Microsoft’s Windows Defender antivirus software is—perhaps ironically—flagging CCleaner, a junk file remover, as a “Potentially Unwanted Application."

The alerts occur when Windows Defender scans certain installers for the free and 14-day trial versions of CCleaner. The same installers can bundle third-party software from CCleaner’s parent company, antivirus provider Avast.  

But according to Microsoft, these other applications are not required to install and run the junk file removing program. “While the bundled applications themselves are legitimate, bundling of software, especially products from other providers, can result in unexpected software activity that can negatively impact user experiences,” Redmond says in its notice. 

The other applications CCleaner can try to install include Avast Free Antivirus, AVG AntiVirus—which Avast also owns—along with Google Chrome and Google Toolbar. (Avast gets a fee from Google for distributing the Chrome browser.)

“While the CCleaner installers do provide an option to opt out, some users can easily inadvertently install these bundled applications,” Microsoft claims in the notice. 

The company also provided screenshots that illustrate how the alleged bundling works. On install, CCleaner uses a plugin to contact Avast.com and download an additional file called Microstub.exe. “When it is launched, it provides a preselected option to install Avast Free Antivirus,” Microsoft claims.

“If users choose to continue, the bundled antivirus product installs in the background. Existing antivirus software, including Microsoft Defender Antivirus, might be turned off or uninstalled during this process,” the company added. 

However, Avast says Microsoft's description of the bundling is inaccurate. Users are simply asked whether to accept or decline installing the antivirus products, not automatically opted in.

"Aside from Chrome, these are handled via an accept decline opt-in," Avast said, noting the screenshots supplied by Microsoft are outdated. We've reached out to Microsoft for comment on the discrepancies.

Being classified as a Potentially Unwanted Application (PUA) doesn’t mean CCleaner is malware. However, the Windows Defender flag is still not a great look for the cleaning utility app, and may prompt some users to remove the product from their computers. 

In response, a CCleaner spokesperson said: “Our products and our partners’ products are genuine and so we are in discussions with Microsoft to resolve this issue as soon as we can.”

We downloaded CCleaner today, and noticed the installer was not bundling other applications during the setup process.

Editor's Note: This story has been updated with comment from Avast.

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