Exploit Monday: Code Integrity on Nano Server: Tips/Gotchas

Although it's not explicitly called out as being supported in Microsoft documentation, it turns out that you can deploy a code integrity policy to Nano Server, enabling enforcement of user and kernel-mode code integrity. It is refreshing to know that code integrity is supported across all modern Windows operating systems now (Win 10 Enterprise, Win 10 IoT, and Server 2016 including Nano Server) despite the fact that Microsoft doesn't make that fact well known. Now, while it is possible to enforce code integrity on Nano Server, you should be aware of some of the caveats which I intend to enumerate in this post.

Code Integrity != Device Guard

Do note that until now, there has been no mention of Device Guard. This was intentional. Nano Server does not support Device Guard - only code integrity (CI), a subset of the supported Device Guard features. So what's the difference you ask?

There are no ConfigCI cmdlets. These cmdlets are what allow you to build code integrity policies. I'm not going to try to speculate around the rationale for not including them in Nano Server but I doubt you will ever see them. In order to build a policy, you will need to build it from a system that does have the ConfigCI cmdlets.
Because there are no ConfigCI cmdlets, you cannot use the -Audit parameter of Get-SystemDriver and New-CIPolicy to build a policy based on blocked binaries in the Microsoft-Windows-CodeIntegrity/Operational event log. If you want to do this (an extremely realistic scenario), you have to get comfortable pulling out and parsing blocked binary paths yourself using Get-WinEvent. When calling Get-WinEvent, you'll want to do so from an interactive PSSession rather than calling it from Invoke-Command. By default, event log properties don't get serialized and you need to access the properties to pull out file paths.
In order to scan files and parse Authenticode and catalog signatures, you will need to either copy the target files from a PSSession (i.e. Copy-Item -FromSession) or mount Nano Server partitions as a file share. You will need to do the same thing with the CatRoot directory - C:\Windows\System32\CatRoot. Fortunately, Get-SystemDriver and New-CIPolicy support explicit paths using the -ScanPath and -PathToCatroot parameters. It may not be obvious, but you have to build your rules off the Nano Server catalog signers, not some other system because your other system is unlikely to contain the hashes of binaries present on Nano Server.
There is no Device Guard WMI provider (ROOT\Microsoft\Windows\DeviceGuard). Without this WMI class, it is difficult to audit code integrity enforcement status at scale remotely.
There is no Microsoft-Windows-DeviceGuard/Operational event log so there is no log to indicate when a new CI policy was deployed. This event log is useful for alerting a defender to code integrity policy and virtualization-based security (VBS) configuration tampering.
Since Nano Server does not have Group Policy, there is no way to configure a centralized CI policy path, VBS settings, or Credential Guard settings. I still need to dig in further to see if any of these features are even supported in Nano Server. For example, I would really want Nano Server to support UEFI CI policy protection.
PowerShell is not placed into constrained language mode even with user-mode code integrity (UMCI) enforcement enabled. Despite PowerShell running on .NET Core, you still have a rich reflection API to interface with Win32 - i.e. gain arbitrary unsigned code execution. With PowerShell not in constrained language mode (it's in FullLanguage mode), this means that signature validation won't be enforced on your scripts. I tried turning on constrained language mode by setting the __PSLockdownPolicy system environment variable, but PowerShell Core doesn't seem to acknowledge it. Signature enforcement of scripts/modules in PowerShell is independent of Just Enough Administration (JEA) but you should also definitely consider using JEA in Nano Server to enforce locked down remote session configurations.

Well then what is supported on Nano Server? Not all is lost. You still get the following:

The Microsoft-Windows-CodeIntegrity/Operational event log so you can view which binaries were blocked per code policy.
You still deploy SIPolicy.p7b to C:\Windows\System32\CodeIntegrity. When SIPolicy.p7b is present in that directory, Nano Server will begin enforcing the rules after a reboot.

Configuration/deployment/debugging tips/tricks

I wanted to share with you the way in which I dealt with some of the headaches involved in configuring, deploying, and debugging issues associated with code integrity on Nano Server.

Event log parsing

Since you don't get the -Audit parameter in the Get-SystemDriver and New-CIPolicy cmdlets, if you choose to base your policy off audit logs, you will need to pull out blocked binary paths yourself. When in audit mode, binaries that would have been blocked generate EID 3076 events. The path of the binary is populated via the second event parameter. The paths need to be normalized and converted to a proper file path from the raw device path. Here is some sample code that I used to obtain the paths of blocked binaries from the event log:

$BlockedBinaries = Get-WinEvent -LogName 'Microsoft-Windows-CodeIntegrity/Operational' -FilterXPath '*[System[EventID=3076]]' | ForEach-Object {

$UnnormalizedPath = $_.Properties[1].Value.ToLower()

$NormalizedPath = $UnnormalizedPath

if ($UnnormalizedPath.StartsWith('\device\harddiskvolume3')) {

$NormalizedPath = $UnnormalizedPath.Replace('\device\harddiskvolume3', 'C:')

} elseif ($UnnormalizedPath.StartsWith('system32')) {

$NormalizedPath = $UnnormalizedPath.Replace('system32', 'C:\windows\system32')

}

$NormalizedPath

} | Sort-Object -Unique

Working through boot failures

There were times when the system often wouldn't boot because my kernel-mode rules were too strict when in enforcement mode. For example, when I neglected to add hal.dll to the whitelist, obviously, the OS wouldn't boot. While I worked through these problems, I would boot into the advanced boot options menu (by pressing F8) and disable driver signature enforcement for that session. This was an easy workaround to gain access to the system without having to boot from external WinPE media to redeploy a better, bootable CI policy. Note that the advanced boot menu is only made available to you if the "Enabled:Advanced Boot Options Menu" policy rule option is present in your CI policy. Obviously, disabling driver signature enforcement is a way to completely circumvent kernel-mode code integrity enforcement.

