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.
Found by:
