Researchers revealed earlier variants of Stuxnet were in operation as early as 2005.
Researchers at security firm, Symantec, have discovered an early version of the Stuxnet computer virus. The virus was used to attack Iran's nuclear-enrichment facilities in Natanz in November 2007, two years earlier than it was initially thought.
Stuxnet, which is reportedly believed to have been developed by the US and Israel, was first discovered in 2010.
Stuxnet is a malware that infects computers through a control system targeted towards industries that control water supplies, oil rigs, and power plants.
Symantec said its researchers have uncovered a piece of software, dubbed Stuxnet 0.5, which was in operation between 2007 and 2009.
Researchers said earlier variants of Stuxnet were in operation as early as 2005.
According to the researchers, Stuxnet 0.5 was built using the Flamer platform and spreads by infecting Step 7 projects, including through USB keys.
Stuxnet 0.5 does not contain Microsoft exploits and has a full working payload against Siemens 417 programmable logic controllers (PLCs) that was incomplete in Stuxnet 1.x versions.
Symantec said a few things changed between Stuxnet 0.5 and 1.001, with later versions increasing their spreading capability and use of vulnerabilities.
According to Symantec, the Flamer platform code was replaced with the Tilded platform code and later versions adopted an alternative attack strategy from uranium enrichment valve disruption to centrifuge speed modification.
Earlier this year, a US General said Iran has strengthened its cyber threat protection following the Stuxnet cyber attack in 2010.