Cross Site Request Forgery also known as CSRF (XSRF) is a widely exploited website vulnerability. In a CSRF attack, a malicious site instructs a victim's browser to send a request to an honest site, as if request were part of the victim's interaction with the honest site, leveraging the victim's network connectivity and the browser's state, such as cookies, to disrupt the integrity of the victim's session with the honest site. One of the popular technique to prevent CSRF attack is by using security tokens (from here).

ASP.NET MVC suports prevention against CSRF through the AntiForgeryToken html helper and ValidateAntiForgeryToken filter. The AntiForgeryToken is supported only for the POST requests and not for GET and this makes sense because the GET operation has to used only for safe operations (as per HTTP spec.).

In some applications we need all the POST operations should be validated for the anti-forgery token and in those cases instead of decorating all the POST actions in the application with the ValidateAntiForgeryTokenAttribute we can create a custom authorization filter and apply it globally, that's what we are going to see in this article. We will also see how to create a html helper that renders form along with the hidden field that contains security token.

When a user request for a static resource like an image, video etc. that is located in a particular folder the ASP.NET happily serves that resource to the user unless we have set some restrictions. Sometimes we need to protect these folders from delivering these resources to users other than the owner. In simple cases we can prevent this through web.config settings but in complex cases like it would be nice if we could control the accessibility through an action/filter and for that we have to direct those requests through MVC pipeline and there comes the RouteExistingFiles property. By setting this property to true we can say MVC to handle those requests instead of giving that responsibility to IIS.

In this article we will see how we can utilize the RouteExistingFiles property with an authorization filter to prevent users from accessing unauthorized resources.