A recent update to Windows Defender and Microsoft Security Essentials is causing widespread problems for Windows users trying to scan systems for malicious files.
The glitch in Microsoft’s anti-malware is being reported by Windows 7, Windows 8.1, and Windows 10 users.
As per German tech blog Borncity, the file scanning issues emerged after Microsoft released an update to the Microsoft Malware Protection Engine (MsMpEng.exe) on September 16 that brought it up to version 4.18.1908.7.
The engine ships with several Microsoft anti-malware products for consumer and enterprise customers, including Windows Defender, Security Essentials and Forefront. And unlike normal Windows security updates, which give users some choice over when they're installed, typically Windows Defender users don’t choose when the engine is updated because Microsoft deploys them automatically through an update manager.
The issue is affecting the 'quick scan' and 'full scan' options, with quick scans ending in seconds and only scanning between five and 64 files. Normally, thousands of files should be scanned in a process that takes around 30 minutes to complete.
But the bug means that Microsoft's anti-malware isn't doing the job it was intended for and as users point out, the software still displays a green tick mark and states the scan is 'complete', even though it's clearly not.
Readers of the AskWoody Windows-focussed blog have reported that both the full scan and quick scan quit menus after combing through just 29 files.
The same complaints are being reported on Microsoft’s Answers website and Reddit, where some users also complain about high CPU usage after installing the last Patch Tuesday update earlier this month, but that's a separate issue.
The Patch Tuesday update fixed a glitch with Windows Desktop Search causing high CPU usage, but the same fix also broke Windows Desktop Search in the process for users of Windows 10 version 1903, the newest release of Windows 10.
It's a similar story for the latest Windows Defender update. As Borncity notes, the KB4052623 update fixed a Windows Defender bug in the product's System File Checker (SFC) that Microsoft acknowledged in August, but didn't get around to fixing until this week. And now users are complaining the fix has caused new problems.
The Windows Defender bug comes at a time when Microsoft has been bragging about its higher protection ratings by antivirus testers, such as AV-Test and AV-Comparatives. A few years ago Windows Defender and Security Essentials rated poorly compared to Kaspersky, F-Secure, Norton and McAfee, but these days it's consistently at the top of the industry.
Microsoft claims these improvements are why today over half of all Windows users -- including Windows 10, Windows 7, and Windows 8.1 -- now actively use Windows Defender.
Update: Microsoft told CSO Online that it has now fixed the issue and that more details can be found at this page on its Answers forum.