All Stuxnet worm attacks -- which targeted a nuclear facility in Iran -- were launched by infecting a total of just 10 machines. In other words, the more than 100,000 Stuxnet infections spotted by September 2010 can be traced back to a handful of infections, which likely targeted peripheral facilities to ultimately infect the true target.
That's the surprise finding from a new Symantec report on Stuxnet, released Friday.
Stuxnet was designed to sabotage the high-frequency convertor drives used in a uranium enrichment facility in Iran. The malware adjusts the automated control system's user interface to make it appear that the drives are running normally. But in reality, the malware is quickly adjusting the speed of the drives to very high and low frequencies. As a result, not only does the uranium not get enriched, but the drive motors are permanently damaged.
In November, Symantec's researchers discovered that Stuxnet records a timestamp every time it infects a new machine. "However, at the time, this information was largely useless as we did not have enough samples to draw any meaningful conclusions," they said in a blog post.
Since then, however, numerous antivirus firms have been feeding the researchers every Stuxnet variant they capture, enabling them to amass a collection of 3,280 unique samples, which would have generated about 12,000 infections. Studying this subset, they found that every infection could be traced back to just one of 10 machines.
Since attackers may not have had direct access to the enrichment facility, it’s possible they targeted machines in five related facilities, in an attempt to spread the malware to the enrichment facility. Interestingly, in June 2009, and again in April 2010, the same PC appears to have been infected with different versions of Stuxnet. Meanwhile, another facility was targeted once, but had an initial three infections, which Symantec said suggests that a USB key carrying Stuxnet was inserted into three different PCs.
Symantec said the last Stuxnet attack appeared to have been launched in May 2010, while the earliest known attack dates from June 2009.
Iran began containing Stuxnet in August 2010. "Looking at newly infected IP addresses per day, on August 22 we observed that Iran was no longer reporting new infections," said Symantec's researchers. "This was most likely due to Iran blocking outward connections to the command and control servers, rather than a drop-off in infections."
But Iran's response came too late. According to news reports, the Stuxnet attacks appear to have been successful.