Azure AD Pass-through Authentication – How does it work? Part 1

Hi everyone. I decided to take a break from the legacy and jump back to modern. Today I’m going to do some digging into Microsoft’s Azure AD Pass-through Authentication solution. The feature was introduced into public preview in December of 2016 and was touted as the simple and easy alternative to AD FS. Before I jump into the weeds of pass-through authentication, let’s do a high level overview of each option.

I will first cover the AD FS (Active Directory Federation Services) solution. When AD FS is used a solution for authentication to Azure Active Directory, it’s important to remember that AD FS is simply a product that enables the use of a technology to solve a business problem. In this instance the technology we are using is modern authentication (sometimes referred to as claims-based authentication) to solve the business problem of obtaining some level of assurance that a user is who they say they are.

When Azure AD and AD FS are integrated to enable the use of modern authentication, the Windows Services Federation Language (WS-FED) standard is used. You are welcome to read the standard for details, but the gist of WS-FED is a security token service generates logical security tokens (referred to assertions) which contain claims. The claims are typically pulled from a data store (such as Active Directory) and contain information about the user’s identity such as logon ID or email address. The data included in claims has evolved significantly over the past few years to include other data about the context of the user’s device (such as a trusted or untrusted device) and user’s location (coming from a trusted or untrusted IP range). The assertions are signed by the security token service (STS) and delivered to an application (referred to as the relying party) which validates the signature on the assertion, consumes the claims from the assertion, and authorizes the user access to the application.

You may have noticed above that we never talked about a user’s credentials. The reason for that is the user’s credentials aren’t included in the assertion. Prior to the STS generating the assertion, the user needs to authenticate to the STS. When AD FS is used, it’s common for the user to authenticate to the STS using Kerberos. Those of you that are familiar with Active Directory authentication know that a user obtains a Kerberos ticket-granting-ticket during workstation authentication to a domain-joined machine. When the user accesses AD FS (in this scenario the STS) the user provides a Kerberos service ticket. The process to obtain that service ticket, pass it to AD FS, getting an assertion, and passing that assertion back to the Azure AD (relying party in this scenario) is all seamless to the user and results in a true single sign-on experience. Additionally, there is no need to synchronize a user’s Active Directory Domain Services password to Azure AD, which your security folk will surely love.

The challenge presented with using AD FS as a solution is you have yet another service which requires on-premises infrastructure, must be highly available, and requires an understanding of the concepts I have explained above. In addition, if the service needs to be exposed to the internet and be accessible by non-domain joined machines, a reverse proxy (often Microsoft Web Application Proxy in the Microsoft world) which also requires more highly available infrastructure and the understanding of concepts such as split-brain DNS.

Now imagine you’re Microsoft and companies want to limit their on-premises infrastructure and the wider technology mark is slim in professionals that grasp all the concepts I have outlined above. What do you do? Well, you introduce a simple lightweight solution that requires little to no configuration or much understanding of what is actually happening. In come Azure AD Pass-through authentication.

Azure AD Pass-through authentication doesn’t require an STS or a reverse proxy. Nor does it require synchronization of a user’s Active Directory Domain Service password to Azure AD. It also doesn’t require making changes to any incoming flows in your network firewall. Sounds glorious right? Microsoft thinks this as well, hence why they’ve been pushing it so hard.

The user experience is very straightforward where the user plugs in their Active Directory Domain Services username and password at the Azure AD login screen. After the user hits the login screen, the user is logged in and go about their user way. Pretty fancy right? So how does Microsoft work this magic? It’s actually quite complicated but ingeniously implemented to seem incredibly simplistic.

The suspense is building right? Well, you’ll need to wait until my next entry to dig into the delicious details. We’ll be using a variety of tools including a simple packet capturing tool, a web proxy debugging tool, and an incredibly awesome API monitoring tool.

See you next post!

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