Two security researchers name MY123 and Slipstream both discovered a way to bypass Secure Boot after Microsoft accidentally leak its Secret Keys that allows attackers to Install Bootkit/Rootkit to the target devices.
Secure Boot was developed to protects Computers from Malwares like Bootkits which can hijack the booting process of the computer.
Microsoft added a new Secure Policy Called Supplemental Policies which was added on Microsoft Windows 10 v1607 Redstone and was discovered by MY123 and Slipstream, Both of these Security Researchers claimed that one of the Supplemental Policies can be used to disabled the Secure Boot features that checks if files executed during booting are signed by Microsoft.
So it means some one having a physical access to the device can install malwares to the device through unsigned binaries or allows him to install other Operating System apart from Windows like Linux, Android etc on the device.
Although Microsoft has released August Patch Yesterday, which includes the designing flaw in Secure Boot for which unlikely not going to fix everything for now.