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WIN-FU OFFICIAL BLOG

WEDNESDAY, FEBRUARY 1, 2017

THE TRUE STORY OF WINDOWS 10 AND THE DMA-PROTECTION

This blog post will tell you if / how Windows 10 protects against DMA (Direct
Memory Access) bases attacks used against BitLocker and other encryption
mechanisms by stealing the encryption key from the memory of a running computer.
The story might be long(ish) but rest assured you want to read it through.

It all actually started when I was delivering a session on Windows 8.1 in
TechEd. I believed what the documentation says and told people that in Windows
8.1 never before seen DMA-enabled devices would not be usable on the logon
screen. So if your computer had no one logged on or the computer was locked we
would not need to worry about DMA-attacks anymore. As I soon learned this did
not actually work in Windows 8.1 and Microsoft told me that it had "skipped"
from the RTM build without them (that I was interacting with) knowing about it.
I felt horrible as I had given misinformation but more that I had "skipped" the
vital "Always test - Don't just trust" policy of mine.

Now the story continued when things like this showed up:

Quote from:
https://technet.microsoft.com/en-us/itpro/windows/whats-new/whats-new-windows-10-version-1507-and-1511

NEW BITLOCKER FEATURES IN WINDOWS 10, VERSION 1507

* DMA port protection. You can use the DataProtection/AllowDirectMemoryAccess
MDM policy to block DMA ports when the device is starting up. Also, when a
device is locked, all unused DMA ports are turned off, but any devices that
are already plugged into a DMA port will continue to work. When the device is
unlocked, all DMA ports are turned back on.

So I decided that I would this time show how it finally worked at Microsoft
Ignite. Nowadays the need for this is much bigger as before we could just block
FireWire and ThunderBolt as no one used them - but now most of my customers have
ThunderBolt 3 docking stations so we can't just disable the bus anymore. I
started experimenting with this and soon found out something that I showed on my
Ignite session for 3000 people (https://myignite.microsoft.com/videos/15848). It
still didn't work! At least By default.

So the problem with Windows 10 was that Microsoft gave misinformation to my
customers and on their websites that Windows 10 would now protect them from the
DMA-attacks as wasn't the case by default. Now the bigger problem with this is
that MS only supports settings this ON via MDM. Now honestly how many of my
customers have MDM? Almost none :( There is no support to set it via SCCM (as it
doesn't support custom URIs), Provisioning package or most of all Group
Policy...

I got a friend of mine (thanks to Petri Paavola @petripaavola) to help me and
build me a PowerShell script so I could experiment without InTune.

I set the setting but DMA still worked. I thought maybe I really need InTune so
I installed InTune and set the setting from there... Still nothing... Now I got
really worried. Was the setting done wrong or was this yet again a "skipped"
feature. Now I needed to get secure@microsoft.com and the product Group on board
with this as this seemed.. well.. fishy...

I would like to thank Microsoft for working with me on this. It took a long time
but now finally we have some results. First of all the DMA-protection is not
FULL. Quote from MS:

”This mitigation only protects PCI-based buses, for example, ExpressCard,
Thunderbolt, & some docking stations (PCIe based). Older, non-PCI busses such
as 1394 and CardBus are still vulnerable.”

That is why I got it working all the time as I was using FireWire to steal the
memory.

So the story continues By Microsoft providing me instructions to deal with this:

1. Set the DMA protection on
https://msdn.microsoft.com/en-us/library/dn904962(v=vs.85).aspx#DataProtection_AllowDirectMemoryAccess
2. Use Group Policy to block Firewire like we have done for years: Blocking the
SBP-2 driver …

So I started to experiment again. Sadly this information is not complete either
and I know most of my customers have them deployed incomplete as well and have
had for many years :( When I used the instructions as such the TB3-devices
didn't work (as I expected). When I used the recommended GP-settings to block
just FireWire my TB3-devices and Dock now worked but so did PassWare Memory
Imager... This is in turn because the instructions don't include all 1394
devices that you can find from here:
https://msdn.microsoft.com/en-us/library/windows/hardware/ff553426(v=vs.85).aspx

I have reported this to Microsoft as well and I hope the instructions are fixed
soon.

