Kerberos delegation lets a service act on behalf of a user toward another service, so a front-end (a web app, say) can reach a back-end (a database) as the user. The feature necessarily lets one account obtain tickets for another identity, and every delegation type is therefore an impersonation primitive when the delegating account is compromised or its configuration is writable.
Lineage. Unconstrained delegation was the original, coarse mechanism and is still the most dangerous. Constrained delegation and protocol transition (S4U) were added to narrow it, and resource-based constrained delegation, formalised offensively in 2018, moved the control onto the target object, which is what makes a single writable computer object a takeover today.
The three types#
- Unconstrained: the delegating host receives and stores the user's TGT, so it can act as that user anywhere. Compromise such a host (or coerce a DC to authenticate to it) and you capture TGTs, including a DC's.
- Constrained: the account may request tickets for a user only to a listed set of SPNs, using the S4U extensions, often with protocol transition that lets it do so without the user ever authenticating.
- Resource-based constrained (RBCD): the permission lives on the target object (
msDS-AllowedToActOnBehalfOfOtherIdentity), so write access to a computer object lets you nominate an account that may impersonate anyone to it.
The S4U mechanism#
Constrained and resource-based delegation both run on the Service-for-User extensions:
- S4U2self: the service asks the KDC for a ticket to itself as an arbitrary user (no user involvement). This yields an impersonation ticket.
- S4U2proxy: the service uses that ticket to request a ticket to the back-end service as the user.
Whether the resulting ticket is forwardable (and so usable onward) depends on protocol transition and account flags, which is the crux of what each abuse can reach.
Pages#
- Unconstrained delegation: capturing TGTs, including via coercion.
- Constrained delegation: S4U2self/S4U2proxy and protocol transition abuse.
- Resource-based constrained delegation: writing the delegation attribute to impersonate.