Completed code integrity policy

After going through many of the phases of an initial deny-all approach as described in my previous post on code integrity policy development, this is the relatively locked CI policy that I got to work on my Nano Server bare metal install (Intel NUC):

<?xml version="1.0" encoding="utf-8"?>

<Rules>

<Rule>

<Option>Enabled:Unsigned System Integrity Policy</Option>

</Rule>

<Rule>

<Option>Enabled:Advanced Boot Options Menu</Option>

</Rule>

<Rule>

<Option>Enabled:UMCI</Option>

</Rule>

<Rule>

<Option>Disabled:Flight Signing</Option>

</Rule>

</Rules>

</FileRules>

</Signer>

</Signer>

</Signer>

</Signer>

</Signer>

</Signer>

</Signer>

</Signer>

</Signers>

</AllowedSigners>

</ProductSigners>

</SigningScenario>

</AllowedSigners>

</ProductSigners>

</SigningScenario>

</SigningScenarios>

</CiSigners>

</SiPolicy>

I conducted the following phases to generate this policy:

Generate a default, deny-all policy by calling New-CIPolicy on an empty directory. I also increased the size of the Microsoft-Windows-CodeIntegrity/Operational to 20 MB to account for the large number of 3076 events I would expect while deploying the policy in audit mode. I also just focused on drivers for this phase so I didn't initially include the "Enabled:UMCI" option. My approach moving forward will be to focus on just drivers and then user-mode rules so as to minimize unnecessary cross-pollination between rule sets.
Reboot and start pulling out blocked driver paths from the event log. I wanted to use the WHQLFilePublisher rule for the drivers but apparently, none of them were WHQL signed despite some of them certainly appearing to be WHQL signed. I didn't spend too much time diagnosing this issue since I have never been able to successfully get the WHQLFilePublisher rule to work. Instead, I resorted to the FilePublisher rule.
After I felt confident that I had a good driver whitelist, I placed the policy into enforcement mode and rebooted. What resulted was nonstop boot failures. It turns out that if you're whitelisting individual drivers, critical drivers won't show up in the event log in audit mode like ntoskrnl.exe and hal.dll. So I explicitly added rules for them and Nano Server still wouldn't boot. What made things worse is that even if I placed the policy back into audit mode, there were no new blocked driver entries but the system still refused to boot. I rolled the dice and posited that there might be an issue with certificate chain validation at boot time so I created a PCACertificate rule for ntoskrnl.exe (The "Microsoft Code Signing PCA 2010" rule). This miraculously did the trick at the expense of creating a more permissive policy. In the end, I ended up with roughly the equivalent of a Publisher ruleset on my drivers with the exception of my Intel NIC driver.
I explicitly made a FilePublisher rule for my Intel NIC driver as it was the only 3rd part, non-OEM driver I had to add when creating my Nano Server image. I don't need to allow any other code signed by Intel so I explicitly only allow that one driver.
After I got Nano Server to boot, I started working on user-mode rules. This process was relatively straightforward and I used the Publisher rule for user-mode code.
After using Nano Server under audit mode with my new rule set and not seeing any legitimate binaries that would have been blocked, I felt confident in the policy and placed it into audit mode and I haven't run into any issues and I'm using Nano Server as a Hyper-V server (i.e. with the "Compute" package).

I still need to get around to adding my code-signing certificate as an authorized policy signer, sign the policy, and remove "Enabled:Unsigned System Integrity Policy". Overall though, despite the driver issues, I'm fairly content with how well locked down my policy is. It essentially only allows a subset of critical Microsoft code to execute with the exception of the Intel driver which has a very specific file/signature-based rule.

Conclusion

I'm not sure if we'll see improved code integrity or Device Guard support for Nano Server in the future, but something is at least better than nothing. As it stands though, if you are worried about the execution of untrusted PowerShell code, unfortunately, UMCI does nothing to protect you on Nano Server. Code integrity still does a great job of blocking untrusted compiled binaries though - a hallmark of the vast majority of malware campaigns. Nano Server opens up a whole new world of possibilities from a management and malware perspective. I'm personally very interested to see how attackers will try to evolve and support their operations in a Nano Server environment. Fortunately, the combination of Windows Defender and code integrity support offer a solid security baseline.

Exploit Monday

Monday, November 28, 2016

Code Integrity on Nano Server: Tips/Gotchas

No comments:

Post a Comment