Now to give you what you are probably here for :) First how to set the
DMA-protection on without InTune:

* In a few days/weeks you will get an Insider Build that has a Group Policy
settings to set this! Thanks to a lot of feedback from MVPs and customers.
* Until then the registry key you can set with any method you want is this:
* HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci
* DisableExternalDMAUnderLock (DWORD) = 1

Second, here is recommendation from now on to my customers to block DMA but
allow the use of ThunderBolt 3 devices:

a. Have UEFI+SecureBoot+TPM+NoAdminRights
b. Block DMA for ThunderBolt by using the registry key until we get the GPO

i. Or MDM of
course if you have one…

c. Use Group Policy to disable FireWire

i. See the
old article:
https://support.microsoft.com/en-us/help/2516445/blocking-the-sbp-2-driver-and-thunderbolt-controllers-to-reduce-1394-dma-and-thunderbolt-dma-threats-to-bitlocker

ii. But block
these ClassIDs:

* {7ebefbc0-3200-11d2-b4c2-00a0C9697d07}

* {c06ff265-ae09-48f0-812c-16753d7cba83}

* {d48179be-ec20-11d1-b6b8-00c04fa372a7}

* {6bdd1fc1-810f-11d0-bec7-08002be2092f}

For some cases if the customer really requires it: add a PIN code protector and
disable standby.

Hope this clears things out and sorry it took a while but there is coordinated
disclosure procedure I want to respect. If you found this helpful please enrol
to my newsletter at: http://eepurl.com/F-GOj

And remember my training videos on PluralSight and my Dojo at
https://win-fu.com/dojo/

Sami

Posted by Sami Laiho at 11:01 AM
Email ThisBlogThis!Share to TwitterShare to FacebookShare to Pinterest

21 COMMENTS:

1. SCCM FTWFebruary 18, 2017 at 12:14 AM

Ok, so someone had to be first to ask so no throwing, but for a., how
critical is the NoAdminRights part. If you do absolutely everything and the
user still has admin rights, is it all for nothing?

ReplyDelete
Replies
Sami LaihoFebruary 18, 2017 at 2:31 PM

Well in Security I'd say it's never for nothing. It's always better to
increase Security than not. But the #1 rule since 1993 has been that there
can't be good Security in Windows unless you don't run admin rights.

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2. UnknownMarch 13, 2017 at 6:02 PM

Sami.
Are you able to clarify how the Windows Hardware Compatibility Requirement
System.Fundamentals.Firmware.NoExternalDMAOnBoot protects(or not) systems?
Just for completeness sake.
Many thanks

ReplyDelete
Replies
Sami LaihoMarch 13, 2017 at 8:42 PM

This one is something that Windows has always really adheared to as Windows
hasn't been able to use DMA-enabled devices for booting anyway. Mac OSX has
supproted booting from FireWire or ThunderBolt but not Windows. The real
trouble really starts when the OS is alive but this requirement is important
to have. We need to protect more against DMA when the Computer is a the
logon state rather than physically off. There is more of this in the
documentation of MSDN:
https://msdn.microsoft.com/en-us/ie/dn932805(v=vs.94)#system_fundamentals_firmware_noexternaldmaonboot

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UnknownMarch 14, 2017 at 3:32 PM

Thanks Sami

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3. StephanMarch 21, 2017 at 10:04 AM

There is a lot more wrong with Bitlocker:

2008: Vijay Bharadwaj and Neils Ferguson of Microsoft Corporation Comment on
NIST standardization of XTS
> In our opinion, one serious shortcoming of the proposal is that it does
not contain a clear statement of what application-level security goals XTS
aims to achieve.
> The proposal appears to miss the effect of temporal effects on the
security of XTS. It is possible for an attacker to observe a disk for a
period of time and thereby gain a significant advantage in cryptanalysis.
> An attack on large data units [...] This shows that large data units
significantly weaken the system.
> AES in XTS mode works with 16-byte blocks, and this allows for very
fine-grained ciphertext manipulation attacks. We believe this is a
significant problem in practice.
> In a code modification attack the attacker randomizes a block of code and
tries to corrupt the code in such a way as to introduce a security hole in
the system whilst keeping the system functional.
> The small block size of XTS-AES makes this attack rather easy. A larger
block size makes it significantly harder.
> A modern operating system has thousands of settings that are important for
the security of the system.
> Again, a larger block size significantly increases the security in two
ways.
Complete comment here:
http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/comments/XTS/collected_XTS_comments.pdf

2016: XTS is default for Bitlocker in Windows 10
> no argument given why it would be "more secure"
> it saves you two percent of time when converting 10 GB of data
https://blogs.technet.microsoft.com/dubaisec/2016/03/04/bitlocker-aes-xts-new-encryption-type/

ReplyDelete
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Sami LaihoMarch 21, 2017 at 10:35 AM

Don't know who you are but I have two comments:
1. I'll pay you 100€ to show me how you would hack BitLocker.
2. What's your replacement for BitLocker? What do you use?

