The flaw was deemed so dangerous that the US software company 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 17:00 GMT.
People whose machines are not set to automatically update Microsoft software need to attend to the process.
"When we saw the first reports about this vulnerability we decided to fix it and fix it fast," Microsoft general manager of trustworthy computing Adrienne Hall said.
"The security of our products is something we take incredibly seriously," he added.
A US government cyber-security watchdog warned computer users 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 to 11 of the Internet Explorer.
The agency, a unit of the Department of Homeland Security, urged computer users to take protective action and consider employing an alternative Web browser until an official update is available."
Microsoft said that an attacker who successfully exploits the vulnerability could take control of the computer adding in a bulletin at its online security centre that it was aware of limited, targeted attacks that attempt to exploit this flaw.
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.
Source: AFP via I-Net Bridge
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