Kerberos delegation abuse

When it applies

An AD engagement where enumeration (BloodHound, nxc, LDAP) shows a delegation primitive: a host trusted for unconstrained delegation, an account with constrained delegation (msDS-AllowedToDelegateTo), or — most commonly — write access over a computer object so you can set RBCD. Coercion + relay turns "can authenticate a DC" into "control a DC". This is the modern lateral-movement/DA path.

Why it works

Delegation lets a service act on behalf of a user. Kerberos implements it with S4U2Self (get a ticket to yourself as any user) and S4U2Proxy (forward it to a target service). If you control an account permitted to delegate — or can write msDS-AllowedToActOnBehalfOfOtherIdentity on a target (RBCD) — you can mint a service ticket as any user, including Domain Admin, to that target.

Method

1. Enumerate the primitive. BloodHound edges (AllowedToDelegate, AllowedToAct, GenericWrite/GenericAll on a computer), or nxc ldap ... --trusted-for-delegation / LDAP queries for the delegation attributes.
2. Unconstrained — on a host trusted for unconstrained delegation, capture TGTs of any user who authenticates to it (Rubeus monitor), and coerce a DC to authenticate (below) to grab the DC's TGT → DCSync.
3. Constrained (S4U) — with an account that has msDS-AllowedToDelegateTo <spn>: getST.py -spn <spn> -impersonate Administrator '<domain>/<svc>:<pass>' (or Rubeus s4u). altservice/SPN-substitution lets one allowed SPN pivot to others on the same host.
4. RBCD (the write-driven path) — when you control a computer account and can write the target's msDS-AllowedToActOnBehalfOfOtherIdentity:
   • Set it: rbcd.py -delegate-to <TARGET$> -delegate-from <YOURMACHINE$> -action write ...
   • Impersonate: getST.py -spn cifs/<target> -impersonate Administrator '<domain>/<YOURMACHINE$>:<pass>'
   • Use the ticket (KRB5CCNAME=...) with psexec.py -k -no-pass, secretsdump.py -k, etc.
5. Coercion → relay → RBCD (no creds needed to start) — coerce a target/DC to authenticate to you (PetitPotam.py, coercer), relay that NTLM to LDAP(S) and grant yourself RBCD: ntlmrelayx.py -t ldaps://<dc> --delegate-access, then S4U as above. (See network-ntlm-relay.)

Gotchas

• altservice only works within the same host's SPNs — you can swap cifs→host→http on the target, not jump machines.
• ProtectedUsers / "Account is sensitive and cannot be delegated" blocks impersonating that principal — pick another admin, or a computer account.
• Clock skew breaks every Kerberos step — sync to the DC (ntpdate/faketime) first.
• RC4 disabled / AES-only domains need AES keys in your tooling flags; RC4 tickets get rejected.
• RBCD needs write on the target computer, and creating a fake computer needs ms-DS-MachineAccountQuota > 0 (or use a machine account you already own).

Verify success

A service ticket as a privileged user to the target service, used to run a command / dump secrets (secretsdump.py -k) you could not before — e.g. local admin on the target or DCSync from a coerced DC.

References

Elad Shamir "Wagging the Dog" (RBCD); Impacket getST.py/rbcd.py/ntlmrelayx.py; Rubeus S4U docs; MITRE ATT&CK T1558.003. Coercion+relay mechanics: network-ntlm-relay.