Windows users have a built-in antivirus program that is expected to be on the job looking for viruses and other threats that could compromise the security of a computer. A major bug has been reported in Windows Defender, the built-in antivirus program that many Windows users take advantage of. It's easy enough to see if a computer is impacted by the bug by running a quick or full scan.
On machines that are impacted, the scans will fail after a few seconds and only go through a handful of files. Depending on the number of files on the computer, this sort of scan can last hours. The bug has been widely reported over the last few days on Microsoft tech support forums, Reddit, and other places.
The bug impacts Windows Defender version 4.18.1908.7 and later. Reports indicate that the bug surfaced as a side effect to another bug that Microsoft was trying to fix that landed along with the July 2019 Patch Tuesday update. The bug broke the "sfc /scannow" command that Windows System File Checker needed to scan and fix corrupted files.
The bug caused the file checker utility to flag some Windows Defender modules as corrupted, leading to incorrect error messages. Microsoft announced a fix for the System File checker bug in August, but the patch was delayed. When the patch did land earlier this week, it didn't address the bug as expected.
The bug also impacts Security Essentials on older versions of Windows. Microsoft has noted that on impacted systems, the bug only affects manual or scheduled scans and that the real-time scanning protection works normally and would detect malware if it reached the system. Users have found that setting up a custom scan gets around the bug. Microsoft announced earlier this summer that Windows Defender would change names to Microsoft Defender Antivirus and Exploit Guard.
Updated 9/19/2019 @ 9:45am
Microsoft has addressed this bug with with the latest "Security Intelligence Update for Windows Defender Antivirus - KB2267602 (Version 1.301.1684.0)" definitions.