Microsoft has uncovered a software bug in old versions of its Windows operating system that could be used to carry out a global-scale cyber attack.
If the bug is discovered by hackers, they could unleash a computer virus similar to the Wannacry attack that crippled the NHS and thousands of other machines in 2017.
"Any future malware that exploits this vulnerability could propagate from vulnerable computer to vulnerable computer," said Simon Pope, Microsoft's director of incident response, in a blog post.
"While we have observed no exploitation of this vulnerability, it is highly likely that malicious actors will write an exploit for this vulnerability and incorporate it into their malware."
Microsoft said the bug affects machines running Windows 2003, Windows XP, Windows 7 and Windows Server 2008.
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Windows 7 or Windows Server 2008 are still supported by Microsoft so, as long as you have automatic updates enabled, you are automatically protected.
Windows 2003 and Windows XP are no longer supported, but the company has released a special one-off critical security update for these operating systems.
If you're still running one of these systems, you can download the patch here .
Data from NetMarketShare suggests that about 3.57% of desktop computers are still running Windows XP.
"We strongly advise that all affected systems should be updated as soon as possible," said Pope.
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Commenting on the news, cyber security expert Graham Cluley said the Windows bug is potentially a big problem, because of the rapid speed with which a these types of virus can spread.
"You can tell just how serious Microsoft believes the wormable vulnerability to be because it has also issued fixes for operating systems that the company no longer officially supports," he said.
"Yes, you thought Windows XP was dead (and good riddance, by the way).
"But no, Microsoft is so worried that another WannaCry-style worm outbreak might be around the corner, fuelled by out-of-date computers that are still riskily connected to the internet, that it will even roll out a patch for Windows XP."
He added that Windows users should update their systems at the earliest opportunity, and consider enabling automatic updates if they haven't already configured your computer to stay up-to-date.
