CCleaner has become flagged like a Potentially Unwanted Application, or PUA, by Microsoft's Windows Defender antivirus, and it is installation is thus blocked on Windows 10 devices.

As the software originally added CCleaner to the threat database with only several details, Microsoft published an update on July 29 to describe why precisely the application is blocked by Windows Defender.

Microsoft says CCleaner itself is not really a dangerous program but its installer includes certain software, such as Google Chrome, google's Toolbar, Avast and AVG antivirus that users may not want.

"Certain installers free of charge and 14-day trial versions of CCleaner include bundled applications, including applications that are not required by CCleaner or produced by the same publisher Piriform. While the bundled applications are legitimate, bundling of software, especially products from other providers, can lead to unexpected software activity that may negatively impact user experiences," Microsoft says.

CCleaner installation blocked

They are all clean apps, Microsoft emphasizes, and regardless of the CCleaner installer offering a setting to opt out and stop them from downloading and installing, quite a few users may not necessarily spot these options and wind up running software they didn't ask for in the first place.

"Some installers for that free form of CCleaner, also according to Nullsoft, have an installer for Google Chrome using the file name PF-Chrome-2019.exe. After installing Google Chrome, the installer sets the bundled application because the default web browser. Some variants of the identical installer include the Google Toolbar to Chrome as an extension," Microsoft says.

Consequently, Windows Defender just blocks CCleaner completely, so long as the installer itself includes such software bundled by default. However, worthwhile to learn isn't that all installers are blocked, as Piriform, the Avast-owned maker of CCleaner, can also be offering separate packages which come without any other programs provided to users throughout the installation.