The researchers' writeup, in a very fun form, can be found at https://rol.im/securegoldenkeyboot/

With text as follows for those whom the joviality of the original presentation is undesirable:

irc.rol.im #rtchurch :: https://rol.im/chat/rtchurch

Specific Secure Boot policies, when provisioned, allow for testsigning to be
enabled, on any BCD object, including {bootmgr}. This also removes the NT loader
options blacklist (AFAIK). (MS16-094 / CVE-2016-3287, and MS16-100 / CVE-2016-3320)

Found by my123 (@never_released) and slipstream (@TheWack0lian)
Writeup by slipstream (@TheWack0lian)

First up, "Secure Boot policies". What are they exactly?

As you know, secureboot is a part of the uefi firmware, when enabled, it only
lets stuff run that's signed by a cert in db, and whose hash is not in dbx
(revoked).

As you probably also know, there are devices where secure boot can NOT be
disabled by the user (Windows RT, HoloLens, Windows Phone, maybe Surface Hub,
and maybe some IoTCore devices if such things actually exist -- not talking
about the boards themselves which are not locked down at all by default, but end
devices sold that may have secureboot locked on).

But in some cases, the "shape" of secure boot needs to change a bit. For example
in development, engineering, refurbishment, running flightsigned stuff (as of
win10) etc. How to do that, with devices where secure boot is locked on?

Enter the Secure Boot policy.

It's a file in a binary format that's embedded within an ASN.1 blob, that is
signed. It's loaded by bootmgr REALLY early into the windows boot process. It
must be signed by a certificate in db. It gets loaded from a UEFI variable in
the secureboot namespace (therefore, it can only be touched by boot services).
There's a couple .efis signed by MS that can provision such a policy, that is,
set the UEFI variable with its contents being the policy.

What can policies do, you ask?

They have two different types of rules. BCD rules, which override settings
in the on-disk BCD, and registry rules, which contain configuration for the
policy itself, plus configuration for other parts of boot services, etc. For
example, one registry element was introduced in Windows 10 version 1607
'Redstone' which disables certificate expiry checking inside mobilestartup's
.ffu flashing (ie, the "lightning bolt" windows phone flasher); and another one
enables mobilestartup's USB mass storage mode. Other interesting registry
rules change the shape of Code Integrity, ie, for a certain type of binary,
it changes the certificates considered valid for that specific binary.

(Alex Ionescu wrote a blog post that touches on Secure Boot policies. He teased a
followup post that would be all about them, but that never came.)

But, they must be signed by a cert in db. That is to say, Microsoft.

Also, there is such a thing called DeviceID. It's the first 64 bits of a salted
SHA-256 hash, of some UEFI PRNG output. It's used when applying policies on
Windows Phone, and on Windows RT (mobilestartup sets it on Phone, and
SecureBootDebug.efi when that's launched for the first time on RT). On Phone,
the policy must be located in a specific place on EFIESP partition with the
filename including the hex-form of the DeviceID. (With Redstone, this got
changed to UnlockID, which is set by bootmgr, and is just the raw UEFI PRNG
output.)

Basically, bootmgr checks the policy when it loads, if it includes a DeviceID,
which doesn't match the DeviceID of the device that bootmgr is running on, the
policy will fail to load.

Any policy that allows for enabling testsigning (MS calls these Retail Device
Unlock / RDU policies, and to install them is unlocking a device), is supposed
to be locked to a DeviceID (UnlockID on Redstone and above). Indeed, I have
several policies (signed by the Windows Phone production certificate) like
this, where the only differences are the included DeviceID, and the signature.

If there is no valid policy installed, bootmgr falls back to using a default
policy located in its resources. This policy is the one which blocks enabling
testsigning, etc, using BCD rules.

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.

Another thing: I saw some additional code in the load-legacy-policy function in
redstone 14381.rs1_release. Code that wasn't there in 14361. Code that
specifically checked the policy being loaded for an element that meant this was
a supplemental policy, and erroring out if so. So, if a system is running
Windows 10 version 1607 or above, an attacker MUST replace bootmgr with
an earlier one.

On August 9th, 2016, another patch came about, this one was given the designation
MS16-100 and CVE-2016-3320. This one updates dbx. The advisory says it revokes
bootmgrs. The dbx update seems to add these SHA256 hashes (unless I screwed up
my parsing):
<snip>

I checked the hash in the signature of several bootmgrs of several
architectures against this list, and found no matches. So either this
revokes many "obscure" bootmgrs and bootmgfws, or I'm checking the wrong hash.

Either way, it'd be impossible in practise for MS to revoke every bootmgr
earlier than a certain point, as they'd break install media, recovery partitions,
backups, etc.

- RoL

disclosure timeline:
~march-april 2016 - found initial policy, contacted MSRC
~april 2016 - MSRC reply: wontfix, started analysis and reversing, working on
almost-silent (3 reboots needed) PoC for possible emfcamp demonstration
~june-july 2016 - MSRC reply again, finally realising: bug bounty awarded
july 2016 - initial fix - fix analysed, deemed inadequate. reversed later rs1
bootmgr, noticed additional inadequate mitigation
august 2016 - mini-talk about the issue at emfcamp, second fix, full writeup
release

credits:
my123 (@never_released) -- found initial policy set, tested on surface rt
slipstream (@TheWack0lian) -- analysis of policies, reversing bootmgr/
mobilestartup/etc, found even more policies, this writeup.

tiny-tro credits:
code and design: slipstream/RoL
awesome chiptune: bzl/cRO <3