S4U2Pwnage

[Edit 9/29/18] For a better weaponization of constrained delegation abuse, check out the “s4u” section of the From Kekeo to Rubeus post.

Several weeks ago my workmate Lee Christensen (who helped develop this post and material) and I spent some time diving into Active Directory’s S4U2Self and S4U2Proxy protocol extensions. Then, just recently, Benjamin Delpy and Ben Campbell had an interesting public conversation about the same topic on Twitter. This culminated with Benjamin releasing a modification to Kekeo that allows for easy abuse of S4U misconfigurations. As I was writing this, Ben also published an excellent post on this very topic, which everyone should read before continuing. No, seriously, go read Ben’s post first.

Lee and I wanted to write out our understanding of the technology and how you can go about abusing any misconfigurations while on engagements. Some of this will overlap with Ben’s post, but we have incorporated a few different aspects that we think add at least a bit of value. Ben also covers the Linux exploitation aspect, which we won’t touch on in this post.

At the heart of this matter is the delegation of privileges – allowing one user to pretend to be another in Active Directory. This delegation (currently) comes in two flavors: unconstrained and constrained delegation. If you don’t care about the technical details, skip to the Abusing S4U section.

Unconstrained Delegation

Say you have a server (or service account) that needs to impersonate another user for some reason. One common scenario is when a user authenticates to a web server, using Kerberos or other protocols, and the server wants to nicely integrate with a SQL backend. Active Directory grants two general ways to go about this: constrained and unconstrained delegation.

Unconstrained delegation used to be the only option available in Windows 2000, and the functionality has been kept (presumably for backwards compatibility reasons). We’ll only briefly cover this delegation type as Sean Metcalf has a great post that covers it in depth. In that article Sean states, “When Kerberos Unconstrained Delegation is enabled on the server hosting the service specified in the Service Principal Name referenced in the TGS-REQ (step 3), the Domain Controller the DC places a copy of the user’s TGT into the service ticket. When the user’s service ticket (TGS) is provided to the server for service access, the server opens the TGS and places the user’s TGT into LSASS for later use. The Application Server can now impersonate that user without limitation!“.

Here’s a graphical overview of the protocol from Microsoft:

Tl;dr: The TGT will be stuffed into memory where an attacker can extract and reuse it if:

1. You are able to compromise a server that has unconstrained delegation set.
2. You are able to trick a domain user that doesn’t have ‘Account is sensitive and cannot be delegated’ enabled (see Protections below) to connect to any service on the machine. This includes clicking on \\SERVER\Share.

This allows an attacker to impersonate that user to any service/machine on the domain! Obviously bad mmmkay. To contrast, if unconstrained delegation isn’t enabled, just a normal service ticket without a TGT stuffed inside it would be submitted, so the attacker would get no additional lateral spread advantage.

How can you tell which machines have unconstrained delegation set? This is actually pretty easy: search for any machine that has a userAccountControl attribute containing ADS_UF_TRUSTED_FOR_DELEGATION. You can do this with an LDAP filter of ‘(userAccountControl:1.2.840.113556.1.4.803:=524288)’, which is what PowerView’s Get-DomainComputer function does when passed the -Unconstrained flag:

Constrained Delegation

Obviously unconstrained delegation can be quite dangerous in the hands of a careless admin. Microsoft realized this early on and released ‘constrained’ delegation with Windows 2003. This included a set of Kerberos protocol extensions called S4U2Self and S4U2Proxy. These extensions also enable something called protocol transition, which we’ll go over in a bit.

In essence, constrained delegation is a way to limit exactly what services a particular machine/account can access while impersonating other users. Here’s how a service account configured with constrained delegation looks in the Active Directory GUI:

The ‘service’ specified is a service principal name that the account is allowed to access while impersonating other users. This is HOST/PRIMARY.testlab.local in our above example. Before we get into the specifics of how this works, here’s how that target object looks in PowerView:

The field of interest is msds-allowedtodelegateto, but there’s also a modification to the account’s userAccountControl property. Essentially, if a computer/user object has a userAccountControl value containing TRUSTED_TO_AUTH_FOR_DELEGATION then anyone who compromises that account can impersonate any user to the SPNs set in msds-allowedtodelegateto. Ben mentions SeEnableDelegationPrivilege being required to actually modify these parameters, which I’ll go over in more depth next week.

