Claims Transformation and Custom Attribute Stores in Active Directory Federation Services 2

Active Directory Federation Services 2 has an amazing amount of power when it comes to claims transformation.  To understand how it works lets take a look at a set of claims rules and the flow of data from ADFS to the Relying Party:

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We can have multiple rules to transform claims, and each one takes precedence via an Order:

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In the case above, Transform Rule 2 transformed the claims that Rule 1 requested from the attribute store, which in this case was Active Directory.  This becomes extremely useful because there are times when some of the data you need to pull out of Active Directory isn’t in a useable format.  There are a couple options to fix this:

  • Make the receiving application deal with it
  • Modify it before sending it off
  • Ignore it

Lets take a look at the second option (imagine an entire blog post on just ignoring it…).  ADFS allows us to transform claims before they are sent off in the token by way of the Claims Rule Language.  It follows the pattern: "If a set of conditions is true, issue one or more claims."  As such, it’s a big Boolean system.  Syntactically, it’s pretty straightforward.

To issue a claim by implicitly passing true:

=> issue(Type = "http://MyAwesomeUri/claims/AwesomeRole", Value = "Awesome Employee");

What that did was ignored the fact that there weren’t any conditions and will always pass a value.

To check a condition:

c:[Type == "http://schemas.microsoft.com/ws/2008/06/identity/claims/role", Value == "SomeRole"]
    => issue(Type = "http://MyAwesomeUri/claims/AwesomeRole", Value = "AwesomeRole");

Breaking down the query, we are checking that a claim created in a previous step has a specific type; in this case role and the claim’s value is SomeRole.  Based on that we are going to append a new claim to the outgoing list with a new type and a new value.

That’s pretty useful in it’s own right, but ADFS can actually go even further by allowing you to pull data out of custom data stores by way of Custom Attribute Stores.  There are four options to choose from when getting data:

  1. Active Directory (default)
  2. LDAP (Any directory that you can query via LDAP)
  3. SQL Server (awesome)
  4. Custom built store via custom .NET assembly

Let’s get some data from a SQL database.  First we need to create the attribute store.  Go to Trust Relationships/Attribute Stores in the ADFS MMC Console (or you could also use PowerShell):

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Then add an attribute store:

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All you need is a connection string to the database in question:

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The next step is to create the query to pull data from the database.  It’s surprisingly straightforward.  This is a bit of a contrived example, but lets grab the certificate name and the certificate hash from a database table where the certificate name is equal to the value of the http://MyCertUri/UserCertName claim type:

c:[type == http://MyCertUri/UserCertName]
   => issue(store = "MyAttributeStore",
         types = ("http://test/CertName", "http://test/CertHash"),
         query = "SELECT CertificateName, CertificateHash FROM UserCertificates WHERE CertificateName='{0}'", param = c.value);

For each column you request in the SQL query, you need a claim type as well.  Also, unlike most SQL queries, to use parameters we need to use a format similar to String.Format instead of using @MyVariable syntaxes.

In a nutshell this is how you deal with claims transformation.  For a more in depth article on how to do this check out TechNet: http://technet.microsoft.com/en-us/library/dd807118(WS.10).aspx.

Converting Claims to Windows Tokens and User Impersonation

In a domain environment it is really useful to switch user contexts in a web application.  This could be if you are needing to log in with credentials that have elevated permissions (or vice-versa) or just needing to log in as another user.

It’s pretty easy to do this with Windows Identity Foundation and Claims Authentication.  When the WIF framework is installed, a service is installed (that is off by default) that can translate Claims to Windows Tokens.  This is called (not surprisingly) the Claims to Windows Token Service or (c2WTS).

Following the deploy-with-least-amount-of-attack-surface methodology, this service does not work out of the box.  You need to turn it on and enable which user’s are allowed to impersonate via token translation.  Now, this doesn’t mean which users can switch, it means which users running the process are allowed to switch.  E.g. the process running the IIS application pools local service/network service/local system/etc (preferably a named service user other than system users).

