Microsoft has fixed a flaw in Windows Defender which was being actively exploited in the wild.
The Microsoft Defender Remote Code Execution Vulnerability (CVE-2021-1647) would see Windows Defender turn into the attacker, triggering the execution of malware when the malware was scanned, instead of quarantining and deleting it.
This means a file could be sent by email or USB drive, and when downloaded and automatically scanned be immediately triggered.
The exploit was fixed as part of Patch Tuesday on the 12th and was one of 80 flaws which were addressed.
To check if you are protected, checked the version number of the scan engine in the Windows Security app by searching for Windows Security in the start menu, opening the app, and going to Settings and About. Version 1.1.17700.4 and above are not vulnerable to the exploit.