Microsoft Windows Containers Privilege Escalation Vulnerability

Windows Containers: ContainerUser has Elevated Privileges

Windows Containers: ContainerUser has Elevated Privileges
Platform: Windows 10 20H2 (not tested other versions)
Class: Elevation of Privilege
Security Boundary: User

Summary:
 The standard user ContainerUser in a Windows Container has elevated privileges and High integrity level which results in making it administrator equivalent even though it should be a restricted user.

Description:
Windows Containers setup two default user accounts, which are created by the CExecSvc service. The first user, ContainerAdministrator is a full administrator. The second user, ContainerUser you would assume is not supposed to be an administrator. However, both users are created as virtual service accounts, this results in ContainerUser being granted SeImpersonatePrivilege, SeCreateGlobalPrivilege and runs with a High integrity level. 

At a minimum having SeImpersonatePrivilege is considered administrator equivalent. You can use public exploits such as RogueWinRM (https://github.com/antonioCoco/RogueWinRM) to get SYSTEM from ContainerUser, but I'm sure there are other techniques to do the same thing.

For example the following is the output from a ServerCore 20H2 image running in Hyper-V (but the container isolation doesn't seem to matter).

C:\\> ver
Microsoft Windows [Version 10.0.19042.685]

C:\\> whoami /groups /priv
GROUP INFORMATION
-----------------

Group Name                           Type             SID          Attributes
==================================== ================ ============ ==================================================
Mandatory Label\\High Mandatory Level Label            S-1-16-12288
...

PRIVILEGES INFORMATION
----------------------
Privilege Name                Description                               State
============================= ========================================= ========
SeChangeNotifyPrivilege       Bypass traverse checking                  Enabled
SeImpersonatePrivilege        Impersonate a client after authentication Enabled
SeCreateGlobalPrivilege       Create global objects                     Enabled
SeIncreaseWorkingSetPrivilege Increase a process working set            Disabled

The issue is as far as I understand it ContainerUser should not be administrator equivalent otherwise there seems little point to have two separate users. I'd recommend ensuring SeCreateGlobalPrivilege is also removed, while it's not as easy to exploit it could still provide a means of compromising the container. Also the CExecSvc looks to be code written and signed by Microsoft so I assume it's in scope as a Microsoft product.

Proof of Concept:

I just installed Docker Desktop on Windows 10 20H2. Then ran the command:
C:\\> docker run --user=ContainerUser -it mcr.microsoft.com/windows/servercore:20H2

Then in the container I ran the following command to display the user's details.
C:\\> whoami /groups /priv

Expected Result:
The ContainerUser does not have elevated privileges.

Observed Result:
The ContainerUser has SeImpersonatePrivilege.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse,
the bug report will become visible to the public. The scheduled disclosure
date is 2021-03-14. Disclosure at an earlier date is also possible if
agreed upon by all parties.


Related CVE Numbers: CVE-2021-26891.