Windows Defender took action against malicious software: Here's what you need to know

Since mid-February, many Windows 10 users have encountered a strange Windows Defender message, informing them that the antivirus took action against malicious software. The only problem is that after a thorough search, no malware appears on the list. The results in the Windows Defender’s history confirm that the scan did not detect anything, but the notification shown in the action center suggests that the antivirus took action against malicious software.

As a result, users are left in the unknown as to what malicious software has been detected. Also, the fact that this message occurs daily makes many users wonder whether Windows Defender works properly or not.

Here’s how one user describes this ambiguous Windows Defender message:

I scan using windows defender and get a message “WD scanned your computer … and took action against malicious software”.
Good, my computer is now clean, right? Apparently not, because when I scan again, I get the same result.

So, to see what the problem is, I check “history” in WD, looking at quarantined items and deleted items – but nothing shows, so it seems that no action was in fact taken. So, is my computer clean or not? And what has WD actually done?

What we know about this mysterious message

This ambiguous message appears both after a full and a quick scan. After using an additional anti-malware tool to perform a system scan, users report that Windows Defender continues to display the same intriguing message.

Most likely this is a bug in Windows Defender itself. The issue could be just a glitch in Defender’s notifications feature after a recent update.

However, many users believe this message is triggered by malware that Defender is not able to completely remove from the system. The malware re-installs itself upon the next boot. This may not be a far-fetched hypothesis because right after boot up, when users run a quick scan with Defender, they get the message indicating that the tool took action against malicious software. When they run the scan again, Windows Defender doesn’t find any threats.

However, if users restart their computers and follow the same steps, they get the same results. The initial scan indicates that Defender found and took action against malicious software, but then the second scan indicates no threats have been found.

All’s well that ends well

The good news is that apparently Microsoft has recently solved this problem. Many users confirm they haven’t encountered this error message since the beginning of March. Try installing the latest Windows Defender updates on your computer, and the annoying “Windows Defender took action against malicious software” message should be history.

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