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SSO for a website with mixed forms and basic authentication

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We have tried to achieve Single Sign-On for a while consolidating 3 of our websites into one. The challenge was that 1 website was using Forms based authentication (fed by Active Directory through Dotnetnuke), 1 with basic authentication and classic ASP and 1 is a document repository with basic authentication.

My first challenge was to enable cross-application authentication for all applications (ASP and .NET) hosted outside the DotNetNuke pool (but still in the same website).
Frankly I had never had to do this before and I was surprised by simplicity of it all:
All we need in this case is to have the same <machinekey> in the web.config of every web application.
The machinekey is the key that is used to encrypt the authentication cookie, having the same machinekey allows other applications to decrypt the same cookie and find out who the user is.
IIS by default generates the machinekey on the go, if you want to achieve SSO you will have to specify it explicitly in the web.config (you can generate the keys from many websites like

<machineKey validationKey=”yourvalidationkey” decryptionKey=”youdecryptkey” decryption=”3DES” validation=”SHA1″ />
<authentication mode=”Forms”>
<forms name=”whatever” protection=”All” timeout=”60″ cookieless=”UseCookies” />

Once I did this I could easily retrieve the username from an ASP.NET page (using User.Identity.Name).
On classic ASP (using Request.ServerVariables(“AUTH_USER”)) is also easy but I had to configure IIS to process requests coming for .asp through .NET Framework. In IIS7.5 this is easy if you use its integrated pipeline (application pool mode=integrated) but it is also possible to achieve with the classic mode.
I followed this article: Wildcard mapping
Obviously once you did all this, you will have to update your apps to retrieve the username from the variables above.

So, at this stage I was already quite happy as I had achieved SSO throughout all our web applications but still I couldn’t find a way to do the same for our document repository as it uses built in (i.e.: not adaptable) functionalities of IIS such as basic authentication coupled with NTFS security.
Then I started thinking that this could be achieved by developing my own isapi filter that would do the checking on NTFS on the user behalf.
On IIS7.5 isapi have been replaced by http handlers and http modules. What I needed was an HTTP module that would:

  • Decrypt the cookie and retrieve the user
  • Redirect to the login page if the user is not logged in
  • Check the NTFS effective permissions of the user against the file he’s trying to download
  • If he doesn’t have read rights, redirect the user to the “not authorised page”
  • If he does have read rights, let the request go (the download is actually operated under the application pool identity)

This would not only achieve SSO for the document repository but would also give the user a better experience as if he is missing rights, he is not presented with the never ending login prompts typical of basic authentication but he’s given a clear “not authorised” message.

Here is the code I’ve put together (wordpress doesn’t allow me to upload actual code files or text so I had to PDF it): ntfs_checker
You can either compile it into dll and put in the bin folder or rename it to .vb and put it in the app_code folder and add the following tags in the web.config:

<add name=”ntfs_checker” type=”ntfs_checker” />
<add key=”Domain” value=”AD_Domain_Netbios”/>
<add key=”LoginPage” value=”/login.aspx”/>
<add key=”NotAuthPage” value=”/NotAuthorized.aspx”/>

If anyone tries it out and finds new ways to improve it, please post it back.

P.S.: one little update: the application pool of the NTFS checker must run with 32-bit apps enabled or it won’t work.


Written by zantoro

March 22, 2014 at 10:10 pm