An Iranian official on Sunday acknowledged that the stuxnet worm had affected personal computers at the Bushehr nuclear plant but asserted that the malware had not caused major damage.

Stuxnet is believed to have been created last year and was first detected by a security firm in Belarus in June, according to ICSA Labs. Once introduced to a computer system via a USB drive, among other attack vectors, it is designed to exploit as many as four different vulnerabilities in various versions of Microsoft Windows and to infect Supervisory Control and Data Acquisition control systems (SCADA) made by Siemens. These systems control critical infrastructure at facilities like power plants.

To date, Microsoft has patched two vulnerabilities exploited by stuxnet.

The malware has been characterized as being exceptionally sophisticated, prompting speculation that it could only be the product of an organization backed by a nation-state, such as an intelligence agency.

No proof of such claims has yet been made public.

The targeted nature of the malware, in the opinion of Siemens, means that it was created to attack a specific facility or industrial process.

"Stuxnet is obviously targeting a specific process or a plant and not a particular brand or process technology and not the majority of industrial applications," the company said in updated information it posted last week.

Symantec, which plans to present a paper on stuxnet at the Virus Bulletin Conference on September 29th, said in June that the majority of stuxnet infections it could detect (59%) were in Iran.

Attacks on critical infrastructure like the Bushehr nuclear plant are just the sort of cyber warfare that U.S. officials have long feared could occur in the U.S. and have sought to prevent through funding and legislation.

The Pentagon has refused to comment on whether or not it launched the stuxnet attack.