Windows: CmKeyBodyRemapToVirtualForEnum Arbitrary Key Enumeration EoP
Platform: Windows 10 1809 (not tested earlier)
Class: Elevation of Privilege
Security Boundary (per Windows Security Service Criteria): User boundary

Summary: 

The kernel’s Registry Virtualization doesn’t safely open the real key for a virtualization location leading to enumerating arbitrary keys resulting in EoP.

Description:

When the virtualization flag is set on the primary token certain parts of the HKLM\Software hive are virtualized to a per-user location under Software\Classes. If the key exists in HKLM (and can be virtualized) then a handle to the HKLM key is opened read-only and the virtualized key is only created if any modification is made to the key, such as writing a value.

However, if a virtualized key already exists then that key is opened and the real key is only opened on demand. One reason to open the backing key is if the virtual key is enumerated, to provide compatibility the kernel will merge the key/value information from the real key into the virtual key. The real key is opened every time a call is made to NtEnumerateKey, NtQueryValue etc.

The open of the real key is performed in CmKeyBodyRemapToVirtualForEnum. It first constructs the real path to the key using CmpReparseToVirtualPath then opens the key object using ObReferenceObjectByName. The problem here is two fold:
1) The access mode passed to ObReferenceObjectByName is KernelMode which means security checking is disabled.
2) The open operation will follow symbolic links in the registry.

When combined together these two issues allow a normal user to redirect a real key to an arbitrary registry location, as security checking is disabled then it will open any key including the SAM or BCD hives. The only requirement is finding a virtualizable key inside HKLM which is writable by the normal user. There’s a number of examples of this, but the easiest and ironic one to exploit is the HKLM\SOFTWARE\Microsoft\DRM key. In order to get the virtualization to work you do need to create a new subkey, without any virtualization flags (the DRM key can be virtualized anyway) with a security descriptor which limits the user to read-only but grants the administrator group full access. This will meet the virtualization criteria, and as the key is in HKLM which is a trusted hive then any symbolic link can reparse to any other hive. This can be exploited as follows:

1) Create a new subkey of DRM which can only be written to by an administrator (just pass an appropriate security descriptor). This should be done with virtualization disabled.
2) Open the new subkey requesting read and write access with virtualization enabled. Write a value to the key to cause it to be virtualized then close it.
3) Reopen the subkey requesting read and write access with virtualization enabled.
4) Replace the new subkey in DRM with a symlink to \Registry\Machine\SAM\SAM. 
5) Enumerate keys or values of the virtual key, it should result in the SAM hive being opened and enumerated. Repeat the process to dump all data from the hive as needed.

Fixing wise, I’m not really sure why the real key is opened without any access checking as the code should have already checked that the user could open the real key for read-only in order to create the virtual key and if the call fails it doesn’t seem to impact the enumeration process, just it doesn’t return the data. You might try and block symbolic link reparsing, but passing OBJ_OPEN_LINK isn’t sufficient as you could replace a key higher up the key path which is the actual symbolic link.

These operations can’t be done from any sandbox that I know of so it’s only a user to system privilege escalation. 

Proof of Concept:

I’ve provided a PoC as a C# project. It will use the vulnerability to enumerate the top level of the SAM hive.

1) Compile the C# project. It’ll need to pull NtApiDotNet from NuGet to build.
2) As a normal user run the PoC. 
3) The PoC should print the subkeys of the SAM hive.

Expected Result:
The query operation should fail.

Observed Result:
The SAM hive key is opened and enumerated.

Some additional notes.

I said this wasn’t exploitable from a sandbox but that turns out to be incorrect. It’s possible to mark a registry key as being a virtual store key using NtSetInformationKey with the KeySetVirtualizationInformation and passing a value of 1. When you do this the kernel always considers it to be a virtualized key for the purposes of enumeration, as long as the virtualization enabled flag is set when calling NtEnumerateKey it’ll call CmKeyBodyRemapToVirtualForEnum. 

The path to the real registry key is generated by CmVirtualKCBToRealPath (not CmpReparseToVirtualPath as I said in the original report as that's the other direction) which just removes the first 4 path elements from the virtual key path and prepends \Registry. For example if you open the key \Registry\User\S-1-1-1\SOFTWARE\MACHINE\XYZ it’ll get mapped to \Registry\MACHINE\XYZ. 

You can exploit this in an AC by creating a new application hive through RegLoadAppKey which will be mapped to \Registry\A\XYZ then creating a directory structure underneath that. For example if you load the app key, then create the subkeys ABC\MACHINE\SAM\SAM and mark the last one as a virtualized key then when opened with virtualization enabled you can now enumerate the SAM hive. I expect this can even be done from an Microsoft Edge Content Process as loading an application hive isn’t restricted, in fact it’s important for AC functionality.

There’s a few places that call CmVirtualKCBToRealPath so I’d probably check their usage is correct as this behavior is odd. Of course I’d argue that CmVirtualKCBToRealPath should be more rigorous and also at a minimum you probably shouldn’t be able to set virtualization flags on application hives in general.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/46912.zip