Microsoft rushed to create the fix after learning of the bug in the operating system over the weekend when cybersecurity firm FireEye Inc warned that a sophisticated group of hackers had exploited the bug to launch attacks on U.S. companies in a campaign dubbed "Operation Clandestine Fox."
It was the first high-profile threat to emerge after the software maker stopped providing support to the 13-year-old operating system on April 8. Microsoft said in a statement on Wednesday that the company would not provide the remedy to Windows XP users because it had stopped supporting the product.
But on Thursday, as Microsoft started releasing the fix through the company's automated Windows Update system, a company spokeswoman said the fix would be pushed out to customers still using XP.
"We decided to fix it, fix it fast, and fix it for all our customers," spokeswoman Adrienne Hall said in a statement.
The company was under pressure to move quickly as the U.S., UK and German governments advised computer users on Monday to consider using alternatives to Microsoft's Explorer browser until it released a fix.
Microsoft first warned that it was planning to end support for Windows XP in 2007, but security firms estimated that 15 to 25 percent of the world's personal computers still run on the version of the operating system that was released in October 2001.