A Kerberos realm is a logical group of networked computers that share a common authentication database. The authentication database is used to store the Kerberos tickets that are issued to users and services when they authenticate to the network.
In a Kerberos environment, each networked computer is a member of a realm. The realm is responsible for authenticating users and services and for issuing and managing Kerberos tickets.
A Kerberos realm can be implemented on any type of network, including networks that are not running Windows. In fact, the Kerberos protocol was developed as an open standard and is used by many different types of operating systems and networks.
On a Windows network, a Kerberos realm is typically equivalent to a domain. Each domain in a Windows network is a member of a realm, and the domain controller is responsible for authenticating users and services and for issuing and managing Kerberos tickets.
However, on a non-Windows network, a Kerberos realm can be implemented independently of any domain structure. In this case, the Kerberos server is responsible for authenticating users and services and for issuing and managing Kerberos tickets.
The Hacker Recipes mainly covers Kerberos attacks in a Windows context, but some attacks can also be conducted in non-Windows networks.
Kerberos is an authentication protocol based on tickets. It basically works like this (simplified process):
- 1.Client asks the KDC (Key Distribution Center, usually is a domain controller) for a TGT (Ticket Granting Ticket). One of the requesting user's keys is used for pre-authentication. The TGT is provided by the Authentication Service (AS). The client request is called
AS-REQ, the answer is called
- 2.Client uses the TGT to ask the KDC for a ST (Service Ticket). That ticket is provided by the Ticket Granting Service (TGS). The client request is called
TGS-REQ, the answer is called
- 3.Client uses the ST (Service Ticket) to access a service. The client request to the service is called
AP-REQ, the service answer is called
- 4.Both tickets (TGT and ST) usually contain an encrypted PAC (Privilege Authentication Certificate), a set of information that the target service will read to decide if the authentication user can access the service or not (user ID, group memberships and so on).
A Service Ticket (ST) allows access to a specific service.
Again, Microsoft has poorly implemented the zero-knowledge proof concept in Kerberos. An attacker knowing a user's NT hash could use it to ask the KDC for a TGT (if RC4 key is accepted). This is called Overpass-the-hash.
Users are not the only ones whose NT hashes can be used to abuse Kerberos.
- A TGT is encrypted with the
krbtgt's NT hash. An attacker knowing the
krbtgt's NT hash can forge TGTs impersonating a domain admin. He can then request STs as a domain admin for any service. The attacker would have access to everything. This forged TGT is called a Golden ticket.
- A ST is encrypted with the service account's NT hash. An attacker knowing a service account's NT hash can use it to forge a Service ticket and obtain access to that service. This forged Service ticket is called a Silver ticket.
If Kerberos preauthentication is disabled for a user, it is possible to request a TGT for that specific user without knowing any credentials. When the TGT is requested, the KDC sends it along with a session key in the
KRB_AS_REPmessage to the requesting client. The session key being encrypted with the requested user's NT hash, it is possible to crack that session key offline in a an attempt to find the user's password. This is called ASREProasting.
If an attacker finds himself in a man-in-the-middle position, effectively capturing Kerberos messages, he could capture
KRB_AS_REQmessages and operate a similar cracking attempt.
When attackers have a foothold in the domain (i.e. valid domain credentials), they have the (intended) ability to request a service ticket (ST) for any valid SPN (ServicePrincipalName), or SAN (samAccountName). The ST being encrypted with the service account's NT hash, when that service account's password is weak, it is then possible to crack the ST offline in an attempt to find the password. This is called Kerberoasting. On a side note, obtaining a service ticket for a service specified by its SAN in an attempt to Kerberoast the account will only work if the service has at least one SPN.
As it turns out, AS-REQ messages can not only be used to request TGTs but can be invoked to ask for Service Tickets as well. One of the consequences of this is that Kerberoast can be conducted without prior foothold to the domain if the attacker knows the service to target (its SPN or name) as well as an ASREProastable username: Kerberoastwithout-pre-authentication.
Kerberos delegations allow services to access other services on behalf of domain users. For instance, this allows services to require access to other services' data on the authenticated user's behalf in order to pull data that only the said user is supposed to have access to.
In some situations, Kerberos delegations can be abused by attackers to operate lateral movement or privilege escalation.
- Service for User to Self (S4U2self): allows a service to obtain a Service Ticket, on behalf of another user (called "principal"), to itself.
The resulting Service Ticket is
forwardable(i.e. can be used with S4U2Proxy to access another service) if and only if:
- the service is configured for constrained delegation (KCD) with protocol transition
- the principal is not "sensitive for delegation"
- the principal is not a member of the Protected Users group
- Service for User to Proxy (S4U2proxy): allows a service to obtain a Service Ticket, on behalf of a user to a different service.
For this extension to work properly, the service needs to supply a Service Ticket as "additional-ticket" (i.e. used as an evidence that the service using S4U2proxy has the authority to do it on behalf of a user).
For an S4U2proxy request to work and have the KDC issue a Service Ticket:
- the ST used as "additional-ticket" must have the forwardable flag set.
- alternatively, in the
TGS-REQ, in the pre-authentication data, the
PA-PAC-OPTIONSstructure must contain a padata value with the resource-based constrained delegation bit set (doc). nota bene 1: this only applies if the resource-based constrained delegation (RBCD) is actually possible and authorized in the proper AD objects attributes. nota bene 2: Rubeus and Impacket's getST always set that bit when doing S4U2proxy.
On a side note, S4U2Proxy always results in a forwardable ST, even when the ticket used as evidence wasn't forwardable.
S4U2self and S4U2proxy are variations of Service Ticket requests (
KRB_TGS_REQ). Below is what differentiates a S4U2self from a S4U2proxy from a standard
cname(user name authenticating) and the
sname(service name being requested) are the same. In order to succeed and not have the KDC throw an
snameshould refer to an account that has at least one SPN (
Service Principal Name) set.
- Request contains an
additional-ticketsfield containing a service ticket. In order to succeed and not have the KDC throw an
KDC_ERR_BADOPTION, the ticket should have the
forwardableflag set. In a standard constrained delegation or rbcd scenario, the ticket added in the
additional-ticketsfield is the one obtained through S4U2self.
U2U has nothing to do with S4U mechanisms and plays no part in delegation internals.
[U2U] allows users to host secure application services on their desktop machines. [...] In the user-to-user protocol, one user acts as a server, and the other user acts as a client. (Frequently Asked Questions about Kerberos).
A U2U request is a variation of a common Service Ticket request (
KRB_TGS_REQ). Below is what differentiates a U2U from a standard
KRB_TGS_REQ. It allows a user to request a service ticket to another user.
- Request contains an
additional-ticketsfield containing the target user TGT.
snamerefers to a UPN (
User Principal Name) of an account that doesn't necessarily have to have an SPN set.
A ticket produced with u2u is encrypted with the session key of the TGT included as
additional-ticketinstead of the target service long-term key (i.e. DES, RC4 or AES128/256 key).
In some specific scenarios, S4U2self and U2U can be combined, in which case the flags and structures bot mechanisms include in their requests are combined.
This allows to