Level 24
Microsoft has released a new update for Windows Defender that has broken both the Quick and Full antivirus scans. When users use these scan options, Windows Defender will only scan 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".

With the release of version 4.18.1908.7, Microsoft may have fixed that bug, but have now created a new one.

Windows Defender Version 4.18.1908.7 Installed
Starting yesterday, when that version was released, users have started reporting [1, 2, 3, 4, 5, 6] 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.

BleepingComputer has been able to independently confirm that this is happening on our own computers that have version 4.18.1908.7 (KB4052623) installed. As you can see below, when we performed a Full scan, it only scanned 44 files and lasted 9 seconds.

Full Scan only scanning 44 files
This same behavior occurs when performing a Quick scan. BleepingComputer readers have also stated that this is affecting 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 were installed today.
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Level 23
It has also become a hot topic on Japanese bulletin boards. In my environment, a safety declaration came out after four files were scanned instantly. Today, after updating and scanning again, this time I got into an unstoppable state.:censored:

....Windows Destroy:sleep: