Deep Dive Into Resource-Based Constrained Delegation – Part 1

Hello all.

Today I wanted to take a step back from cloud and step back into the world of “legacy” authentication protocols. Many of you reading this blog should be familiar with the Kerberos authentication protocol, its usage, and its plusses and minuses. If you’re unfamiliar with the protocol there are plenty of resources online that will walk you through the basics.

The best overview I’ve seen is the Mark Minasi “Cracking Open Kerberos: Understanding How Active Directory Knows Who You Are” presentation. The challenge there is you’ll need to find a working stream of the video as the Channel 9 video seems quite dead. If you enjoy nerding out like I do you can read through the following RFCs and standards:

  • The Kerberos Network Authentication Service V5 RFC 4120
  • MS-KILE Kerberos Protocol Extensions
  • MS-SFU Kerberos Protocol Extensions for Service for User and Constrained Delegation Protocol

For this blog post I’ll be focusing on Resource-Based Kerberos Constrained Delegation (RBKCD). RBKCD provides new capabilities to the Microsoft implementation of Kerberos and was introduced in Windows Server 2012. I am not a fan of re-creating documentation when someone has already done a wonderful job putting together the information, so take a read through the links below for a wonderful overview of the different types of delegation and information as to how RBKCD works.

So what types of business problems could we solve with RBKCD? Let’s suppose two organizations wish to collaborate and leverage a single shared solution to provide data analysis and the solution must meet the following requirements:

  1. The solution will be owned by Company A and hosted using Company A’s infrastructure.
  2. The solution must be capable of querying data sources in both Company A and Company B.
  3. The solution must be accessible by users within both Company A and Company B.
  4. The solution must only return the data the user is authorized to see.
  5. The solution must make security a top priority.

In the above scenario we can satisfy requirement 3 through a standard Active Directory trust between Company A and Company B. Requirement 1, 2, and 4 could be satisfied by Kerberos Unconstrained (Open) Delegation. However, requirement 5 requires we leverage the more secure Kerberos Constrained Delegation. The problem we have with that solution is Kerberos Constrained Delegation can’t satisfy requirement 2 when combined with requirements 3 and 4. Prior to Windows Server 2012, we’d have to accept the risk and loss of features such as protocol transition when using Kerberos Unconstrained Delegation. With Windows Server 2012 and later, we can leverage RBKCD to satisfy all five requirements. There are additional benefits to RBKCD in simplifying administration, but I won’t go into those, you can read about them in the links above.

So now we know what value it can bring to the business, but how does it work and what does it look like? The links I provide above answer a bit of the first question, but not much of the second. We’ll take a look at the latter question in my next blog post.

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