What makes Claims Based Authentication Secure?

Update: I should have mentioned this when I first posted, but some of these thoughts are the result of me reading Programming Windows Identity Foundation.  While I hope I haven’t copied the ideas outright, I believe the interpretation is unique-ish. Smile

One of the main reasons we as developers shy away from new technologies is because we are afraid of it.  As we learned in elementary school, the reason we are afraid usually boils down to not having enough information about the topic.  I’ve found this especially true with anything security related.  So, lets think about something for a minute.

I’m not entirely sure how valid a method this is for measure, but I like to think that as developers we measure our understanding of something by how much we abstract away the problems it creates.  Now let me ask you this question:

How much of an abstraction layer do we create for identity?

Arguably very little because in most cases we half-ass it.

I say this knowing full well I’m extremely guilty of it.  Sure, I’d create a User class and populate with application specific data, but to populate the object I would call Active Directory or SQL directly.  That created a tightly coupled dependency between the application and the user store.  That works perfectly up until you need to migrate those users in a SQL database to Active Directory.  Oops.

So why do we do this?

My reason for doing this is pretty simple.  I didn’t know any better.  The reason I didn’t know better was also pretty simple.  Of the available options to abstract away the identity I didn’t understand how the technology worked, or more likely, I didn’t trust it.  Claims based authentication is a perfect example of this.  I thought to myself when I first came across this: “are you nuts?  You want me to hand over authentication to someone else and then I have to trust them that what they give me is valid?  I don’t think so.”

Well, yes actually.

Authentication, identification, and authorization are simply processes in the grand scheme of an application lifecycle.  They are privileged, but that just means we need to be careful about it.  Fear, as it turns out, is the number one reason why we don’t abstract this part out.*

With that, I thought it would be a perfect opportunity to take a look at a few of the reasons why Claims based authentication is reasonably secure.  I would also like to take this time to compare some of these reasons to why our current methods of user authentication are usually done wrong.


First and foremost we trust the source.  Obviously a bank isn’t going to accept a handwritten piece of paper with my name on it as proof that I am me.  It stands to reason that you aren’t going to accept an identity from some random 3rd party provider for important proof of identity.

Encryption + SSL

The connection between RP and STS is over SSL.  Therefore no man in the middle attacks.  Then you encrypt the token.  Much like the SSL connection, the STS encrypts the payload with the RP’s public key, which only the RP can decrypt with its private key.  If you don’t use SSL anyone eavesdropping on the connection still can’t read the payload.  Also, the STS usually keeps a local copy of the certificate for token encryption.

How many of us encrypt our SQL connections when verifying  the user’s password?  How many of us use secured LDAP queries to Active Directory?  How many of us encrypt our web services?  I usually forget to.

Audience whitelist

Most commercial STS applications require that each request come from an approved Relying Party.  Moreover, most of those applications require that the endpoint that it responds to also be on an approved list.  You could probably fake it through DNS poisoning, but the certificates used for encryption and SSL would prevent you from doing anything meaningful since you couldn’t decrypt the token.

Do we verify the identity of the application requesting information from the SQL database?  Not usually the application.  However, we could do it via Kerberos impersonation.  E.g. lock down the specific data to the currently logged in/impersonated user.

Expiration and Duplication Prevention

All tokens have authentication timestamps.  They also normally have expiration timestamps.  Therefore they have a window of time that defines how long they are valid.  It is up to the application accepting the token to make sure the window is still acceptable, but it is still an opportunity for verification.  This also gives us the opportunity to prevent replay attacks.  All we have to do is keep track of all incoming tokens within the valid time window and see if the tokens repeat.  If so, we reject them.

There isn’t much we can do in a traditional setting to prevent this from happening.  If someone eavesdrops on the connection and grabs the username/password between the browser and your application, game over.  They don’t need to spoof anything.  They have the credentials.  SSL can fix this problem pretty easily though.


Once the token has been created by the STS, it will be signed by the STS’s private key.  If the token is modified in any way the signature wont match.  Since it is being signed by the private key of the STS, only the STS can resign it, however anyone can verify the signature through the STS’s public key.  And since it’s a certificate for the STS, we can use it as strong proof that the STS is who they say they are.  For a good primer on public key/private key stuff check out Wikipedia.

