Devil is in the (Owin/Katana Redirect) Details

Some time ago I wrote about a bug that took a month to be solved, involving a 401 - Unauthorized Access to an Azure AppService.

After lots of troubleshooting, that issue got a solution from Microsoft support with a little code snippet that handles the AAD redirection at run-time, rather than relying on the config file value.

Turns out that this code snippet caused another issue, namely a Owin/Katana bug.

This time the same AppService (that runs just fine on Azure), cannot run anymore locally under IIS, as it generates an infinite redirect loop between the Azure login page and the AppService.

After another month of troubleshooting with Microsoft support, bouncing from one team to another (AppService, IIS, AAD, you name it), they finally were able to reproduce the core issue, which eventually got acknowledged as an official bug (I will update this post with the link to it as soon as I will receive it from Microsoft support):

Symptom:
The MVC project stopped working in IIS 10, and was entering into an infinite loop.
Cause:
There is a problem with OWIN and IIS when specifying either a CallbackPathURL or a Redirect URI. After the authentication happens > OWIN receives the response and drops the cookie > because of that the user is being set as “Not Authenticated”, as there is no cookie to trace the session that just happened. At that point is when the infinite loop starts. There is a problem with Microsoft.Owin.Host.SystemWeb and is being considered as a bug and being investigated. Seems that when mixing usage like HttpContext.GetOwinContext().Response.Cookies and HttpContext.Response.Cookies in some cases OWIN cookies are "lost".

Resolution:
For now as a work-around if you could not specify any callbackPath or Redirect URI in the request and let Azure decide where to send the response it will work just fine for your project in IIS.
So, long story short: if you have multiple Domains in your Azure AppService, and you implement the code snippet shown before, you have to remove the "RedirectUri" and "CallbackPath" from the default "OpenIdConnectAuthenticationOptions" created for you by the ActiveDirectory Client NuGet package, and then you can run your AppService locally under IIS as usual.

In the picture below you can see the red lines over the code to be removed, and the blue line around the code snippet to fix the issue reported here.



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