To allow users to do this go to C:\Program Files\Windows Identity Foundation\v3.5\c2wtshost.exe.config and add in the service users to <allowedCallers>:

<windowsTokenService>
  <!--
      By default no callers are allowed to use the Windows Identity Foundation Claims To NT Token Service.
      Add the identities you wish to allow below.
    -->
  <allowedCallers>
    <clear/>
    <!-- <add value="NT AUTHORITY\Network Service" /> -->
    <!-- <add value="NT AUTHORITY\Local Service" /> –>
    <!-- <add value="nt authority\system" /> –>
    <!-- <add value="NT AUTHORITY\Authenticated Users" /> -->
  </allowedCallers>
</windowsTokenService>

You should notice that by default, all users are not allowed.  Once you’ve done that you can start up the service.  It is called Claims to Windows Token Service in the Services MMC snap-in.

That takes care of the administrative side of things.  Lets write some code.  But first, some usings:

using System;
using System.Linq;
using System.Security.Principal;
using System.Threading;
using Microsoft.IdentityModel.Claims;
using Microsoft.IdentityModel.WindowsTokenService;

The next step is to actually generate the token.  From an architectural perspective, we want to use the UPN claims type as that’s what the service wants to see.  To get the claim, we do some simple LINQ:

IClaimsIdentity identity = (ClaimsIdentity)Thread.CurrentPrincipal.Identity;
string upn = identity.Claims.Where(c => c.ClaimType == ClaimTypes.Upn).First().Value;

if (String.IsNullOrEmpty(upn))
{
    throw new Exception("No UPN claim found");
}

Following that we do the impersonation:

WindowsIdentity windowsIdentity = S4UClient.UpnLogon(upn);

using (WindowsImpersonationContext ctxt = windowsIdentity.Impersonate())
{
    DoSomethingAsNewUser();

    ctxt.Undo(); // redundant with using { } statement
}

To release the token we call the Undo() method, but if you are within a using { } statement the Undo() method is called when the object is disposed.

One thing to keep in mind though.  If you do not have permission to impersonate a user a System.ServiceModel.Security.SecurityAccessDeniedException will be thrown.

That’s all there is to it.

Implementation Details

In my opinion, these types of calls really shouldn’t be made all that often.  Realistically you need to take a look at how impersonation fits into the application and then go from there.  Impersonation is pretty weighty topic for discussion, and frankly, I’m not an expert.

Build your own Directory Federation Service

This is more of a random collection of thoughts because earlier today I came to the conclusion that I need something very similar to Active Directory Federation Services, except for non-domain users.  This is relatively easy to do; all I need is to create a Secure Token Service with a user store for the back end. 

The simplest approach is to use ASP.NET Membership and Roles with SqlProvider’s wrapped up by some WIF special sauce.  Turns out Dominick Baier already did just that with StarterSTS.

The problems I have with this is that it’s a pain to manage when you start getting more than a hundred or so users.  Extending user properties is hard to do too.  So my solution is to use something that is designed for user identities… an LDAP directory.  If it’s good enough for Active Directory, it’ll be plenty useful for this situation.

Reasoning
As an aside, the reason I’m not using Active Directory in the first place is because I need to manage a few thousand well known users without CAL’s.  This would amount to upwards of a couple hundred thousand dollars in licensing costs that just isn’t in the budget.  Further, most of these users probably wouldn’t use any of our systems that use Active Directory for authentication, but nevertheless, we need accounts for them.

Also, it would be a lot easier to manage creation and modification of user accounts because there are loads of processes that have been designed to pull user data out of HR applications into LDAP directories instead of custom SQL queries.

So lets think about what makes up Active Directory Federation Services.  It has roles that provides:

  • Token Services
  • A Windows Authentication end-point
  • An Attribute store-property-to-claim mapper (maps any LDAP properties to any claim types)
  • An application management tool (MMC snap-in and PowerShell cmdlets)
  • Proxy Services (Allows requests to pass NAT’ed zones)

That’s a pretty lightweight product when you compare it to the other services in Microsoft’s Identity stack. 

We can simplify it even further by breaking down the roles we need.

Token Services

This is actually pretty easy to accomplish.  Refer back to the WIF magic sauce.

