Security researchers from CyberArk have discovered a new technique that allows malware to bypass Windows Defender, the standard security software that comes included with all Windows operating systems. The technique nicknamed Illusion Gap relies on a mixture of both social engineering and the use of a rogue SMB server. The attack exploits a design choice in how Windows Defender scans files stored on an SMB share before execution. Microsoft does not view this as a security issue but a feature request which I have forwarded to the engineering group.
Source: https://www.bleepingcomputer.com/news/security/new-illusion-gap-attack-bypasses-windows-defender-scans/