Updated, 9/11/2016, 11:20am PT: Microsoft sent us a statement shortly after we published this article. The statement is below, and we've adjusted the article copy to reflect the new information.
“The jailbreak technique described in the researchers’ report on August 10 does not apply to desktop or enterprise PC systems. It requires physical access and administrator rights to ARM and RT devices and does not compromise encryption protections," said a Microsoft spokesperson.
Two security researchers, MY123 and Slipstream, uncovered multiple security vulnerabilities in Microsoft’s Secure Boot policies on Windows 8, Windows 8.1, and Windows 10 devices. The flaws can now allow anyone to unlock devices that were supposed to load only the signed Windows operating system. Because Secure Boot can now be bypassed, it also means the devices are vulnerable to bootkit and rootkit types of malware, much like PCs were in the pre-Secure Boot era.
Windows’ “Golden Key" Backdoor
The researchers described the vulnerabilities as a “golden key” backdoor that Microsoft created, supposedly to allow developers to debug the devices. However, as expected with any type of backdoor, once it’s created, anyone that discovers it gets access whether they're supposed to or not.
This seems to be the point the researchers were also trying to make:
“A backdoor, which MS put in to Secure Boot because they decided to not let the user turn it off in certain devices, allows for Secure Boot to be disabled everywhere! You can see the irony. Also the irony in that MS themselves provided us several nice 'golden keys' (as the FBI would say) for us to use for that purpose,” wrote the security researchers on their website.“About the FBI: are you reading this? If you are, then this is a perfect real world example about why your idea of backdooring cryptosystems with a 'secure golden key' is very bad! Smarter people than me have been telling this to you for so long, it seems you have your fingers in your ears.You seriously don't understand still? Microsoft implemented a 'secure golden key' system. And the golden keys got released from MS['s] own stupidity. Now, what happens if you tell everyone to make a 'secure golden key' system? Hopefully you can add 2+2....,” they added.
A Backdoor That Can’t Be Closed
To make matters worse, according to the researchers, this isn’t even a backdoor that can be closed. That means Windows devices that have a locked Secure Boot, such as smartphones and tablets, may have become permanently vulnerable to physical unlocking or to bootkits and rootkits.
The security researchers said that it would be "impossible in practise [sp] for MS to revoke every bootmgr earlier than a certain point, as they'd break install media, recovery partitions, backups, etc."
Microsoft Hasn’t Tackled The Issue Head-On
The two researchers discovered the vulnerabilities in March of this year and alerted Microsoft about them soon after. However, according to the researchers, Microsoft didn’t seem too responsive to these bug reports, but it eventually awarded them the bug bounties in June. Microsoft then followed up with a few patches in July and August, in the latest "Patch Tuesday" update.
The security researchers noted that these patches don’t do much at all to resolve the issue. According to them, Microsoft blacklisted most of the Secure Boot policies that made the boot process vulnerable in the first place, but not all of them. Also, before the blacklist file is loaded, the boot process loads up a vulnerable Secure Boot policy, so the fix isn’t much of a fix right now.
On the latest Windows 10 build 1607 (Anniversary Update), an attacker would have to replace the existing boot manager with an older boot manager for the attack to work. That should be relatively easy, though, if the attacker has physical access to the device.