Monday, June 22, 2015

ASP.NET MVC 5 and the [AllowAnonymous] attribute

In ASP.NET MVC 4 the [AllowAnonymous] attribute was introduced. This attribute made it a lot easier to secure your whole ASP.NET MVC application. What it allowed you to do is to use a global authentication filter for your application and only allow anonymous access to your login(and register) page. Before you had to add the authorize filter to every controller as there was no way to override the filter when registered globally.

So far the good news… The problem is that in ASP.NET MVC 5, the security model was slightly changed with the introduction of authentication filters. Prior to authentication filters, developers used the Authorization filters to drive some of the authentication tasks for the current request. These tasks can now be separated out to a new custom authentication filter and authorization related tasks can be performed using authorization filters which provide a clean separation of concerns.

The problem is that this new separation doesn’t play nicely with the [AllowAnonymous] attribute. Although you would expect that AllowAnonymous would work at the authentication level, it doesn’t. Instead it is still coupled to the AuthorizationFilter. Why? To not break backwards compatibility.

To fix this and make AllowAnonymous work with the AuthenticationFilter I created my own implementation:

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