A Look At : Federated Authentication

More and more organisations are looking to collaborate with partners and customers in their ecosystem to help them achieve mutual goals. SharePoint is a great tool for enabling this collaboration but many organisations are reluctant to create and maintain identities for users from other organisations just to allow access to their own SharePoint farm. It’s hardly surprising; identity management is complex and expensive.

You have to pay for servers to host your identity provider (Microsoft Active Directory if you are using Windows); you have to keep it secure; you have to back it up and ensure that it is always available, and you have to pay for someone to maintain and administer it. Identity management becomes even more complicated when your organisation wants to give external users access to SharePoint; you have to ensure that they can only access SharePoint and can’t gain access to other systems; you have to buy additional client access licenses (CALs) for each external user because by adding them to your Active Directory you are making them an internal user.

 

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Microsoft, Google and others all offer identity providers (also known as IdPs or claims providers) that are free to use, and by federating with a third party IdP you shift the ownership and management of identities on to them. You may even find that the partner or customer you are looking to collaborate with may offer their own IdP (most likely Active Directory Federation Services if they themselves run Windows). Of course, you have to trust whichever IdP you choose; they will be responsible for authenticating the user instead of you so you must be confident that they will do a good job. You must also check what pieces of information about a user (also known as claims; for example, name, email address etc) IdPs offer to ensure they can tell you enough about a user for your purposes as they don’t all offer the same.

Having introduced support for federated authentication in SharePoint 2010, Microsoft paved the way for us to federate with third party IdPs within SharePoint itself. Unfortunately, configuring SharePoint to do this is fiddly and there is no user interface for doing so (a task made more onerous if you want to federate with multiple IdPs or tweak the configuration at a later date). Fortunately Microsoft has also introduced Azure Access Control Services (ACS) which makes the process of federating with one or more IdPs simple and easy to maintain. ACS is a cloud-based service that enables you to manage the IdPs used by your applications. The following diagram illustrates, at a high-level, the components of ACS.

An ACS namespace is a container for mappings between IdPs and one or more relying parties (the applications that want to use ACS), in our case SharePoint. Associated with each mapping is a rule group with defines how the relying party handles the individual claims associated with an identity. Using rule groups you can choose to hide or expose certain claims to specific relying parties within the namespace.

So by creating an ACS namespace you are in effect creating your own unique IdP that encapsulates the configuration for federating with one or more additional IdPs. A key point to remember is that your ACS namespace can be used by other applications (relying parties) that want to share the same identities, not just SharePoint. 

Once your ACS namespace has been created you need to configure SharePoint to trust it, which most of the time will be a one off task and from that point on you can manage and maintain the IdPs you support from within ACS. The following diagram illustrates, at a high-level, the typical architecture for integrating SharePoint and ACS.

 

In the scenario above the SharePoint web application is using two different claims providers (they are referred to as claims providers in SharePoint rather than IdPs). One is for internal users and trusts an internal AD domain and another is for external users and trusts an ACS namespace.

When a user tries to access a site within the web application they will get the default SharePoint Sign In page asking them which provider they want to use.

This page can be customised and branded as required. If the user selects Windows Authentication they will get the standard authentication dialog. If they select Azure Provider (or whatever you happen to have called your claims provider) they will be redirected to your ACS Sign In page.

Again this page can be customised and branded as required. By clicking on one of the IdPs the user will be redirected to the appropriate Sign In page. Once they have been successfully authenticated by the IdP they will be redirected back to SharePoint.

 

Conclusion

By integrating SharePoint with ACS you can simplify the process of giving external users access to SharePoint. It could also save you money in licence fees and administration costs[i].

An important point to bear in mind when planning federated authentication for SharePoint is that in order for Search to be able to index content within SharePoint, you must enable Windows authentication on at least one zone within your web application. Also, if you use a reverse proxy to perform authentication, such as Microsoft Threat Management Gateway, before allowing traffic to hit your SharePoint servers, you will need to disable the authentication checks

 

[i] The licensing model for external users differs between SharePoint 2010 and SharePoint 2013. With SharePoint 2010 if you expose your farm to external users, either anonymously or not, you have to purchase a separate licence for each server. The license covers you for any number of external users and you do not need to by a CAL for each user. With SharePoint 2013, Microsoft did away with the server license for external users and you still don’t need to buy CALs for the external users.

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