An antivirus service used by tens of thousands of businesses and millions of home users shut down an untold number of computers around the world Monday after it mistakenly identified core parts of Microsoft Windows as threats, the company confirmed.

Webroot Inc. of Broomfield, Colorado, didn't immediately respond to a request for comment. But it confirmed on its support forum for customers that it issued an updated detection rule that "identified false positives" for critical Windows operating files Monday afternoon, resulting in those files' being "quarantined" and inaccessible to Windows.

@SwiftOnSecurity, an anonymous but well-respected tech security Twitter account, reported that it appeared that the rule somehow allowed genuine "signed Microsoft files to be removed."