Back to the Basics; Ownership Matters

TL;DR It is a bit of a truism, but the owner of an object in Windows always has WriteDACL, regardless of the object’s ACL. This shows why that matters in terms of system administration and security.

Welcome to Part II of our ‘Back to the Basics’ series!

Part I: NTDS.dit vs SAM

Part II: Ownership Matters

Part III: Recovering from a Crash

Part IV: Setting up a Simple Honeypot Account

Part V: Automating DC Deployment

Part VI: Sometimes it’s the dumbest thing

Background

This was one of those ideas that was banging around in my head. We touched on it in a prior article where we kinda spitballed an idea about how an attacker could maintain persistence via ownership of HVT objects in AD. I remembered something that had happened ‘Back in the Day’ that was related and figured WTH, let’s turn this idea into Part II of Back to the Basics.

We will examine three scenarios regarding ownership of objects:

  • Scenario I; an odd mashup of AD privileges
  • Scenario II; a junior sysadmin screws up an OU
  • Scenario III; a section FUBARs their own data’s ACL

Scenario I; the odd mashup of AD privileges

First we will consider the following example:

  • A user has been placed in 2 groups; GroupX and GroupY
  • Group X has been denied ExtendedRight with ObjectType all 0s & WriteDACL on the domain root
  • Group Y has been allowed FullControl on the domain root

Please note that this was simply an exercise proposed by a vendor. This is not something that one is likely to see in the wild, at least I hope not.

If one wants to follow along with this scenario simply copy/paste the below:

We can then verify by running the below query:

Next login to a domain workstation as Malicious Insider and play along. A clever attacker who knows how to Google will enumerate their privileges and then simply seize ownership of the domain root and make whatever changes they want.

Wait, how was Insider able to modify the ACL on the domain root even though they are a member of a group which is specifically denied WriteDACL? They didn’t even remove the ACE that denies them WriteDACL, they just removed the ACE that denies GroupX ExtendedRight with ObjectType all 0s.

This is because owners of objects have implicit WriteDACL and all read rights, regardless of what is or isn’t in said object’s ACL. This seems to be by design, as we shall see in our next 2 scenarios.

On a sidenote, SDProp doesn’t seem to change ownership of objects back to the default, in this case the builtin Administrators group. This lab/range’s domain root was still owned by the fake admins group we used in a prior howto, although SDProp had removed that lab project’s modifications to the ACL itself. This is interesting since that project was on testing out a theory for sneaky persistence via changing ownership of objects without changing their ACLs.

The security aspect of this scenario should be obvious. Our Malicious Insider can now run secretsdump, grab everyone’s NTLM, start forging tickets, and can also feed everyone’s NTLM into crackstation.net, hashcat, john, etc and see if they get lucky. They will probably find at least a few passwords and given human nature these passwords may get them into third party systems like HR portals, the org’s bank account, etc.

For example crackstation.net find johnsmith’s password in seconds: Example10

JamesGoodman’s password is HelloThere!

Scenario II; a junior sysadmin screws up an OU

In this scenario we have an OU containing some VIP accounts. This scenario was inspired by a question on Microsoft Technet. They had a former sysadmin who had denied Everyone read rights to some domain workstation accounts in AD before leaving their organization.

If one wants to follow along just copy/paste the below:

This assumes you have an account named “mishka”. If not change the SamAccountName accordingly.

Next login to a domain workstation as mishka and go ahead and screw up the OU by disabling inheritance and removing all ACEs from the OU’s ACL.

Please note some screenshots are redacted to omit Internet accessible FQDN’s and kindergartner’s real names.

Hey we all make mistakes sometimes and she just turned 6. It happens.

Let’s suppose that the junior sysadmin was let go following this snafu and our other sysadmin didn’t follow best practice, i.e. they deleted mishka’s account rather than disabling it. She was the owner of the OU, so she was the only one who had WriteDACL on it. We can create another account with the same UPN, but the SID won’t be the same. This is why best practice dictates to disable accounts until you are 100% sure the person is gone for good and you do not need access to anything they might have controlled.

(Obviously we didn’t actually let her go. She’s the only user and the only administrator in test.local, besides the ‘Break Glass’ admin account.)

So is this OU hosed? That’s a negative as we can seize ownership of it using a Domain Admin account and then fix the ACL.

Google was mixed on whether seizing ownership requires any member of Domain Admins or if it necessitates the builtin SID 500 Administrator account. I am happy to say that using any Domain Admin account worked fine in test.local, which is great because we limited SID 500 to local login only. This is because the SID 500 account does not lockout no matter what Group Policy is set to.

