An Iranian double agent implanted the Stuxnet virus, which attacked Iran's nuclear program. The double agent was from group Mujahedeen-e-Khalq (M.E.K.) and used a corrupted "memory stick.32" to impose the virus, at Natanz. Both former and current U.S. intelligence officials have now confirmed this.
The program came under attack in 2009, when the code was implemented. The virus affected the nuclear equipment that controlled the centrifuges, to enrich uranium. It seemed to "force a change in the centrifuge's rotor speed, first raising the speed and then lowering it, likely with the intention of inducing excessive vibrations or distortions that would destroy the centrifuge," explained a report issued by the Institute for Science and International Security (ISIS).
Listed, as a "foreign terrorist organisation" since 1992, due to deadly attacks on Americans outside the U.S, the M.E.K. has been training members at a secret location in Nevada. The group has also been connected to the killing of five Iranian nuclear scientists, since 2007. It is also considered "the assassination arm of Israel's Mossad intelligence service", by the U.S. - according to Sale.
Back in November 2010, the Nantanz plant had to be closed for seven days as the virus was still causing negative effects on Iran's nuclear program.
Source: Business Insider