Now, for Microsoft's screwups. During the development of Windows 10 v1607 'Redstone', MS added a new type of secure boot policy. Namely, "supplemental" policies that are located in the EFIESP partition (rather than in a UEFI variable), and have their settings merged in, dependant on conditions (namely, that a certain "activation" policy is also in existance, and has been loaded in). Redstone's bootmgr.efi loads "legacy" policies (namely, a policy from UEFI variables) first. At a certain time in redstone dev, it did not do any further checks beyond signature / deviceID checks. (This has now changed, but see how the change is stupid) After loading the "legacy" policy, or a base policy from EFIESP partition, it then loads, checks and merges in the supplemental policies. See the issue here? If not, let me spell it out to you plain and clear. The "supplemental" policy contains new elements, for the merging conditions. These conditions are (well, at one time) unchecked by bootmgr when loading a legacy policy. And bootmgr of win10 v1511 and earlier certainly doesn't know about them. To those bootmgrs, it has just loaded in a perfectly valid, signed policy. The "supplemental" policy does NOT contain a DeviceID. And, because they were meant to be merged into a base policy, they don't contain any BCD rules either, which means that if they are loaded, you can enable testsigning. Not just for windows (to load unsigned driver, ie rootkit), but for the {bootmgr} element as well, which allows bootmgr to run what is effectively an unsigned .efi (ie bootkit)!!! (In practise, the .efi file must be signed, but it can be self-signed) You can see how this is very bad!! 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
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 own stupidity. Now, what happens if you tell everyone to make a "secure golden key" system? Hopefully you can add 2+2... Anyway, enough about that little rant, wanted to add that to a writeup ever since this stuff was found
Anyway, MS's first patch attempt. I say "attempt" because it surely doesn't do anything useful. It blacklists (in boot.stl), most (not all!) of the policies. Now, about boot.stl. It's a file that gets cloned to a UEFI variable only boot services can touch, and only when the boot.stl signing time is later than the time this UEFI variable was set. However, this is done AFTER a secure boot policy gets loaded. Redstone's bootmgr has extra code to use the boot.stl in the UEFI variable to check policy revocation, but the bootmgrs of TH2 and earlier does NOT have such code. So, an attacker can just replace a later bootmgr with an earlier one.
@movdqa in the same boat it seems.
