxnet, considered by many security researchers to be the most sophisticated malware ever , targeted Windows PCs that managed large-scale industrial-control systems in manufacturing and utility companies. Those control systems, called SCADA, for "supervisory control and data acquisition," operate everything from power plants and factory machinery to oil pipelines and military installations.
Since the worm was first detected in June, researchers have come to believe that it was crafted by a state-sponsored team of programmers, and designed to cripple Iran's nuclear program.
In September, Iran officials confirmed that Stuxnet infected 30,000 PCs in the country, but have denied that the worm had caused any significant damage or infiltrated the SCADA systems at the Bushehr nuclear reactor.
Symantec's latest analysis indicates that the reactor was not the target. Instead, Stuxnet aimed to disrupt uranium enrichment efforts.
Stuxnet looks for devices called "frequency converter drives" connected to a SCADA system, said Chien. Such drives take electrical current from a power grid, then change the output to a much higher frequency, typically 600 Hz or higher.
"The high-frequency output from the frequency changer is fed to the high-speed gas centrifuge drive motors (the speed of an AC motor is proportional to the frequency of the supplied current)," states the Federation of American Scientists (FAS) in an explanation of uranium production on its website. "The centrifuge power supplies must operate at high efficiency, provide low harmonic distortion, and provide precise control of the output frequency."
Stuxnet, however, monkeys with the output frequency over a period of months, Symantec said in its revised paper ( download PDF ).
When it finds converter drives operating between 807 Hz and 1210 Hz, the worm resets the frequency to 1410 Hz, then after 27 days, drops the frequency to just 2 Hz and later bumps it up to 1064 Hz. It then repeats the process.
"Interfering with the speed of the motors sabotages the normal operation of the industrial control process," said Chien.
Sabotaging centrifuge motor speed will do more than that, said Ivanka Barzashka, a research assistant with the Strategic Security Program of FAS, and an expert on gas centrifuges. "A centrifuge is a delicate piece of equipment and operating a centrifuge at the right frequency is extremely important," Barzashka said in an e-mail Sunday. "Problems controlling the operating frequency can cause the machines to fly apart."
Although Symantec did not claim outright that Iran's uranium enrichment operations were the target, it provided tantalizing clues.
The latter is significant, as researchers have said Iran was the country hardest hit by the initial wave of Stuxnet infections.
The high frequency of the targeted converters also points to a possible gas centrifuge application. For example, the U.S. Nuclear Regulatory Commission (U.S. NRC) restricts exports of converter drives able to output at frequencies higher than 600 Hz because they are required to operate gas centrifuges.
Gas centrifuges are hollow tubes that spin at very high speeds, and are used to separate the fissionable U-235 isotope from the much more prevalent U-238 found in natural uranium. Iran, which first started its centrifuge project in 1987, has installed centrifuges in an underground facility at Natanz in central Iran.
According to a paper published in Physics Today , the flagship journal of the American Institute of Physics, Iran began testing a 164-centrifuge assembly in April 2006, and soon after reported it had produced minute quantities of low-enriched uranium. A year later, Iran's president, Mahmoud Ahmadinejad, boasted that Iran had begun production of enriched uranium using 3,000 centrifuges.
By September 2008, the International Atomic Energy Agency reported that Iran was using nearly 4,000 centrifuges and was in the process of adding several thousand more.
Symantec's Chien acknowledged that he and his colleagues were not experts in industrial control systems, and that there may be other uses for high-frequency converter drives outside of gas centrifuges. He called on experts in their use to contact Symantec.
"Symantec's findings about Stuxnet are very interesting indeed," said Barzashka.