Microsoft (MSFT) Windows 10—which, incidentally, is apparently at least 3.22 times better than Windows 3.1, the OS we knew and loved circa 1992—sooner or later will be upon us. And that could be bad news for open source fans, since the jury is out on whether the new Windows platform’s UEFI Secure Boot feature will play nicely with third-party operating systems, such as Linux, that users want to install on their computers.

Secure Boot became an issue at the time of the Windows 8 release in fall 2012. Back then, Microsoft required hardware vendors that wanted their devices to carry the “Designed for Windows 8” logo to enable Secure Boot, which prevented computers from booting operating systems if they lacked a compatible cryptographic signature. Since independent open source developers did not have a way of assuring that their platforms would sport the appropriate signatures, Secure Boot posed the danger of making it impossible to boot Linux or other open source systems on PCs sold with Windows 8.

Fortunately for the open source folk, Secure Boot turned out to be a non-issue, because Microsoft required that hardware vendors provide a way for end users to disable the feature—and therefore boot any OS they chose—if they wished.

Fast-forward to the present, and the future may not look so bright. At a recent conference in which it outlined the specifications for Secure Boot in Windows 10, Microsoft suggested that it may allow hardware manufacturers to require Secure Boot—and give users no way to disable the feature—if they chose.

Lest the alarm bells start ringing, it’s important to note that Microsoft has not yet confirmed the final design specifications for Secure Boot in Windows 10. They could turn out to be just as flexible for Windows 10 as they were for its predecessor.

And, even if Secure Boot became mandatory, that wouldn’t necessarily make Windows 10 PCs totally incompatible with open source operating systems. Some Linux platforms—especially the big-name ones, such as Red Hat (RHT) and Ubuntu—likely will be able to obtain the signatures they need to boot. And PC vendors may provide tools for creating custom signatures for any alternative OS.

Still, this is a reminder that, even in an age when Canonical and Microsoft have become close partners, and in which the cloud has made cross-platform interoperability much simpler, there is no guarantee the worlds of proprietary and open source software will always get along.