Microsoft Update Breaks Windows Defender
A broken Microsoft Windows Defender signature file that was causing system file checks to fail was patched but it made Defender antivirus scans fail. Microsoft re-patched it a day later but not before the press reported on the mistake.
The patch was for the administrative prompt sfc/scannow which had been out of commission since July. SFC is used to find corrupted files and fix them after installing updates. Since the July Patch Tuesday updates though, it had been failing. SFC was flagging internal Windows PowerShell files within Defender as malformed. The system caused SFC to cancel itself. In August, Microsoft issued a notice, explaining that the System File Checker tool flags files that are located in the %windir%System32WindowsPowerShellv1.0ModulesDefender folder as corrupted or damaged. When this issue occurs, there were error entries about Hashes for file member not matching.
The fix for the bug was issued on Tuesday in a silent update, but that update ended up causing manual or scheduled Defender malware scans to fail. This caused users to post on online forums to report the bug. Real-time scanning was still enabled and the Custom scan option was also still working.