Microsoft is patching a bug in Windows XP, two years after the last one, and five years after it officially abandoned the operating system.
The patch comes as Microsoft fixes 78 other bugs across its other operating systems, but Windows XP being updated is the most worrying, Wired reports.
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Windows XP was released in 2001, and has been followed by four newer systems - Vista, 7, 8 and 10. Microsoft announced in 2014 it planned to end support for Windows XP.
The latest patch is only the second since then, with one performed in May 2017 to address vulnerabilities to WannaCry ransomware. WannaCry targeted companies worldwide and blocked files until users paid a ransom of thousands of dollars in bitcoin.
"Any future malware that exploits this vulnerability could propagate from vulnerable computer to vulnerable computer in a similar way as the WannaCry malware spread across the globe in 2017," Simon Pope, director of incident response for the Microsoft Security Response Center, wrote in a statement on Tuesday.
The Microsoft website lists the severity of the security update as "critical".
But users have to manually download the patch and install it, prompting concerns people won't, leaving their computers vulnerable.
An estimated 3.57 percent of all desktops and laptops run Windows XP. It was revealed in 2016 that Britain's doomsday submarines still used the software.