Microsoft has issued an emergency patch program to fix a weakness in several Windows browsers, created by the company's own antivirus software.
Two Google Project Zero researchers, Tavis Ormandy and Natalie Silvanovich, discovered the flaws in software like Windows Defender over the weekend and drew attention to them on Twitter.
If Microsoft hadn't acted on the problem, it could allow hackers to remotely access a user's computer and even infiltrate other PCs on a network, because of how "wormable" the weakness is.
The term to describe it is 'remote code execution', which refers to the ability of a hacker to break into a device and make changes, irregardless of geographical location.
Silvanovich called it the "worst Windows remote code (execution vulnerability)" in recent memory.
Microsoft quickly responded to the problem, releasing the patch to cover the weakness.
It will be available to users as '1.1.13704.0', and should be automatically downloaded onto affected computers within one or two days.