Microsoft has carried out an emergency update of Windows after discovering that the makers of a spy virus had exploited a software bug.
The Flame espionage tool infected PCs across the Middle East by tricking computer security systems into accepting it as a genuine Windows product.
Mike Reavey, a senior director with Microsoft's security team, said the attacks were targeted and "highly sophisticated".
As a result of the bug fix, any viruses that bears the fake Microsoft code are likely to stop working.
Microsoft declined to comment on whether other viruses had exploited the same flaw in Windows, or whether the company was looking for similar bugs in the operating system.
Experts said the method had probably been used to deliver other viruses that have not yet been identified.
"It would be logical to assume that (the virus creators) would have used it somewhere else at the same time," said Mikko Hypponen, chief research officer for security software maker F-Secure.
Flame has been in circulation since 2010 but because of its complexity was only discovered last week.
It was aimed primarily at Iran, but also affected Israeli and Palestinian territories, Sudan, Syria and Lebanon.
Researchers say that technical evidence suggests it was built on behalf of the same nation that commissioned the Stuxnet worm that attacked Iran's nuclear program in 2010.
Information about the virus is still being gathered by computer analysts.
