In a surprising twist to a possible forthcoming antitrust trial, Microsoft Corp. has admitted that it does disable some third-party antivirus software, but only because of compatibility issues.

The admission comes following a complaint Russian antivirus software maker Kaspersky Lab filed with the European Commission and the German federal cartel office. It claims that Microsoft prevents makers of security software from competing on equal footing and is abusing its dominant position to promote its own Windows Defender security software.

The complaint, and possible antitrust trial, is highly reminiscent of similar claims made against Microsoft in the 1990s over its installation and promotion of its Internet Explorer web browser. In that case, the software giant ending up paying hundreds of millions in fines in both the United States and Europe.

Without referencing the case directly, Rob Lefferts, director of security in the Windows and Devices group, wrote in a post on the Microsoft Security blog that the company disables third-party antivirus software installations, but only to make sure customers are safe.

“Microsoft’s application compatibility teams found that roughly 95 percent of Windows 10 PCs had an antivirus application installed that was already compatible with Windows 10 Creators Update,” Lefferts wrote. “For the small number of applications that still needed updating, we built a feature just for AV apps that would prompt the customer to install a new version of their AV app right after the update completed. To do this, we first temporarily disabled some parts of the AV software when the update began.”

Lefferts also argued that Microsoft gives companies ample opportunities to make sure that their software is compatible with newer releases of Windows through the Windows Insider program. That point has been disputed by Kaspersky, which claims that it’s often given as little as six days in advance before an update to make sure that their software works. It claims the move is intentionally anti-competitive given that Microsoft itself has an unfair advantage because it has months in advance to tweak Windows Defender to work with new Windows releases or updates.

The admission from Microsoft that it does indeed disable third-party software, on any grounds, puts it in an interesting position going forward as it is essentially a partial admission of guilt. Microsoft used the consumer safety and performance argument in the Internet Explorer case and that didn’t turn out so well.

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