Authentication end-point

This is just (well, you know what I mean) a web page login control.  We can’t do Windows Authentication without Kerberos (or NTLM), and we can’t do Kerberos without Active Directory (technically it could be done, but you’d be crazy to try).

Attribute store-property-to-claim mapper

ADFS can connect to a bunch of different attribute stores, including custom built stores if you provide assemblies.  We only really need to map to a few LDAP properties, and make it easy to map to other properties in the future.

Application management tool

This would be to manage the mapper and a few STS settings like URI names and certificates.  This, I think, would be a relatively simple application if we designed the configuration database properly.

Proxy Services

Proxies are a pain in the butt.  Useful in general, but we don’t really need to think about this at the moment.

Some Warnings

There are some things that are worth mentioning.  We have to be really careful about what we create because we are developing a serious piece of the security infrastructure.  Yes, it is for a group of employees that won’t have much access to anything dangerous (if they need access, they’d be migrated to Active Directory), but nevertheless we are creating the main ingress point for the majority of our employees.  It also needs to be accessible from the internet.

It may sound like I think it’ll be a synch to develop this system and have it work securely, but in reality there is a lot that will need to go into it to protect the network, the employees, and the data this could possibly interact with.  It is tough to develop applications securely.  It is far harder to develop secure applications whose sole responsibility is security related.

Next Steps

The next step is to design the thing.  I know how it will exist in relation to the systems it will be used to provide identity to, but aside from that, the architecture of the thing is still unknown.  With any luck I can accomplish rough designs tomorrow on the train, on my way to visit family for the holiday.

Better yet, maybe while visiting with family. Winking smile

Single Sign-On Between the Cloud and On-Premise using ADFS 2

One of the issues I hear about hosting services in the cloud has to do with managing Identity.  Since the service isn’t local, it’s harder to tie it into services like Active Directory.  What do I mean by this?

I’m kind of particular how certain things work.  I hate having more than one set of credentials across applications.  Theoretically since we can’t join our Azure Servers to our domain, there’s a good chance we will need separate credentials between our internal domain and Cloud Services.  However, it’s possible to make our Cloud Applications use our Active Directory credentials via a Claims Service.

With Federation Services it’s surprisingly easy to do.  Yesterday we talked about installing Active Directory Federation Services and federating an application.  Today we will talk about what it takes to get things talking between Azure and ADFS.

As a recap, yesterday we:

  1. Installed prerequisites
  2. Installed ADFS 2.0 on a domain joined server
  3. Created a relying party
  4. Created claims mappings to data in Active Directory
  5. Created a simple Claims-Aware application

So what do we need to do next?  There really isn’t much we need to do:

  1. Build Azure App
  2. Federate it using FedUtil.exe

Building an Azure application isn’t trivial, but we don’t need to know much to Federate it. 

How do we federate it?  Follow these steps providing the Azure details for the application URI and the Federation Metadata from ADFS. 

One of the gotcha’s with deploying to Azure though is that the Microsoft.IdentityModel assembly is not part of the GAC, and it’s not in Azure builds.  Therefore we need to copy the assembly to the bin folder for deployment.  We do that by going to the Microsoft.IdentityModel reference properties and setting Copy Local to true:

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That isn’t the only gotcha.  We need to keep in mind how data is transferred between Cloud and intranet.  In most cases, nothing goes on behind the scenes; it passes across the client’s browser through POST calls.  If the client’s browser is on the local intranet, when it hits the cloud app it will redirect to an intranet location.  This works because the client has access to both the cloud app and can access ADFS.  This isn’t necessarily the case with people who work offsite, or are partners with the company.

We need to have the ADFS Server accessible to the public.  This is kind of an ugly situation.  Leaving the politics out of this, we are sticking a domain joined system out in the public that’s sole responsibility is authentication and identity mapping.

One way to mitigate certain risks is to use an ADFS Proxy Service.  This service will sit on a non-domain joined system sitting on an edge network that creates a connection to the ADFS Server sitting inside the corporate network.  External applications would use the Proxy service.

Installing the Proxy service is relatively simple, but a topic for another post.