Pass-through authentication

In my previous blogpost I discussed Azure AD Connect Pass-Through Authentication (PTA), how it works and how it can be configured. In that blogpost I did not enable Single Sign-On (SSO) and that was also the first comment I got, within one or two days. Enabling SSO and how it works it this blogpost’s topic.

Authentication flow

I already explained the authentication flow when using PTA. When accessing a service in Office 365 you are redirected to Azure AD, you enter your credentials and the credentials are placed in the Azure Service Bus. The Azure AD Connect server retrieves these credentials from the Service Bus and presents them to the on-premises Domain Controller. The result is returned to the service bus and you’re granted access, or denied when something is wrong of course.

So, what happens if you enable SSO in the Azure AD Connect wizard? Enabling SSO is just a matter of checking the Enable single sign-on checkbox in the Azure AD Connect wizard:

Note. I skipped most of the configuration steps since this is identical to the configuration steps in the previous blogpost.

During the Azure AD Connect wizard you also must enter your on-premises administrator credentials, these are needed to configure your on-premises Active Directory to enable SSO with PTA.

To be fair, it’s not true SSO as with federation (through ADFS), but it is seamless Single-Sign On (sSSO). When enabling SSO in the Azure AD Connect wizard, users only need to enter their logon name when accessing services in Office 365, for example with Outlook Web App:

When you are on a domain joined workstation that has access to a Domain Controller, you only have to select the appropriate user account. The password is automatically returned to Azure AD and if all is well you are granted access to Outlook Web App.

If you don’t want to select or enter a logon name you can also use domain hints. In combination with Outlook Web App you would use a URL like https://outlook.office.com/owa/contoso.com. If you do so your current credentials (again, on a domain joined workstation that has access to a Domain Controller) are automatically passed through and you are granted access.

Seamless Single-Sign On under the hood

But how does this sSSO actually work under the hood.

When enabling SSO in the Azure AD Connect wizard you have to enter your on-premises domain administrator account. This is used to create an additional computer object in Active Directory called AZUREADSSOACC.

This computer account is used to create a shared Kerberos key between your on-premises Active Directory and Azure Active Directory, needed for creating the sSSO experience.

During logon in this scenario, the following 8 steps occur:

The client accesses a service in the Microsoft cloud, for example OWA via https://outlook.office.com/owa.
The request is redirected from Office 365 to Azure Active Directory.
Access is denied, and a 401 error is returned to the client.
The client accesses a local Domain Controller and requests a Kerberos token.
A Kerberos session ticket is returned to the client.
The session ticket is presented to Azure Active Directory. Since Azure Active Directory has a shared ticket with your on-premises Active Directory is can generate a Kerberos token for the client to use.
The Kerberos token is returned to the client
The Kerberos token is presented to Office 365 and access is granted. The user can now start using OWA.

As you can see this only works for domain joined clients that have access to a local Domain Controller. If they don’t have access to a local Domain Controller the regular PTA process as shown in the beginning of this blogpost (and previous blogpost) is followed.

Note. For the client to automatically pass the credentials, the Azure AD endpoints must be in the intranet zone

Tip. Use GPO to change this for all clients in your network.

If you use your browser and navigate to Exchange Online you will still be prompted to enter your username (or select a username when used previously) but you are not required to enter your password:

If you use a domain hint in your URL like https://outlook.office.com/owa/inframan.nl, then the account is automatically logged on. One small but strange note, this is supported by Microsoft Internet Explorer, but not by the Microsoft Edge browser (at least not at the moment of writing, early November 2017). This might change in the future though.

To get this working with Outlook 2016 (or fully patched Outlook 2013 that supports Modern Authentication) we need to enable OAuth on a tenant level. To achieve this, logon use Remote PowerShell in Exchange Online using the following commands:

$Cred= Get-Credential 'administrator@contoso.onmicrosoft.com'
$Session= New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/PowerShell-LiveID -Credential $Cred -Authentication Basic -AllowRedirection
Import-PSSession -Session $Session

And when logged in enter the following command:

Set-OrganizationConfig -OAuth2ClientProfileEnabled:$TRUE

Next time you start Outlook you will see that it will automatically logon to Exchange Online (whereas it didn’t when Oauth was not enabled).

Note. As outlined earlier in this post there’s a shared key between the computer account in your on-premises Active Directory and Azure Active Directory. It is strongly recommended to roll-over these keys every 30 days. For more information check the Microsoft FAQ on this: https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-sso-faq

Summary

You can use Pass-through authentication if you have a requirement to keep all user passwords on-premises (and thus not store them in the Microsoft cloud). When using Pass-through authentication you can also enable seamless Single-Sign On or sSSO. This way domain joined clients (that have access to a Domain Controller) can use Kerberos authentication to access services in the Microsoft cloud.

A number of issues to be aware of: not all clients do support PTA or sSSO as outlined in this article. For example, Internet Explorer does support it, but the Edge browser doesn’t. Outlook 2013/2016 do support it (modern authentication) but Outlook 2010 does not. Also, the Lync/Skype for business clients do not support this at all. I expect this to change in the (near) future, and when it does I will update this article.

More information

Azure Active Directory Seamless Single Sign-On: Frequently asked questions – https://docs.microsoft.com/en-us/azure/active-directory/connect/active-directory-aadconnect-sso-faq

At Ignite 2017 it was announced that Pass Through Authentication (PTA) has reached General Availability (GA) so it is a fully supported scenario now.

But what is PTA? If Office 365 there are Cloud Identities, Synced Identities and Federated Identities. The first two are authenticated in Azure Active Directory, the last one is authenticated against on-premises Domain Controllers. For this to happen you need an ADFS infrastructure, consisting of multiple internal ADFS servers and multiple WAP (Windows Application Proxy) servers in the DMZ acting as ADFS proxies. Oh, and all servers need to be load balanced as well to provide redundancy and scalability.

PTA on the other hand is built on top of Azure AD Connect, and as such an interesting extension of the Synced Identities. PTA installs an agent on the Azure AD Connect server (AuthN agent) which accepts authentication requests from Azure AD and sends these to on-premises Domain Controllers. The advantage of authentication against on-premises Domain Controllers is that no passwords (or password hashes to be more precise) are stored in Azure Active Directory.

My first thought was how an authentication mechanism based on an asynchronous replication tool (Azure AD Connect synchronizes accounts every 30 minutes, and passwords within 2 minutes) ever be a reliable and safe solution. The last thing you want to happen is that you cannot authenticate to any service in the Microsoft cloud, because your Azure AD Connect server is busy doing other stuff (like automatically updating its engine for example ).

My second thought was how secure this could be. There’s no inbound connection to the Azure AD Connect server, there’s only an outbound connection on ports 80 (only used for SSL certificate revocation lists) and 443. And the communication itself should be secured as well, so…. But now that PTA is generally available more information becomes available, and things become clearer.

Authentication flow

For authentication to happen PTA uses a ‘service bus’ in Azure. The service bus is a standard Azure solution where application can store system messages in the service bus and where other applications can use these system messages. Using a service bus, you can create an asynchronous but reliable communication mechanism.

When logging to an Office 365 service the credentials are requested by Azure Active Directory, nothing new here. The credentials are encrypted and stored in the service bus. The AuthN agent on the Azure AD Connect server has a persistent connection to Azure AD and to the service bus, and retrieves the encrypted credentials from the service bus, decrypts them and presents them to the on-premises Domain Controller. The Domain Controller response (success, failure, password expired or user locked out) is returned to the AuthN agent and stored it on the service bus. Azure AD picks up this response and the user can continue working (or not of course, depending on the Domain Controller response).