Microsoft has long pooh-poohed Linux as a desktop competitor. But a recent filing with the Security and Exchange Commission has the company admitting for the first time that Linux represents a significant threat to Windows.

TechFlash reports that in Microsoft's recent annual filing with the SEC, it listed for the first time Ubuntu's maker Canonical, and Linux distributor Red Hat as competitors to its Client division, which makes Windows. Previously, TechFlash says, Red Hat was listed but only as a competitor to its Business and Server & Tools divisions.