One of Windows’ most important security features is BitLocker support, which has provided full-disk encryption since Windows Vista first rolled out. Coupled with a compatible Trusted Platform Module, which is now required for new Windows 10 machines, BitLocker theoretically provides solid protection for a Windows machine that’s lost or stolen.
However, any security feature is only as good as the entire system that surrounds it, and any weak link can present a vulnerability that renders it less than secure. For Windows 10, the weak link involves the fact that the operating system turns off Bitlocker during Feature Updates, aka upgrades, creating a potential exploit, as the official Win-Fu blog reports.
According to Windows trainer and MVP Sami Laiho, the vulnerability occurs due to the ability to hit SHIFT+F10 during the reimaging process performed during a Feature Update and access the command prompt. This result in access by the non-admin account that’s in use during the update to the root SYSTEM folder and to all of the contents of the non-BitLocker-protected hard drive.
The following video provides an overview of the process:
According to The Register, security experts further maintain that anyone with physical access to the machine could exploit the bug to access the BitLocker encryption keys. Fortunately, Microsoft is working on fixing the bug, which affects all relevant versions of Windows 10 including the production versions 1511 (November Update) and 1607 (Anniversary Update), as well as newer Windows Insider builds.
The bug does require physical access to the Windows 10 machine, but once that’s accomplished, for example via theft or by an internal employee, then the bug allows admin access to the system once an upgrade is kicked off. Until Microsoft issues a fix, Laiho recommends disallowing unattended upgrades and using the Long-term Servicing Branch version of Windows 10. That’s not much help to nonenterprise Windows 10 users, however, and so maintaining physical control over a Windows 10 machine becomes that much more important.