The worm initially spreads indiscriminately, but includes a highly specialized malware payload that is designed to target only Siemens Supervisory Control And Data Acquisition (SCADA) systems that are configured to control and monitor specific industrial processes. Stuxnet infects PLCs by subverting the Step-7 software application that is used to reprogram these devices.[8]
Different variants of Stuxnet targeted five Iranian organizations, with the probable target widely suspected to be uranium enrichment infrastructure in Iran; Symantec noted in August 2010 that 60% of the infected computers worldwide were in Iran. Siemens stated on November 29 that the worm has not caused any damage to its customers, but the Iran nuclear program, which uses embargoed Siemens equipment procured clandestinely, has been damaged by Stuxnet. Russian computer security firm Kaspersky Lab concluded that the sophisticated attack could only have been conducted "with nation-state support"[18] and it has been speculated that Israel and the United States may have been involved.
In May 2011, the PBS program Need To Know cited a statement by Gary Samore, White House Coordinator for Arms Control and Weapons of Mass Destruction, in which he said, "we're glad they [the Iranians] are having trouble with their centrifuge machine and that we - the US and its allies - are doing everything we can to make sure that we complicate matters for them" offered "winking acknowledgement" of US involvement in Stuxnet. According to the British Daily Telegraph, a showreel that was played at a retirement party for the head of the Israel Defence Forces (IDF), Gabi Ashkenazi, included references to Stuxnet as one of his operational successes as the IDF chief of staff. Wikipedia