The OU appears empty as no one except mishka can even view the contents. If there are no Allow statements in an object’s ACL then no one is allowed to do anything. The exception to this rule is the owner. The owner has implicit WriteDACL.

Ignore this information box and hit the Advanced button.

Hit ‘Change’, put ‘Domain Admins’, and hit Ok. We have now seized ownership of the OU.

Notice that Windows helpfully informs us that as owner we can WriteDACL.

Next simply copy/paste the ACL of a known good OU:

We can now access the OU and the user accounts contained therein, the ACL is back, and Inheritance is once again enabled. While I would like to think we’d get a thank you for this, we’d probably just get a “hey hero, why’d you give a junior sysadmin the ability to screw up our account!?”

In hindsight this was probably a funny story for the guy or gal on Technet. They were able to fix their issue by cloning a known good ACL. However the security implications of this should be obvious. Availability is part of the CIA triad and if a disgruntled insider can FUBAR AD then you will very likely have a serious issue with availability for any service that authenticates via AD [Exchange, Sharepoint, probably ESXi in many orgs, Cisco SWs & RTRs in many orgs, AAD if you’re using pass-through authentication or Federation, etc etc].

Scenario III; a section FUBAR’s their own data’s ACL

I have said it numerous times before, but it bears repeating. I am not all that original or creative. Much [most?] of what we do here was inspired by something we saw before. I will spare everyone the “so back in the day” story and just say that a certain section FUBARed their own folder on a share drive downrange once. We will now replicate this in the lab and show how to fix it by, you guessed it, seizing ownership of the data.

Test.local has a share drive auto mapped to all systems via DFS and Group Policy, however I really don’t feel like messing with that for a lab project like this. We’ll just create a quick and dirty share on a member server and abuse that:

‘Set-SmbPathAcl’ is a very handy command which translates the share privileges to NTFS and applies them to the folder’s NTFS ACL. ‘Get-SmbShareAccess’ is used simply to confirm.

Next just login to a domain workstation as any member of Minions and screw up the privileges on the data in the Share. After all, we’re super secret right? We should definitely deny everyone else any rights on our data right?

Go ahead and blow past that warning. We know what we’re doing. Disable inheritance too, after all we don’t want anyone else from outside to see what’s in our folder.

Once this is done they suddenly realize that although they can see still see the files in their folder, they cannot read the contents of them.

So one would think that super special users who were able to figure out how to FUBAR their own data that they were self managing should be able to figure out how to fix it right? Well in the case of this ‘Back in the Day’ story one would be dead wrong.

Luckily Domain Admins are also local admins by default on domain systems. Therefore we can fix this snafu by seizing ownership of the folder.

We can then re-enable inheritance and remove the Deny Everyone everything ACE.

Summary

The moral of this story is that the owner always has implicit WriteDACL. By default Domain Admins can seize ownership of AD objects. They can also seize ownership of NTFS folders and files since by default they are local admins on domain systems.

Be aware though that ACLs can be modified by delegating privileges, and if a prior sysadmin was a bit careless then they could have given out GenericAll, WriteDACL, or WriteOwner to those who probably shouldn’t have such rights. This in turn allows them to seize ownership of the object and then make whatever changes they want.

References:

Ownership of objects explained: https://www.itprotoday.com/strategy/take-control-windows-object-ownership-and-inheritance

SDProp: https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/security-best-practices/appendix-c--protected-accounts-and-groups-in-active-directory

admin screws up AD ACLs: https://social.technet.microsoft.com/Forums/en-US/4ed78f40-fdde-4490-8bc2-fbe885b2d9dc/take-ownership-of-an-ad-object?forum=winserverDS

Well known SIDs: https://docs.microsoft.com/en-us/windows/security/identity-protection/access-control/security-identifiers

Creating folders in PS: https://rdr-it.com/en/howto/create-a-folder-in-powershell/#:~:text=To%20create%20a%20folder%20in%20PowerShell%2C%20use%20the%20New%2DItem,want%20to%20create%20a%20folder.

Setting NTFS ACLs in PS: https://petri.com/how-to-use-powershell-to-manage-folder-permissions/

SMB & PS: https://docs.microsoft.com/en-us/powershell/module/smbshare/new-smbshare?view=windowsserver2022-ps

Changing Share rights in PS: https://docs.microsoft.com/en-us/powershell/module/smbshare/grant-smbshareaccess?view=windowsserver2022-ps

Setting NTFS ACLs in PS: https://petri.com/how-to-use-powershell-to-manage-folder-permissions/

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Rich

I work various IT jobs & like Windows domain security as a hobby. Most of what’s here is my notes from auditing or the lab.