Microsoft Thursday released a patch for a perilous hole in its Internet Explorer browser that hackers could slip through to invade computers.
The flaw was deemed so dangerous that the US software colossus planned to take the unusual step of releasing a fix for its aged Windows XP operating system, which it officially stopped supporting last month.
Microsoft began "pushing" patches as automated updates to a "critical" flaw at 1700 GMT.
People whose machines are not set to automatically update Microsoft software need to tend to the process.
"When we saw the first reports about this vulnerability we decided to fix it, fix it fast, and fix it for all our customers," Microsoft general manager of trustworthy computing Adrienne Hall said in a blog post.
"The security of our products is something we take incredibly seriously."
A US government cybersecurity watchdog warned computer users Monday against using a version of the Microsoft Internet Explorer browser with a security hole.
The government's Computer Emergency Response Team (US-CERT) said in a statement it was aware of "active exploitation" of the security flaw in versions six through 11 of the flagship Explorer browser.
The agency, a unit of the Department of Homeland Security, urged computer users to take protective actions and "consider employing an alternative Web browser until an official update is available."
Microsoft said Saturday that an attacker who successfully exploits the vulnerability could take control of the computer.
On Thursday, it noted in a bulletin at its online security center that it was "aware of limited, targeted attacks that attempt to exploit this vulnerability in Internet Explorer."
The security flaw is of particular concern for computers running Windows XP, an older version of the operating system for which Microsoft had stopped issuing security updates.
Explore further: US warns on use of flawed Microsoft browser
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