Microsoft is now detecting the popular CCleaner Windows optimization and Registry cleaner program as a potentially unwanted application (PUA) in Microsoft Defender. with all files automatically removed from Windows computers. The CCleaner listing was last updated on July 27.

While Microsoft hasn’t provided any information as to why the app is now flagged as a PUA.

In a support document, the company explains that turning to such apps can eventually affect the stability of Windows.

Some products such as registry cleaning utilities suggest that the registry needs regular maintenance or cleaning.  However, serious issues can occur when you modify the registry incorrectly using these types of utilities. These issues might require users to reinstall the operating system due to instability. Microsoft cannot guarantee that these problems can be solved without a reinstallation of the Operating System as the extent of the changes made by registry cleaning utilities varies from application to application,” it says.

Furthermore, Microsoft’s evaluation criteria state that displaying “misleading” or “inaccurate” claims about files and registry entries would cause a program to fall under the classification of a potentially unwanted program.

Piriform on the other hand has told BleepingComputer that they believe this is a false positive and trying to work with Microsoft to remove the detection.

“We discovered this classification on Tuesday, after reports from our customers of difficulties installing CCleaner with Windows Defender.  We believe this is a “false positive” – we are in discussions with Microsoft and expect a resolution to this matter soon”

“We have noted that Windows Defender seems to be classifying the free version of CCleaner as “Potentially Unwanted Software” but not the paid versions.  Also this classification does not seem to be due to any recent changes at our end, since Windows Defender is also flagging other Piriform software such as Recuva (which has not been updated for a while) in the same fashion.”

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“It would also seem odd to describe Recuva – free file recovery software that a user would need to go to the effort of discovering on our website and downloading – as “potentially unwanted,” David Peterson, General Manager CCleaner, told BleepingComputer.