Stuxnet is a malicious computer worm first uncovered on 17 June 2010 and thought to have been in development since at least 2005. Stuxnet targets supervisory control and data acquisition (SCADA) systems and is believed to be responsible for causing substantial damage to the Iran nuclear program after it was first installed on a computer at the Natanz Nuclear Facility in 2009. Although neither the United States nor Israel has openly admitted responsibility, multiple independent news organizations claim Stuxnet to be a cyberweapon built jointly by the two countries in a collaborative effort known as Operation Olympic Games. The program, started during the Bush administration, was rapidly expanded within the first months of Barack Obama's presidency.

Stuxnet specifically targets programmable logic controllers (PLCs), which allow the automation of electromechanical processes such as those used to control machinery and industrial processes including gas centrifuges for separating nuclear material. Exploiting four zero-day flaws in the systems, Stuxnet functions by targeting machines using the Microsoft Windows operating system and networks, then seeking out Siemens Step7 software. Stuxnet reportedly compromised Iranian PLCs, collecting information on industrial systems and causing the fast-spinning centrifuges to tear themselves apart. Stuxnet's design and architecture are not domain-specific and it could be tailored as a platform for attacking modern SCADA and PLC systems (e.g., in factory assembly lines or power plants), most of which are in Europe, Japan and the United States. Stuxnet reportedly destroyed almost one-fifth of Iran's nuclear centrifuges. Targeting industrial control systems, the worm infected over 200,000 computers and caused 1,000 machines to physically degrade.

Stuxnet has three modules: a worm that executes all routines related to the main payload of the attack, a link file that automatically executes the propagated copies of the worm and a rootkit component responsible for hiding all malicious files and processes to prevent detection of Stuxnet. It is typically introduced to the target environment via an infected USB flash drive, thus crossing any air gap. The worm then propagates across the network, scanning for Siemens Step7 software on computers controlling a PLC. In the absence of either criterion, Stuxnet becomes dormant inside the computer. If both the conditions are fulfilled, Stuxnet introduces the infected rootkit onto the PLC and Step7 software, modifying the code and giving unexpected commands to the PLC while returning a loop of normal operation system values back to the users.

Stuxnet, discovered by Sergey Ulasen from a Belarusian antivirus company VirusBlokAda, initially spread via Microsoft Windows, and targeted Siemens industrial control systems. While it is not the first time that hackers have targeted industrial systems, nor the first publicly known intentional act of cyberwarfare to be implemented, it is the first discovered malware that spies on and subverts industrial systems, and the first to include a programmable logic controller (PLC) rootkit.

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.

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 percent of the infected computers worldwide were in Iran. Siemens stated that the worm caused no damage to its customers, but the Iran nuclear program, which uses embargoed Siemens equipment procured secretly, was damaged by Stuxnet. Kaspersky Lab concluded that the sophisticated attack could only have been conducted "with nation-state support". F-Secure's chief researcher Mikko Hyppönen, when asked if possible nation-state support were involved, agreed: "That's what it would look like, yes."

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 U.S. and its allies – are doing everything we can to make sure that we complicate matters for them", offering "winking acknowledgement" of United States involvement in Stuxnet. According to The Daily Telegraph, a showreel that was played at a retirement party for the head of the Israel Defense Forces (IDF), Gabi Ashkenazi, included references to Stuxnet as one of his operational successes as the IDF chief of staff.

On 1 June 2012, an article in The New York Times reported that Stuxnet was part of a US and Israeli intelligence operation named Operation Olympic Games, devised by the NSA under President George W. Bush and executed under President Barack Obama.