But first, a bit more on how Active Directory implements this whole process. Feel free to skip ahead to the Abusing S4U section if you’re not interested.

S4U2Self, S4U2Proxy, and Protocol Transition

So you have a web service account that needs to impersonate users to only a specific backend service, but you don’t want to allow unconstrained delegation to run wild. Microsoft’s solution to how to architect this is through the service-for-user (S4U) set of Kerberos extensions. There’s extensive documentation on this topic; Lee and I were partial to the Microsoft’s “Kerberos Protocol Extensions: Service for User and Constrained Delegation Protocol” ([MS-SFU]). What follows is our current understanding. If we’ve messed something up, please let us know!

The first extension that implements constrained delegation is the S4U2self extension, which allows a service to request a special forwardable service ticket to itself on behalf of a particular user. This is meant for use in cases where a user authenticates to a service in a way not using Kerberos, i.e. in our web service case. During the first KRB_TGS_REQ to the KDC, the forwardable flag it set, which requests that the TGS returned be marked as forwardable and thus able to be used with the S4U2proxy extension. In unconstrained delegation, a TGT is used to identify the user, but in this case the S4U extension uses the PA-FOR-USER structure as a new type in the “padata”/pre-authentication data field.

Note that the S4U2self process can be executed for any user, and that target user’s password is not required. Also, the S4U2self process is only allowed if the requesting user has the TRUSTED_TO_AUTH_FOR_DELEGATION field set in their userAccountControl.

Now, Lee and I first thought that this may be a way to Kerberoast any user we wanted, but unfortunately for us attackers this isn’t the case. The PAC is signed for the source (not the target) user, in this case the requesting service account, so universal Kerberoasting is out of the picture. But we now have a special service ticket that’s forwardable to the target service configured for constrained delegation in this case.

The second extension is S4U2proxy, which allows the caller, the service account in our case, to use this forwardable ticket to request a service ticket to any SPN specified in msds-allowedtodelegateto, impersonating the user specified in the S4U2self step. The KDC checks if the requested service is listed in the msds-allowedtodelegateto field of the requesting user, and issues the ticket if this check passes. In this way the delegation is ‘constrained’ to specific target services.

Here’s Microsoft’s diagram of S4U2self and S4U2proxy:

This set of extensions allows for what Microsoft calls protocol transition, which starts with the first Kerberos exchange during the S4u2Self component. This means that a service can authenticate a user over a non-Kerberos protocol and ‘transition’ the authentication to Kerberos, allowing for easy interoperability with existing environments.

Abusing S4U

If you’re asking yourself “so what” or skipped ahead to this section, we can think of a few ways that the S4U extensions can come into play on a pentest.

The first is to enumerate all computers and users with a non-null msds-allowedtodelegateto field set. This can be done easily with PowerView’s -TrustedToAuth flag for Get-DomainUser/Get-DomainComputer:

Now, remember that a machine or user account with a SPN set under msds-allowedtodelegateto can pretend to be any user they want to the target service SPN. So if you’re able to compromise one of these accounts, you can spoof elevated access to the target SPN. For the HOST SPN this allows complete remote takeover. For a MSSQLSvc SPN this would allow DBA rights. A CIFS SPN would allow complete remote file access. A HTTP SPN it would likely allow for the takeover of the remote webservice, and LDAP allows for DCSync ; ) HTTP/SQL service accounts, even if they aren’t elevated admin on the target, can also possibly be abused with Rotten Potato to elevate rights to SYSTEM (though I haven’t tested this personally).

Luckily for us, Benjamin recently released a modification to Kekeo to help facilitate these types of lateral spread attacks if we know the plaintext password of the specific accounts. Lee and I envision four different specific scenarios involving S4U that you may want to abuse. We have tested two of the scenarios in a lab reliably, but haven’t been able to get the other two working (notes below). [edit]: @gentilkiwi reached out and let Lee and I know that asktgt.exe accepts a /key:NTLM argument as well as a password. This allows us to execute scenarios 3 and 4 below using account hashes instead of plaintexts!

Scenario 1 : User Account Configured For Constrained Delegation + A Known Plaintext

This is the scenario that Benjamin showed in his tweet. If you are able to compromise the plaintext password for a user account that has constrained delegation enabled, you can use Kekeo to request a TGT, execute the S4U TGS request, and then ultimately access the target service.

