Vulnerability when using shared Windows base user profiles

In many Terminal Services/Remote Desktop and Citrix XenApp envrionments, a common practice is to use a single shared base profile for the users, typically a mandatory profile. Since there is effectively a single copy of this profile, the registry hive contained therein, either in an or ntuser.dat file, must have registry permissions such that the user whose profile gets built on top of this base has full access to it. However, since it is shared across many different users, it has to have a fairly lax set of permissions – typically allowing a group such as “Everyone” to have Full Control which is then propagated to each user’s individual profile as they logon.

What does this actually mean then when two or more users are logged on to the same server at the same time? Well one user can access, and change, another user’s HKEY_CURRENT_USER (HKCU) key via HKEY_USERS\<sid>. As a demonstration, see below where user gl-test02 has accessed the registry for user gl-test01 without even having to know any SIDs:

cross user profile

You might counter that registry editing tools are disabled through group policy but unless you are running something like AppSense Application Manager to give an extra layer of security via it’s Trusted Ownership feature, then a “policy proof” version is easy to produce with a binary editor and a little know how (a useful tool for troubleshooting in “locked down” environments). Even with an extra security layer in place, it is still relatively easy to access the registry via macros in any Microsoft Office product.

Being the responsible chap that I am, I wouldn’t write about a vulnerability without also offering a method of fixing it. All we need to do is to change the registry permissions at logon such that the group ACE (Access Control Entry) with Full Control is removed and replaced with one for the logging on user. I would normally use the Microsoft subinacl.exe utility but I couldn’t seem to get it to work on HKCU so I switched to setacl.exe from Helge Klein available here. I find the syntax a little perverse but it does the job so one shouldn’t grumble in what is a very flexible and granular free tool. Add the following line into a logon script, AppSense Environment Manager logon action, or similar, assuming you have the Everyone Full Control ACE in the base profile and your HKCU is secured:


setacl -on hkcu -ot reg -actn trustee 
-trst "n1:everyone;n2:%username%;ta:repltrst;w:d"

This only changes the top level key but as long as inheritance for subkeys is on then this should be enough and is very quick. This then means we get this after logon if we try the same method as above to access gl-test01’s registry hive from gl-test02’s session:

secured hkcu

Note that when running setacl you will probably see the following warnings, since it is hopefully being run as a non-administrative user who doesn’t have these privileges but also doesn’t need them for this particular use of setacl.

Privilege 'Restore files and directories' could not be enabled. 
SetACL's powers are restricted. Better run SetACL with admin rights.
Privilege 'Take ownership of files or other objects' could not be enabled. 
SetACL's powers are restricted. Better run SetACL with admin rights.

The mitigating circumstances here are that the information contained within the registry is probably not that sensitive, although there may be third party products that store passwords which could be used nefariously or even just lists of document names worked on which might interest someone.


Author: guyrleech

I wrote my first (Basic) program in 1980, was a Unix developer after graduation from Manchester University and then became a consultant, initially with Citrix WinFrame, in 1995 and later into Terminal Server/Services and more recently virtualisation, being awarded the VMware vExpert status in 2009 and 2010. I have also had various stints in Technical Pre-Sales, Support and R&D. I work as a Senior Technical Consultant for HCL, live in West Yorkshire, England; have a wife, three children and three dogs and am a keen competitive runner when not injured.

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