Microsoft is releasing an out of schedule security update to the Microsoft Malware Protection Engine which has a critical vulnerability to remote code execution delivered via an email. Windows 8, 8.1, 10 and Windows Server OS are all affected.

[technet.microsoft.com...]

The patch should be delivered if your systems are configured to automatically update.

However, bearing in mind this is a critical issue, users should perhaps consider running the update manually to ensure the system is updated quickly.