Again, if you would like to execute this attack from a Linux system, read Ben’s post.

Scenario 2 : Agent on a Computer Configured For Constrained Delegation

If you are able to compromise a computer account that is configured for constrained delegation (instead of a user account) the attack approach is a bit different. As any process running as SYSTEM takes on the privileges of the local machine account, we can skip the Kekeo asktgt.exe step. You can also use an alternative method to execute the S4U2Proxy process, helpfully provided by Microsoft. Lee and I translated the process from C# into PowerShell as follows:

# translated from the C# example at https://msdn.microsoft.com/en-us/library/ff649317.aspx

# load the necessary assembly
$Null = [Reflection.Assembly]::LoadWithPartialName('System.IdentityModel')

# execute S4U2Self w/ WindowsIdentity to request a forwardable TGS for the specified user
$Ident = New-Object System.Security.Principal.WindowsIdentity @('Administrator@TESTLAB.LOCAL')

# actually impersonate the next context
$Context = $Ident.Impersonate()

# implicitly invoke S4U2Proxy with the specified action
ls \\PRIMARY.TESTLAB.LOCAL\C$

# undo the impersonation context
$Context.Undo()

As detailed by Microsoft, when using WindowsIdentity, an “identify-level” token is returned by default for most situations. This allows you to see what groups are associated with the user token, but doesn’t allow you to reuse the access. In order to use the impersonation context to access additional network resources, an impersonation-level token is needed, which is only returned when the requesting account has the “Act as part of the operating system” user right (SeTcbPrivilege). This right is only granted to SYSTEM by default, but since we need to be SYSTEM already to use the privileges of the machine account on the network, we don’t need to worry.

Also, due to some of the powershell.exe peculiarities I mentioned a bit ago, if you are using PowerShell Version 2, you need to launch powershell.exe in single-thread apartment mode (with the “-sta” flag) in order for the token impersonation to work properly:

Scenario 3 : User Account Configured For Constrained Delegation + A Known NTLM Hash

Our next goal was to execute this transition attack from a Window system only given the the target user’s NTLM hash, which we were unfortunately not able to get working properly with the same method as scenario 2. Our gut feeling is that we’re missing some silly detail, but we wanted to detail what we tried and what went wrong in case anyone had a tip for getting it working properly. [Edit] Ben’s pointed out that /key:NTLM works for asktgt.exe as well, which is covered below.

We attempted to use Mimikatz’ PTH command to inject the user’s hash into memory (assuming you are a local admin on the pivot system) instead of Kekro’s asktgt.exe. One issue here (as in scenario 2) is SeTcbPrivilege, but despite explicitly granting our principal user that right we still ran into issues. It appears that the the S4U2Self step worked correctly:

Despite the necessary privileges/rights, it appeared that the S4U2Proxy process fell back to NTLM instead of Kerberos with some NULL auths instead of the proper process:

[Edit] You can execute this scenario with asktgt.exe/s4u.exe nearly identically to scenario 1. Simply substitute /key:NTLM instead of /password:PLAINTEXT:

Scenario 4 : Computer Account Configured For Constrained Delegation + A Known NTLM Hash

If you compromise a computer account hash through some means, and want to execute the attack from another domain machine, we imagined that you would execute an attack flow nearly identical to scenario 3. Unfortunately, we ran into the same problems. Again, if anyone can give us a tip on what we’re doing wrong, we would be greatly appreciative :) [Edit] This can be executed with /user:MACHINE$ and /key:NTLM for asktgt.exe, identical to scenario 3:

Protections

Microsoft has a great protection already built into Active Directory that can help mitigate delegation abuse. If an account has “Account is sensitive and cannot be delegated” enabled, then “the security context of the user will not be delegated to a service even if the service account is set as trusted for Kerberos delegation“. You can easily check if an account has this set by again examining the userAccountControl attribute, checking for the NOT_DELEGATED value. PowerView allows you to easily search for accounts with this value set or not set (Get-DomainUser -AllowDelegation/-DisallowDelegation) and you can use the ConvertFrom-UACValue function to examine the values set for a particular account, as shown in previous examples.

Next week I will have a post that overlaps a bit with this topic, and presents additional defensive ideas concerning the rights needed to modify these delegation components for user objects.