Problem scanning certain files with windows defender

Hello

My windows defender has started having errors that stop working, stops and cannot be restarted, unless you start it from services.msc. Yesterday the antivirus was working fine, today I have updated to April 14, 2020 — KB4549951 cumulative update. I have applied the last defender definition.

I think that i have located the source of the error and it consists of the antivirus stopping when it tries to scan any file whose name is like this:

filename..ext

For example, filename..txt

If you scan filename.txt or filename...txt it does not produce any error, it is only when the file name ends with a point before the file extension.

Any solution?

Pilovar said:

I think that i have located the source of the error and it consists of the antivirus stopping when it tries to scan any file whose name is like this:
filename..ext
For example, filename..txt
If you scan filename.txt or filename...txt it does not produce any error, it is only when the file name ends with a point before the file extension.
Any solution?

That sounds so ridiculous that I had to test it. I created a filename..txt, right-clicked on it, then selected 'Scan with Windows Defender...'. It scanned perfectly.

But then I checked for updates and got the Security Intelligence update 1.313.1666.0. I can confirm that now it behaves exactly as you describe


As the bug seems to be entirely due to the security intelligence definitions the solution would appear to be in Microsoft's hands, not ours.

This seemed so unlikely that I had to try going back and test it again. I restored my Macrium image from two days ago and confirmed Defender could scan a 'double-dot' file name....



... then updated just the definitions and confirmed that scanning the same file would kill the Windows Defender Antivirus Service. The only difference is the antivirus definitions version, confirming that this was the trigger for the bug.



That was a remarkable piece of detective work @Pilovar, how on earth did you discover that a double-dot file name caused the bug?

I saw the bug appear when I was running a certain program. I excluded the program parent folder from Defender options and the error stopped happening. Well, the problem would be in this folder. Then I have doing individual manual scans of all the subfolders of the program, until I verified that the error only occurred in one subfolder. I repeated the process with all the files in that subfolder (there were only a few dozen ...), seeing that the problem was in certain apparently normal files but with that double dot. Then I changed the name and location of the files with problems and rescaninng them and I verified that the bug was only for the double dot, not file itself or its location.


Note:
I have updated to the latest virus definitions and the bug is still there

New security intelligence version 1.313.1687.0 has just come out, seems to be fixed now

Bree said:

New security intelligence version 1.313.1687.0 has just come out, seems to be fixed now

It's fixed for me after getting 1.313.1687.0.

The tip off was how quickly it returned the 'No current threats'

Before you could tell that it was confused by this 'double-dot' filename because of how long it took to finish what was just a single line text file.