The fear of massive global computer viruses has led Microsoft to issue security updates for very old versions of its Windows software.
A patch is for Windows XP, which debuted in 2001 and Microsoft stopped supporting in 2014.
Microsoft said the patch closed a hole that could be used to spread a virus.
The malicious hackers who exploit it could trigger a global outbreak such as the Wannacry 2017 worm, which affected thousands of machines.
It was "very likely" that the vulnerability would be exploited if it were not patched, wrote Simon Pope, director of incident response at Microsoft, in a blog about the error.
He said that the error could be exploited simply by connecting to a vulnerable machine through the internet.
"Any future malware that exploits this vulnerability could spread from a vulnerable computer to a vulnerable computer in a similar way to the WannaCry malware that spread around the world in 2017," he said.
This danger caused the release of a patch that closes the gap:
- Windows Xp
- Windows 2003
- Windows 7
- Windows Server 2008
The market industry data suggest that about 3.75% of desktop machines currently use XP or its variants.
The Wired technology news site said that the wide range of vulnerable versions of Windows meant that "millions" of machines were at risk.
Although Microsoft ended up supporting Windows XP more than five years ago, the last patch of software issued for the operating system was released just before the Wannacry outbreak.
"We strongly recommend that all affected systems (…) be updated as soon as possible," Mr. Pope said.
Microsoft said there was no evidence that cyber criminals were exploiting the lagoon.
People and organizations that use Windows 8 or 10 were at risk, Microsoft said.
Independent security expert Graham Clule said that the creation of the patch for XP showed that the threat of a virulent worm was "serious".
Many old and vulnerable systems were still "risky" connected to the Internet, so they urged people to update.