PC Periodicals: Microsoft flags CCleaner software

Microsoft is now detecting the popular CCleaner Windows optimization and Registry cleaner program as a potentially unwanted application (PUA) in Microsoft Defender.  Microsoft strongly advises against allowing any program to mess with your registry. CCleaner can do that, so Microsoft does not want you to get the idea that you can use it with impunity or that they endorse it in any way.

Before the acquisition of CCleaner’s maker Piriform by Avast in 2017, CCleaner did have a good reputation. But Avast has done some questionable things since the acquisition and that reputation has suffered.

In September 2017, researchers discovered that CCleaner was compromised by hackers who inserted a backdoor into the software, enabling remote access of infected machines.

It is possible that CCleaner may have tricked you into installing a PUP (potentially unwanted program) when you installed CCleaner. PUPs are not dangerous. Typically, just annoying adware. A scan with the free version of Malwarebytes should enable you to get rid of any PUPs.