Windows Defender crashes: Microsoft fixes bug causing full scans to fail

Microsoft's Windows Defender antivirus was crashing when hitting filenames with two dots.

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Microsoft has rolled out a fix for a recent update that has been causing the Windows Defender built-in antivirus software to crash on Windows 10 PCs when running a full system scan.

With around half a billion Windows 10 PCs using Windows Defender or Microsoft Defender, when things go wrong, lots of users feel the pain. 

Users on Reddit and Microsoft's Answers forum yesterday started reporting error messages that were caused by a recent definition update to Microsoft's antivirus. 

Per BleepingComputer, running a Quick Scan didn't produce any error messages, but a Full Scan resulted in a failure when scanning files and multiple error messages due to an issue affecting the Windows Defender Antivirus service. 

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Users on Reddit discovered Windows Defender was crashing when it reached filenames with two dots (..) in it. Simply removing one of the dots would allow Windows Defender to continue scanning. 

Fortunately, a few hours ago Microsoft released a new Security Intelligence Update for Windows Defender that addresses the two-dot crashes. Since Microsoft automatically issues updates to Windows Defender outside its Patch Tuesday schedule, users should have received it and no longer experience the crashes.

If users are currently experiencing Windows Defender crashes, they can manually grab the latest updates by clicking 'Check for updates' within the Windows Security app.    

Windows Defender users also ran into problems with Microsoft's built-in antivirus in September, though in that case both Quick Scan and Full Scan were complete within seconds because Windows Defender wasn't actually scanning files. 

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While Microsoft fixed the issue fairly quickly this time, any bug it introduces that prevents people from connecting to work machines is an extra problem given remote-working conditions during the COVID-19 coronavirus pandemic

In March Microsoft raced out a patch for a Windows 10 bug that was causing internet connectivity issues for users and preventing some Office 365 setups from reaching the cloud.