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(RBCD) Resource-based constrained

Theory

If an account, having the capability to edit the msDS-AllowedToActOnBehalfOfOtherIdentity attribute of another object (e.g. the GenericWrite ACE, see Abusing ACLs), is compromised, an attacker can use it populate that attribute, hence configuring that object for RBCD.

Machine accounts can edit their own msDS-AllowedToActOnBehalfOfOtherIdentity attribute, hence allowing RBCD attacks on relayed machine accounts authentications.

For this attack to work, the attacker needs to populate the target attribute with the SID of an account that Kerberos can consider as a service. A service ticket will be asked for it. In short, the account must be either (see Kerberos tickets for more information about the following):

The common way to conduct these attacks is to create a computer account. This is usually possible thanks to a domain-level attribute called MachineAccountQuota that allows regular users to create up to 10 computer accounts.

In 2022, Jame Forshaw demonstrated that the SPN requirement wasn't completely mandatory and RBCD could be operated without: Exploiting RBCD using a normal user. While this technique is a bit trickier and should absolutely be avoided on regular user accounts (the technique renders them unusable for normal people), it allows to abuse RBCD even if the MachineAccountQuota is set to 0. The technique is demonstrated later on in this page (RBCD on SPN-less user).

Then, in order to abuse this, the attacker has to control the account (A) the target object's (B) attribute has been populated with. Using that account's (A) credentials, the attacker can obtain a ticket through S4U2Self and S4U2Proxy requests, just like constrained delegation with protocol transition.

In the end, an RBCD abuse results in a Service Ticket to authenticate on the target service (B) on behalf of a user. Once the final Service Ticket is obtained, it can be used with Pass-the-Ticket to access the target service (B).

If the "impersonated" account is "is sensitive and cannot be delegated" or a member of the "Protected Users" group, the delegation will (probably) fail.

Nota bene: the native, RID 500, "Administrator" account doesn't benefit from that restriction, even if it's added to the Protected Users group (source: sensepost.com).

There are a few additional details to keep in mind, valid as of the time of writing this note: Jan. 24th 2023.

  • In December 2020, along with KB4598347 patching the bronze-bit attack (CVE-2020-17049), Microsoft issued KB4577252 patching the CVE-2020-16996 vulnerability. While this second CVE has few information and details about it online, some lab testing indicates it may be linked to the verifications made by KDCs when receiving S4U2proxy TGS-REQ requests.

  • Before this patch, some testing indicates that accounts set as "sensitive and cannot be delegated" wouldn't be delegated (intended behavior), but members of the Protected Users group (and without the "sensitive" setting) would be (unintended !).

  • As it turns out, even after the patch, as of Jan. 24th 2023, members of the Protected Users group are now in fact protected against delegation, except for the native administrator account (RID 500), even if it's a member of the group. No idea if this is intended or not but it seems it's not the only security behavior of that group that doesn't apply for this account (e.g. RC4 pre-authentication still works for the RID-500 admin, even if member of the Protected Users group, source: Twitter).

A technique called AnySPN or "service class modification" can be used concurrently with pass-the-ticket to change the service class the Service Ticket was destined to (e.g. for the cifs/target.domain.local SPN, the service class is cifs).

The msDS-AllowedToActOnBehalfOfOtherIdentity was introduced with Windows Server 2012 implying that RBCD only works when the Domain Controller Functionality Level (DCFL) is Windows Server 2012 or higher.

Practice

1 - Edit the target's "rbcd" attribute (ACE abuse) ✏️ ****

Impacket's rbcd.py script (Python) _c_an be used to read, write or clear the delegation rights, using the credentials of a domain user that has the needed permissions.

# Read the attribute
rbcd.py -delegate-to 'target$' -dc-ip 'DomainController' -action read 'DOMAIN'/'POWERFULUSER':'PASSWORD'

# Append value to the msDS-AllowedToActOnBehalfOfOtherIdentity
rbcd.py -delegate-from 'controlledaccountwithSPN' -delegate-to 'target$' -dc-ip 'DomainController' -action write 'DOMAIN'/'POWERFULUSER':'PASSWORD'

Testers can also use ntlmrelayx to set the delegation rights with the --delegate-access option when conducting this attack from a relayed authentication.

In this example, controlledaccount can be a computer account created for the attack, or any other account -with at least one Service Principal Name set for the usual technique, or without for SPN-less RBCD- which credentials are known to the attacker.

2 - Obtain a ticket (delegation operation) 🎫 ****

Once the attribute has been modified, the Impacket script getST (Python) can then perform all the necessary steps to obtain the final "impersonating" ST (in this case, "Administrator" is impersonated but it can be any user in the environment).

getST.py -spn "cifs/target" -impersonate Administrator -dc-ip $DomainController 'DOMAIN/controlledaccountwithSPN:SomePassword'

In some cases, the delegation will not work. Depending on the context, the bronze bit vulnerability (CVE-2020-17049) can be used with the -force-forwardable option to try to bypass restrictions.

The SPN (Service Principal Name) set can have an impact on what services will be reachable. For instance, cifs/target.domain or host/target.domain will allow most remote dumping operations (more info on adsecurity.org). There however scenarios where the SPN can be changed (AnySPN) to access more service. This technique is automatically tried by Impacket scripts when doing pass-the-ticket.

3 - Pass-the-ticket 🛂 ****

Once the ticket is obtained, it can be used with pass-the-ticket.

Resources

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