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StephanMarch 21, 2017 at 10:56 AM

Sorry, I had not set up my Blogger account yet, that's why my name was
displayed as "Unknown".

Second question first: The problem is not Bitlocker itself, but its default
settings. I'm not using Windows, but if I did, I would switch it back to CBC
mode and be aware that data is encrypted but not authenticated.

Attack would take a lot of preparation, but it could look something like
this:

0. Perform some Windows installations in order to determine which disk
blocks are probably used for interesting files (system executables, DLLs
etc.).
1. When I have access to the victim's laptop, I copy the encrypted disk. At
this point I cannot decrypt anything.
2. I wait for some time until some cricial security issues have been
discovered and patched.
3. When I have access to the victim's laptop again, I surgically implant
disk blocks from my image file in oder to reset particular binaries to the
vulnerable versions. This is particularly easy with XTS because of the small
size of unchained blocks.
4. When the victim is using his laptop again, I exploit the known
vulnerability to take over.

PS: You don't have to pay me, because I am not willing to invest the time
for a proof of concept.

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Sami LaihoMarch 21, 2017 at 11:00 AM

1. Yes, agreed.
2. Luckily you are like the others I've asked

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StephanMarch 21, 2017 at 11:28 AM

It is not an easy attack. You have to prepare, you only have one try and
have to be careful to not write rubbish to the disk.

I totally agree that the DMA issue that you are highlighting is much more
relevant for most users, because it can be done with a stolen laptop without
too much preparations.

The original point was a different one: Microsoft experts were very critical
of XTS cipher mode, yet they have chosen it as their default.

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4. UnknownSeptember 25, 2017 at 8:16 AM

"Great blog created by you. I read your blog, its best and useful
information. You have done a great work. Super blogging and keep it up.php
jobs in hyderabad.
"

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5. UnknownOctober 10, 2017 at 12:14 AM

You have shared the legal things about the windows 10 and its dma
protection, It is good for our knowledge. If you are looking for home and
office based MS Office Project 2016 Online, visit on softwareempire.

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6. Bernd SchwanenmeisterOctober 25, 2017 at 8:14 PM

Sami, this will be of interest: since Win10 v1703, there's the "Disable new
DMA devices when this computer is locked"-GPO as you know. This GPO however
produces weird results on win10 v1709 (RTM and 16299.19)! For me and several
others that I read about, you need to disable the policy in order for other
devices to work - for me it was the sound card, for other it included wifi
("code 10 - device cannot start")! This is reproducible and did not happen
with v1703 on the same hardware. Please try it for yourself.

->This needs be be reported to Microsoft - you have connections, will you
tell the developers? If you need hardware to reproduce: for me it was an
Asrock Motherboard H97 Pro4 with onboard soundcard by realtek.

ReplyDelete
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Sami LaihoOctober 26, 2017 at 12:41 AM

Thanks for the notice. I've forwarded this to Microsoft.

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Sami LaihoOctober 26, 2017 at 1:40 PM

Microsoft asked for you to report this through the Feedback Hub as well, so
please do so.

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Nathan O'MearaDecember 18, 2017 at 9:42 PM

Microsoft's official response to that:
https://support.microsoft.com/en-gb/help/4057300/devices-not-working-before-log-on-a-computer-running-windows-10-1709

This is a big problem, as this setting is definitely an important one, but
many companies will also need to upgrade to 1709, and don't want to wait on
firmware updates that may never arrive.

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UnknownMarch 21, 2018 at 6:22 PM

Based on the wording of this response, is it safe to assume that while the
GPo results in the devices breaking (machine bricking in my case), the
registry key listed in this article does not?

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Sami LaihoMarch 23, 2018 at 7:22 PM

The GPO sets the same registry setting, and nothing else, so it should be
the same result. Mine broke down when ever I set it via MDM, GPO or
manually.

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7. Bernd SchwanenmeisterOctober 30, 2017 at 4:30 PM

Thanks, just did.

ReplyDelete
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8. peyscheNovember 15, 2017 at 1:13 PM

Hi Sami,

I think MS messed up something in 1709 with this setting, please have a look
at this Thread and my response..
https://social.technet.microsoft.com/Forums/en-US/8b03a659-93b3-4d72-b7fd-beca9b136474/win-10-enterprise-x64-fall-creators-update-1629915-no-lan-no-wireless-no-audio-on-lenovo?forum=win10itprohardware&prof=required

THX
Paul

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9. UnknownApril 18, 2018 at 11:02 AM

Looks like in RS4 everything is fixed now. Another question I have is how to
turn on "Kernel DMA Protection", when you open MSINFO32 on Windows 10, for
us it is turned off, allthough Device Guard is ON.

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