In this post I will show you how I discovered a severe vulnerability in the so-called “Group Policy Caching” which was fixed (among other GP vulnerabilities) in CVE-2020-1317
A standard domain user can perform, via the “gpsvc” service, arbitrary file overwrite with SYSTEM privileges by altering behavior of “Group Policy Caching”.
Cool, isn’t it?
The “Group Policy caching” feature is configurable since Windows 2012/8, here you can learn more about it: https://specopssoft.com/blog/things-work-group-policy-caching/
For our goal, all you need is just a domain user and at least the “Default Domain” policy with no special configuration and no special settings in “Group Policy Caching” are needed. Only default configurations 😉
The “Group Policy caching files” are stored under subfolders:
Each time a standard domain user performs ” a gpupdate /force”, the directory is created (if it doesn’t exist) and then populated depending on the group policies which are applied.
The domain user has full control over the “Datastore” directory but only read access under the “0” subfolder.
This can be easilly overriden, if we just rename the “Datastore” folder and create a new “Datastore” folder with the “0” subfolder.
Now let’s create a file in the “0” folder:
and then run a “gpupdate /force” watching what happens in procmon:
As we can see, the file is opened without impersonation (SYSTEM) and 2 SetSecurity calls are made. The resulting perms on the test file are:
Very strange, we lost the full control over the file, probably the second SetSecurity call, but the first one?
Let’s move a step forward, now we create a junction under the “0” folder pointing to a protected directory, for example: “c:\users\administrator\documents”
Note: We obviously need to rename the actual “Datastore” folder and create a new one with the “0” subfolder given that now we have again only read access
Again, in procmon let’s see what happens with a new “gpupdate /force”
A first SetSecurity call is done on “desktop.ini” and then the processing stops due to an access denied on “MyMusic”.
What will be the resulting perms of “desktop.ini”.. ? Guess what, full control for our user!
Now we have a clear vision of the whole picture:
When Group Policy Caching is processed, the “gpsvc” service, running as SYSTEM, lists all the files and folders starting from the local “DataStore” root directory and performs several “SetSecurity” operations on subfolders and files. The first “SetSecurity” will always grant “Full control” to the current user, the second one only read access.
Being able to obtain full control over a SYSTEM protected file (during the first SetSecurity call) EoP should be super-easy, don’t you agree?
For example, we can alter the contents of “printconfig.dll”, start an XPS Print job and gain a SYSTEM shell, as described in my post: https://decoder.cloud/2019/11/13/from-arbitrary-file-overwrite-to-system/
To achieve my goal, I created a very simple POC, here the relevant parts:
When the targetfile is accessed, an “opportunistic lock” is set and after the new junction is moved to “\RPC Control” which will in fact stop the processing and the second SetSecurity call.
Our domain user now has full control over the file!
Microsoft fixed this security “bug” by simply moving the “Group Policy” folders under c:\programdata\microsoft\grouppolicy\users\<sid> which is readonly for standard users….
That’s all 🙂