While playing with Kerberos tickets, I discovered an issue that allowed me to authenticate to other domains within an Active Directory (AD) forest across external non-transitive trusts. This means that there is in fact no such thing as a “non-transitive trust.” The term is at best misleading and offers systems administrators a false sense of security. As part of the issue discussed in this post, attackers can authenticate to other domains across a non-transitive trust and potentially elevate privileges within the forest of the trusting domain. This post details the discovered issue.
After reporting this issue to Microsoft, I received the following response:
Figure 1. MSRC response
As Microsoft determined that the issue does not affect security and therefore does not plan to change the behavior, there is no way to avoid the issue —apart from simply not using external trusts.
Trusts and transitivity
Note: This post assumes a basic understanding of trusts and how they work. (For more information on those topics, I suggest reading Will Schroeder’s great post on trusts.) The post does not cover intra-forest trusts (i.e., trusts within a single forest)—other than as they apply to the specific attack path discussed—or selective authentication trusts – which are incredibly rare in real environments.
The remaining trusts are forest trusts and external trusts.
Forest trusts are trusts between two forests. More accurately, they are transitive trusts between the root domains of two forests. Any user from any domain within the trusted forest can authenticate to any domain within the trusting forest.
External trusts are trusts between two domains. These trusts are non-transitive. Only users within the trusted domain can authenticate against only the trusting domain (Figure 2).
Figure 2. External non-transitive trust
Microsoft describes trust transitivity as follows:
Transitivity determines whether a trust can be extended outside of the two domains with which it was formed.
A transitive trust can be used to extend trust relationships with other domains.
A non-transitive trust can be used to deny trust relationships with other domains.
This description is clear: For non-transitive trusts, only the two domains involved in the trust can authenticate to each other and not beyond.
Unfortunately, as I will demonstrate in this post, this is not the case.
To properly demonstrate the non-transitive trust issue, I set up the lab with several multi-domain forests that share external trusts (Figure 3).
Figure 3. Lab setup
This setup involves three (3) forests. Two (2) of the forests contains three (3) domains. The third forest contains two (2) domains. The domains semperis.lab and treetest.lab share a bidirectional external non-transitive trust (Figure 4).
Figure 4. External Trust 1
The domains grandchild1.child1.semperis.lab and semperisaz.lab also share a bidirectional external non-transitive trust (Figure 5).
Figure 5. External Trust 2
This setup makes it possible to demonstrate the implications and limitations of the non-transitive trust issue.
How Kerberos cross-trust authentication works
To authenticate to a service across a trust using Kerberos, you need a referral (also known as a referral ticket granting ticket [TGT]). This is a ticket requested from your local domain controller (DC) for the foreign domain. To demonstrate how ticket requests are performed across trusts, this section focuses on the semperis.lab forest. However, this information is applicable to any “allowed” trust path.
Note: This post assumes a basic understanding of normal Kerberos authentication flow. (For a detailed explanation of that flow, see Sean Metcalf’s Detecting Kerberoasting Activity post.) The remainder of the post uses Rubeus to request tickets manually.
The simplest example of cross-trust authentication is the authentication of a service on a domain that has a direct trust with the local domain. The trust that the domain grandchild1.child1.semperis.lab has with child1.semperis.lab (Figure 5) is an example of this type of trust. In this situation, after obtaining an initial TGT, the first step in obtaining a service ticket for an account in grandchild1.child1.semperis.lab to a service in child1.semperis.lab is to obtain a referral for child1.semperis.lab (Figure 6, Figure 7).
Figure 6. Referral request for child1.semperis.lab
Figure 7. Referral request
The request was made to a DC (SGC1DC1.grandchild1.child1.semperis.lab) within the domain (grandchild1.child1.semperis.lab) local to the authenticating user (lowpriv). The request was made for the service krbtgt/child1.semperis.lab. The fact that the ServiceRealm (srealm) is the local domain (grandchild1.child1.semperis.lab) within the resulting ticket shows that this ticket is a referral.
This referral can now be used to request service tickets (STs) from the foreign DC (SC1DC1.child1.semperis.lab) for the service host/SC1DC1.child1.semperis.lab (Figure 8, Figure 9).
Figure 8. Example use of referral
Figure 9. Requesting ST using referral
To take this one step further, if a service on the forest root domain (semperis.lab) is required, a referral for this domain cannot be directly requested from the local (grandchild1.child1.semperis.lab) domain. A referral for krbtgt/semperis.lab is requested from the local DC sgc1dc1.grandchild1.child1.semperis.lab. However, the DC returns a ticket for the service krbtgt/child1.semperis.lab, indicating that this referral is for the domain child1.semperis.lab, not for semperis.lab (Figure 10, Figure 11).
