Microsoft routinely releases software patches the second Tuesday of each month, but resorts to making patches available 'out of band' when it deems situations dangerous.
'Several families of malware have been attempting to attack this vulnerability,' Microsoft Trustworthy Computing response manager Christopher Budd wrote in a blog post.
'We firmly believe that releasing the update out of band is the best thing to do to help protect our customers.'
Some attacks have reportedly been directed at power plants and other vital infrastructure. People using computers running on any version of Windows software were urged to apply the update immediately.