Prevent MVC Application from Cross Site Request Forgery Attacks

Introduction

All web application platforms are potentially vulnerable to CSRF (Cross-Site Request Forgery) attacks. The best way to prevent this attack in MVC application is to use Anti-Forgery token.

Consider a banking website "www.bank.com" contains an action method DeleteUser in User Controller. When a web request comes from a client, the controller fetches the user id from session and deletes the user from database. Consider one hacker created a site "www.songs.com" and it contain one button 'Latest songs'. The button click event calls the "www.bank.com/User/DeleteAccount". A user is logged in "www.bank.com" and he is visiting "www.songs.com" using the same browser with another tab. When he clicking the 'Latest songs' button, his account will delete from the bank database. To avoid these type of unwanted requests from other sites, MVC application developers use Anti-Forgery Token.

Anti-Forgery Token is mainly used in form POST actions to verify the source of the POST data. In this method, for each page request, the web server sends a cookie to the client browser. While posting the data or next request time, the web server uses this cookie for client authentication. If the request is coming from an unauthorized site, the cookie will be null or invalid. By adding [ValidateAntiForgeryToken] above the controller and @Html.AntiForgeryToken() in the view page, we can prevent cross site requests forgery.

Using the Code

The below code illustrates how Anti-Forgery Token Cross Site Request Forgery:

Without Anti-Forgery Token

1. Controller (Controller for deleting the user account)

public class UserController : Controller
   {
       public ActionResult DeleteUser()
       {
           var userId = (int)Session["userId"];
           DeleteUserFromDb(userId);  //Function for deleting the user from Database
          return View();
       }
   }

2. View (Button for deleting the user account in Bank page)

@using (Html.BeginForm("DeleteUser", "User"))
{    
    <input type="submit" value="Delete My Account" />
} 

With Anti-Forgery Token

1. Controller

[ValidateAntiForgeryToken] 
public class UserController : Controller
    {
        public ActionResult DeleteUser()
        {
            var userId = (int)Session["userId"];            
            DeleteUserFromDb(userId);//Function for deleting the user from Database
            return View();
         }
    }

2. View

@using (Html.BeginForm("DeleteUser", "User"))
{  
    @Html.AntiForgeryToken()
    <input type="submit" value="Delete My Account" />
} 

Cross Site Request Error

Server Error in '/' Application.

The required anti-forgery cookie "__RequestVerificationToken" is not present.

Description: An unhandled exception occurred during the execution of the current web request. 
Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.Web.Mvc.HttpAntiForgeryException: 
The required anti-forgery cookie "__RequestVerificationToken" is not present.

Points of Interest

You would be wondering that I have mentioned the error and have not mentioned how to resolve that. This is a simple error that arises if you are not using the Antiforgery token attributes at appropriate places. As in, if we specify the [ValidateAntiForgeryToken] in the controller and miss out specifying in the View page posting the form, this gives rise to this exception and also prevents posting the data to the server.

History

I came through this exception after using this valuable asset that MVC provides us with. This is a real handy attribute that is the first stage of security which an MVC developer should have in mind.