Figure 10. Referral request for semperis.lab
Figure 11. Requesting referral for semperis.lab
You can see this issue by trying to use this ticket to request an ST for the semperis.lab domain. Doing so results in an AP_ERR_BAD_INTEGRITY error. This is because the referral ticket is encrypted with the trust key for the grandchild1.child1.semperis.lab -> child1.semperis.lab trust. The DCs in the semperis.lab root domain have no knowledge of this key and so are unable to decrypt the ticket (Figure 12, Figure 13).
Figure 12. Root domain ST request
Figure 13. Requesting ST from semperis.lab
To request a service ticket for the root domain semperis.lab, first a referral for semperis.lab must be requested from a DC in the domain child1.semperis.lab, using this referral. Figure 14 and Figure 15 show a request made to the foreign DC (sc1dc1.child1.semperis.lab), using the referral for the domain child1.semperis.lab to request a further referral for the root domain semperis.lab.
Figure 14. Referral request for semperis.lab
Figure 15. Requesting referral for semperis.lab
Note that to request this ticket, the /targetdomain argument is required. By default, Rubeus uses the domain within the ticket passed to it to fill in the domain in the TGS-REQ. In this case, that domain would be grandchild1.child1.semperis.lab and would result in an ERR_WRONG_REALM error because the domain local to the DC is child1.semperis.lab. This information will be important in the next section.
Lastly, this resulting referral for semperis.lab can be used to request STs for the semperis.lab domain. Figure 16 shows the ST request made from the user email@example.com to the DC SDC1.semperis.lab for the SPN host/SDC1.semperis.lab. Because the trust path is allowed, this request was successful even though the two domains involved do not have a direct trust (Figure 17).
Figure 16. ST request for semperis.lab
Figure 17. ST request for host/sdc1.semperis.lab
This method of requesting referrals for trusting domains can be followed to request STs for any service within any domain for which the trust path is allowed.
Making the non-transitive trust transitive
So, how it is possible to traverse external trusts to authenticate to domains that should be prohibited?
The domains semperisaz.lab and grandchild1.child1.semperis.lab have a bidirectional external trust. Therefore, after retrieving a TGT for any account within the domain semperisaz.lab, it is possible to request a referral for grandchild1.child1.semperis.lab (Figure 18, Figure 19).
Figure 18. Referral to granchild1.child1.semperis.lab
Figure 19. Request referral for grandchild1.child1.semperis.lab
This referral can be used to request STs for services on the domain grandchild1.child1.semperis.lab. However, an attempt to obtain a referral to other domains within the same forest (e.g., child1.semperis.lab) returns an ERR_PATH_NOT_ACCEPTED error, as expected (Figure 20).
Figure 20. Path not accepted error
This error occurs because the semperisaz.lab -> granchild1.child1.semperis.lab trust is non-transitive. Therefore, the path from semperisaz.lab to child1.semperis.lab is not allowed (Figure 21).
Figure 21. Requesting referral for child1.semperis.lab
However, you can request a local TGT for grandchild1.child1.semperis.lab. I call this a local TGT because, unlike the referral—which has a ServiceRealm (srealm) of semperisaz.lab—the ServiceRealm of this TGT is grandchild1.child1.semperis.lab (Figure 22, Figure 23).
Figure 22. Requesting local TGT For grandchild1.child1.semperis.lab
Figure 23. Requesting a local TGT
Using this local TGT, a referral for child1.semperis.lab can now be requested (Figure 24, Figure 25).
Figure 24. Referral for child1.semperis.lab
Figure 25. Local TGT to request referral for child1.semperis.lab
You now have a usable referral that can be used to request STs for child1.semperis.lab, using any account from the semperisaz.lab domain and without making any changes to trusts or accounts within AD. Figure 26 and Figure 27 show an ST request from the DC sc1dc1.child1.semperis.lab for the service host/sc1dc1.child1.semperis.lab as the user firstname.lastname@example.org.
Figure 26. Requesting ST for DC In child1.semperis.lab
Figure 27. Requesting ST for host/sc1dc1.child1.semperis.lab
Furthermore, this method can be used to hop around any domain within the same forest in which grandchild1.child1.semperis.lab exists. We can demonstrate this by requesting a referral to the root domain semperis.lab. A referral was requested for semperis.lab from the DC sc1dc1.child1.semperis.lab as the user email@example.com (Figure 28, Figure 29).
Figure 28. Requesting referral for semperis.lab
Figure 29. Requesting referral to semperis.lab
Fortunately, hopping across further trusts outside of the forest (external or forest trusts) is not possible using this method. As Figure 3 shows, the root domain semperis.lab has a bidirectional external trust with treetest.lab. This trust can be used to demonstrate that limitation (Figure 30, Figure 31).
Figure 30. Requesting referral for treetest.lab
Figure 31. Requesting referral for treetest.lab
This at least stops an attacker using this method from trust hopping into another forest. Still, this non-transitive trust issue is clearly useful for attackers trying to elevate privileges within a forest from across a trust. Attackers could query domain information from supposedly disallowed domains, query more sensitive domains or domains with potentially weaker security, or perform Kerberoasting attacks or NTLM authentication coercion on domains that are assumed to be disallowed.