It's pretty tricky to modify payloads between SQL and an application, but it is certainly possible.  Since we don’t usually encrypt the connections (I am guilty of this daily – It’s something I need to work on Winking smile), intercepting packets and modifying them on the fly is possible.  There isn’t really a way to verify if the payload has been tampered with.

Sure, there is a level of trust between the data source and the application if they are both within the same datacenter, but what if it’s being hosted offsite by a 3rd party?  There is always going to be a situation where integrity can become an issue.  The question at that point then is: how much do you trust the source, as well as the connection to the source?

Authentication Level

Finally, if we are willing to accept that each item above increases the security and validity of the identity, there is really only one thing left to make sure is acceptable.  How was the user authenticated?  Username/password, Kerberos, smart card/certificates, etc.  If we aren’t happy with how they were authenticated, we don’t accept the token.

So now that we have a pretty strong basis for what makes the tokens containing claims as well as the relationship between the RP’s and STS’s secure, we don’t really need to fear the Claims model.

Now we just need to figure out how to replace our old code with the identity abstraction. Smile

* Strictly anecdotal evidence, mind you.

Managing Identity in SharePoint

Yet another presentation on the docket!  I submitted an abstract to SharePoint Summit 2011 and they accepted!  I will be presenting on SharePoint and how it manages Identity.  More specifically, how SharePoint 2010 uses WIF to handle Claims based authentication and Federation.

Here are the details

Event: SharePoint Summit 2011, January 31st 2011 – February 2nd, 2011

When: 11:30 a.m. - 12:45 p.m. February 1st, 2011

Where: Four Seasons Hotel, Toronto

Abstract: Managing identities within an organization is relatively easy. However, as business changes, we need to be able to adapt quickly. Identity is something that often gets overlooked in adaptation. In this session we will discuss the Windows Identity Foundation and how SharePoint uses it to adapt easily to change.

Link: http://www.sharepointsummit2011.com/Toronto/conference_day2.htm#session_7_3

Presenting a TechDays Local Flavours Track Session!

Earlier this morning I got an email from John Bristowe congratulating me on being selected to present a session for the local flavours track at TechDays in Toronto!  This bumps up my count to 2.  Needless to say I am REALLY excited.

I was a little disappointed to find out there weren’t any sessions on the Windows Identity Foundation, so that just meant I had to submit my own to the local flavours track…and they accepted it!  Here are the details:

October 27, 3:40 PM to 4:45 PM

Breakout | LFT330: Windows Identity Foundation Simplified: All the Scary Things Made Un-Scary

The Windows Identity Foundation helps simplify user access for developers by externalizing user access from applications via claims and reducing development effort with pre-built security logic and integrated .NET tools. This presentation is an intimate discussion on the basics of the Windows Identity Foundation and its claims model. In this session, you’ll learn how to refactor an existing sample set of applications to use WIF, to connect identities to the Cloud, and to remove the burden of managing multiple disparate user stores.

Location: Metro Toronto Convention Centre - South Building (255 Front Street West, Toronto)

Room: TBA


Presentation: Changing the Identity Game with the Windows Identity Foundation

Rob Windsor and TVBUG are letting me present on November 8th on Claims-Based Authentication and Identification.  Here are the details:

Location: Room 1, Library, 2nd floor, North York Public Library

Date: Monday, November 8, 2010

6:30 to 6:50 (Pizza - Meet and Greet)
6:50 to 7:00 (Group Business)
7:00 to 9:00 (Presentation)

Topic: Changing the Identity Game with the Windows Identity Foundation

Abstract: Identity is a tricky thing to manage. These days every application requires some knowledge of the user, which inevitably requires users to log in and out of the applications to prove they are who they are as well as keep a record of their accounts. With the Windows Identity Foundation, there is a fundamental shift in the way we manage these users and their accounts. In this presentation we will take a look at the why's and dig into the how's of the Windows Identity Foundation by building an Identity aware application from scratch.


