Brink said:

Read more: https://www.zdnet.com/article/windows-defender-malware-scans-are-failing-after-a-few-seconds

The article is misleading in stating that...

The bug impacts Windows Defender version 4.18.1908.7 and later, released earlier this week. The bug was introduced while Microsoft tried to fix another bug introduced with the July 2019 Patch Tuesday...

The bug affects ALL Microsoft's scan engines in ALL supported OS, that's Defender and Microsoft Security Essentials in Windows 7, Windows 8.x and Windows 10. It appears that the virus definitions are the cause of this bug. The bug is in ANY Microsoft AV that uses a virus definition from about 1.301.1600.0 or newer.


Here I demonstrate the same bug in Windows 7 using MSE.

Bree said:

... 'before and after' Quick Scan test on a different W7/MSE machine.



A subsequent Full Scan on this machine also only checked 50 items.

Windows 10 Defender - Quick/Full Scans Last Only 15 Seconds??

Faith said:

Does this affect automatic checks?

Yes. The bug affects the Quick scan and the Full scan. The automatic scan is a Quick scan. Only the Custom scan works normally.

It appears that the real-time protection is still working normally though.

magilla said:

Thank you Bree. At least some comfort knowing real-time protection appears to be working.

Yes, I have confirmed that with the EICAR test virus.

Bree said:

Yes, I have confirmed that with the EICAR test virus.

Ah awesome. I was worried about that too. Thanks for confirming real time protection is working as usual, Bree.

I also tried using the off-line scan , and I believe that wasn't working correctly either.

Bree said:

It appears that the virus definitions are the cause of this bug. The bug is in ANY Microsoft AV that uses a virus definition from about 1.301.1600.0 or newer.

In support of Bree and his Defender Definition as the cause of this bug you can run the new definitions against the old platform which is in a different version folder:

"C:/ProgramData/Microsoft/Windows Defender/Platform/4.18.1907.4-0/\MpCmdRun.exe" Scan -ScheduleJob -ScanType 1

Same 10 second run as

"C:/ProgramData/Microsoft/Windows Defender/Platform/4.18.1908.7-0/\MpCmdRun.exe" Scan -ScheduleJob -ScanType 1

Code:

------------------------------------------------------------------------------------
MpCmdRun: Command Line: "C:\ProgramData\Microsoft\Windows Defender\platform\4.18.1908.7-0\MpCmdRun.exe" Scan -ScheduleJob -ScanType 1
 Start Time: ‎Wed ‎Sep ‎18 ‎2019 15:07:56

MpEnsureProcessMitigationPolicy: hr = 0x1
Starting RunCommandScan.
Scanning path as file: (null).
Start: MpScan(MP_FEATURE_SUPPORTED, dwOptions=16385, Timeout in days = 1)
MpScan() started
Time Info - ‎Wed ‎Sep ‎18 ‎2019 15:08:07 MpScan() was completed
Finish: MpScanStart(MP_FEATURE_SUPPORTED, dwOptions=16385)
Finish: MpScan(MP_FEATURE_SUPPORTED, dwOptions=16385, Timeout in days = 1)
MpScan() has detected 0 threats.
MpCmdRun: End Time: ‎Wed ‎Sep ‎18 ‎2019 15:08:07
-------------------------------------------------------------------------------------


-------------------------------------------------------------------------------------
MpCmdRun: Command Line: "C:\ProgramData\Microsoft\Windows Defender\platform\4.18.1907.4-0\MpCmdRun.exe" Scan -ScheduleJob -ScanType 1
 Start Time: ‎Wed ‎Sep ‎18 ‎2019 15:10:03

MpEnsureProcessMitigationPolicy: hr = 0x1
Starting RunCommandScan.
Scanning path as file: (null).
Start: MpScan(MP_FEATURE_SUPPORTED, dwOptions=16385, Timeout in days = 1)
MpScan() started
MpScan() was completed
Finish: MpScanStart(MP_FEATURE_SUPPORTED, dwOptions=16385)
Finish: MpScan(MP_FEATURE_SUPPORTED, dwOptions=16385, Timeout in days = 1)
MpScan() has detected 0 threats.
MpCmdRun: End Time: ‎Wed ‎Sep ‎18 ‎2019 15:10:13
-------------------------------------------------------------------------------------