Although attackers cannot hop further using the method described in this post alone, the issue could open new avenues of attack. I previously wrote a post demonstrating the ability to create machine accounts across trusts and the implications of that. By incorporating that method of domain hopping with this new method, it is possible to overcome the limitation described at the end of the previous section.
Using the referral retrieved for semperis.lab (at the end of the previous section), it is possible to request a ticket for the LDAP service on a DC in semperis.lab. Here, a ST for ldap/sdc1.semperis.lab was requested using the referral for semperis.lab as the user firstname.lastname@example.org (Figure 32, Figure 33).
Figure 32. Request ST for ldap
Figure 33. Request ST for ldap/sdc1.semperis.lab
This ST can be injected and used to create a machine account directly in semperis.lab if the configuration allows Authenticated Users, which is the default (Figure 34).
Figure 34. Create machine account in semperis.lab
This action causes the DC sdc1.semperis.lab to create the machine account TestComp within the semperis.lab domain (Figure 35).
Figure 35. Create machine account TestComp
The new TestComp machine account is a local account within the domain semperis.lab. Now, you can retrieve a TGT for that machine account (Figure 36, Figure 37).
Figure 36. Machine account TGT
Figure 37. Requesting TGT for TestComp
This machine account TGT is allowed to request a referral to the trusting domain treetest.lab. The machine account TGT can be used to request a referral for the treetest.lab domain from the DC SDC1.semperis.lab (Figure 38, Figure 39).
Figure 38. Requesting referral for treetest.lab
Figure 39. Request referral for treetest.lab
Lastly, the issue described in this post can be used to gain access to the inaccessible dsptest.lab domain. First, request a local TGT for treetest.lab. The local TGT is requested using the referral for treetest.lab as the account TestCompemail@example.com from the DC TDC1.treetest.lab (Figure 40, Figure 41).
Figure 40. Requesting local TGT for treetest.lab
Figure 41. Requesting local TGT for treetest.lab
Next, use this local TGT to request a referral for dsptest.lab. Figure 42 and Figure 43 shows that a referral was requested for the domain dsptest.lab from the DC TDC1.treetest.lab as the user TestCompfirstname.lastname@example.org.
Figure 42. Requesting referral for dsptest.lab
Figure 43. Requesting referral for dsptest.lab
Clearly, combining these two methods could make it possible to hop very deep into AD enterprise infrastructures, using any low-privileged account on a domain that has an external trust to any domain within a forest.
Detection of trust transitivity vulnerability
Unfortunately, given Microsoft’s response, the only way to avoid the issue is to remove external trusts.
If removing all external trusts is not possible, you’ll need to monitor for Windows event 4769 (A Kerberos service ticket was requested). The first indication to look for is that a local TGT was requested from an account in a different forest (Figure 44).
Figure 44. 4769 for local TGT
Here, the Account Domain field is a domain that belongs to a different forest and the Service Name is krbtgt. This event is followed by another event 4769, requesting a referral (Figure 45).
Figure 45. 4769 for referral
Here, the Account Domain is a domain within a different forest and the Service Name is another domain within the local forest.
Note that these two events can occur on different DCs within the same domain. The events do not need to occur on the same DC. At this point, any requested STs (event 4769) that use this referral will have an Account Domain that contains a domain that should not be allowed to authenticate to the local domain.
Look for monitoring for these events and detection of this type of attack in a future release of Semperis Directory Services Protector (DSP).
Who can you trust?
As you can see, the official description of external non-transitive trusts is misleading. As it stands, when you create an external non-transitive trust, you must accept that any account within the trusted domain will be able to authenticate against any domain within the entire forest in which the trusting domain resides.
As demonstrated in the Hopping Further section of this post, best practice is to disallow Authenticated Users from creating machine accounts. Not doing so not only puts domains within the forest at higher risk of attack, but also puts any domains (and the forest in which they reside) that have an external trust with any domain within the forest at a higher risk due to the ability to create machine accounts across trusts and authenticating against domains that should be disallowed. (More information on the machine account quota and how to prevent low-privileged machine account creation can be found on Kevin Robertson’s excellent post “MachineAccountQuota is USEFUL Sometimes”.)
If nothing else, I hope this post informs systems administrators of the real forest-wide risk involved in implementing external trusts.
May 4, 2022: MSRC case created
May 12, 2022: Case status changed to “Review/Repro”
June 17, 2022: Case status changed to “Develop” with email that stated “We confirmed the behavior you reported. We’ll continue our investigation and determine how to address this issue.”
June 17, 2022: Case status changed to “Complete – Resolved”
August 24, 2022: Comment left on case to find out case status
September 2, 2022: Follow-up comment left on case to find out case status
September 14, 2022: Follow-up email sent to find out case status
September 29, 2022: Email received explaining that issue was not determined to affect security
March 7, 2023: Public disclosure
Will Schroeder for the creation of Rubeus, without which this research might not have been possible
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