All TVBUG meetings are held at the North York Public Library or North York Memorial Hall. Both are located in the same building at Yonge Street and Park Home Avenue (North of the 401 between Sheppard and Finch across from Empress Walk). If you are taking the Subway get off at the North York Centre Station. The library meeting rooms are on the 2nd Floor. Memorial Hall meeting rooms are on the Concourse Level near the food court.

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>:

      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.
    <!-- <add value="NT AUTHORITY\Network Service" /> -->
    <!-- <add value="NT AUTHORITY\Local Service" /> –>
    <!-- <add value="nt authority\system" /> –>
    <!-- <add value="NT AUTHORITY\Authenticated Users" /> -->

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())

    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.

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


A couple of weeks ago I had the opportunity to speak at the Canadian Technology Triangle .NET Users Group.

We had a great crowd of over 60 people eager to learn more about ASP.NET MVC. I was able to do a mix of Visual Studio 2008 and 2010 demos, as well as showcase a production application,  a crowd-sourced translation dictionary for First Nation Languages, currently supporting Maliseet and Mi’kmaw languages (www.sayitfirst.ca).

A question came up in the talk about content management systems and ASP.NET MVC. There was one in particular that intrigued me, but I couldn’t remember the name. After doing some digging it was www.n2cms.com. The interesting angle of N2 is that it is very lightweight, and not meant to be the shell of your entire site, but rather works within your application to serve up content where appropriate. I hope to use this on a future project.

Make sure to check out these other valuable resources as you learn ASP.NET MVC:

Toronto Code Camp 2010: Blendability Follow Up

This past weekend I gave a talk on “Blendability”; The ability to maintain and leverage blend design time compatibility with your WPF and Silverlight projects. Thanks to everybody who came out to the talk, we had some good discussions despite the oppressive heat in the room. You’ll find my slides above. As some of you have requested, here you can find my demos including the MVVM template that was used in the Blend 4 previews that demonstrates the behavior technique for calling methods on your view models.

Toronto Code Camp 2010: Ultimate Architecture Experience Follow Up

This is just a quick follow up post from my demo at the Toronto Code Camp last weekend. Thanks for everybody who came out.

In addition to my slides, here ere are a few resources that will help you learn more about the architecture tools in Visual Studio 2010 Ultimate Edition:

  • The .NET Pet Shop that I used as a sample is available for download.
  • Walkthrough: MSDN How-To’s on Modeling in 2010
  • Blogs: Cameron Skinner, Peter Provost and last but not least Chris Lovett who has some most awesome video demos and tips for dealing with large diagrams. He also provides some samples for those that are interested in learning more about Directed Graph Markup Language for creating their own diagram generators.
  • The Patterns and Practices Team has released a set of templated layered diagrams for various references architectures.

Toronto Code Camp 2010 Keynote: Building the Right Software

Opening Keynote by Joey DeVilla.

I had the privilege to open up the 2010 Toronto Code Camp last week. It’s hard to know what to talk about to such a broad audience getting ready to kick off a great event with so many different sessions and tracks. A topic that is near and dear to my heart is about trying to figure out what makes software relevant. Ultimately it is about knowing your customer, very intimately so you can build the best experience for them.

With that said, I decided to step outside the world of software development to compare to successful invention/design stories and how they differed in their approach to “knowing their customer”.

Ron Popeil the famous pitchman and inventor of the Infomercial. Malcom Gladwell wrote an interesting article/biography Ron and his fellow pitchman. There is a lot to learn about Ron’s successes, but specifically how he focused on the audience that he knew. Himself. The chef, the entertainer, and the balding guy.

The contrasting story of Deborah Adler and her thesis turned Target product for a revolutionary redesign of the common pill bottle, is no less inspiring. You can watch/hear Deborah tell her own story from the Mix 09 keynote. Unlike Popeil, Deborah inspires her design efforts by forming such an intimate understanding of her customers that she refers to it as a “Love Affair”.

These two approaches are so simple and common sense that it’s hard to ignore them when compared to the way most of us build software and disconnect ourselves from our users through layers of requirement and specification documents.

The rest of the conference was a tremendous pleasure and wouldn’t have been possible without the efforts of the numerous volunteers, speakers and most importantly Chris Dufour who has the thankless job of herding this motley crew of geeks :) Thanks to all who made a contribution to this worthwhile event.