Vole apparently released a new update for Windows Defender that has broken both the Quick and Full antivirus scans meaning that Windows Defender was only scanning approximately 40 files.
In August, Microsoft announced that they were releasing Windows Defender 4.18.1908.7 to fix a bug that was causing the SFC /scannow command to fail with an error stating that "Windows Resource Protection found corrupt files but was unable to fix some of them."
However when the release of version 4.18.1908.7 came, users started reporting that they are unable to perform Quick or Full scans. When they attempt to do so, instead of scanning the thousands of files that they are used to, Windows Defender is only scanning about 40 files.
This same behavior occurs when performing a Quick scan, the Offline Scan feature and Microsoft Security Essentials (MSE) in Windows 7. For Windows 7, the user has MSE definition 1.301.1645.0 that was installed today.
Custom scans were you specify the drive were not borked.
Vole issued a statement saying that the issue has been fixed.
"Microsoft Defender AV employs real-time scanning of endpoints, which was not impacted by this update. Only manual or scheduled scans conducted by administrators were temporarily impacted and we are working to resolve it. The issue in the definitions and has been resolved in "Security Intelligence Update for Windows Defender Antivirus - KB2267602 (Version 1.301.1684.0)".