ProxyApi & Anti-Forgery Tokens

Anti-Forgery Tokens?

Good question.  Anti-forgery tokens are a recommended way of preventing one of the OWASP Top Ten security vulnerabilities: Cross Site Request Forgery, or CSRF.

CSRF works on the basis that once you have logged into YourSite using your browser, any request to that domain will share the authentication information.  Normally, requests to YourSite would come from YourSite, but other developers are perfectly capable of writing some code on their site that tries to make a request to YourSite to do something evil.

Though there are a few ways of preventing or reducing the risk of CSRF attacks, anti-forgery tokens are the currently recommended approach.

So how do they work?  Whenever the server serves up a page that may result in a submission (e.g. a page that contains a form) it sets a randomly-generated cookie value.  The client must then include the random value in both a hidden form field and the request cookie; otherwise, the server will reject the request as invalid.  Attackers will not be able to read the cookie value; therefore they cannot include it as a form field and so their attack fails.

ASP.NET MVC Implementation

MVC makes it very easy to implement anti-forgery tokens.  Very easy.

Step 1: add an attribute to your action or controller

[ValidateAntiForgeryToken]
public ActionResult DoSomething()
{
    //…
}

Step 2: include the following within the form on the page

@Html.AntiForgeryToken()

Unfortunately WebAPI does not have a similar implementation, but there are thankfully a lot of examples out there (e.g. Kamranicus’ example & the MVC SPA template ) of how to achieve similar functionality that works with WebAPI.

So how can we adapt these ideas to work with ProxyApi?

ProxyApi Implementation

The intention of this library is to allow you to quickly create proxy classes for WebAPI methods; because it is expected to be running in the browser (it generates JavaScript, after all) it will be using cookie authentication and should therefore consider CSRF.

Ideally, the developer using the library doesn’t want to do anything more than they do for their MVC implementation, so it would seem like that is a good convention to follow.

Setting The Token

As with MVC, setting the cookie token and inserting the hidden form value onto the page is done by calling the Html.AntiForgeryToken() method in your view.  This is deliberately identical to the MVC method to keep things as consistent as possible.

Decorating the Controller

Following the same pattern as MVC and the examples listed above, the ProxyApi implementation uses an attribute that can be specified against a controller or an action:

[ValidateHttpAntiForgeryToken]
public void PostSomething(Something data)
{
    //...
}

This attribute is an extension of AuthorizationFilterAttribute that uses the cookie- and hidden tokens to validate the request.  The second value – the one that would normally be included as a hidden form field – is instead expected as a custom header value: X-RequestVerificationToken.  This approach avoids complications in combining the ProxyApi automatically-generated POST data and a custom form field.

Because WebAPI is often used for non-browser-based access, the attribute also allows you to optionally specify any types of authentication (e.g. Basic) that should be excluded from the verification process.

Passing the Hidden Token to the Server

The JavaScript implementation of the proxy objects allows you to specify either a concrete value or an accessor function to get the form field value:

$.proxies.myController.antiForgeryToken = "1234abc";

// or

$.proxies.myController.antiForgeryToken = function() { 
    return $("#someField").val();
};

By default, this function will use jQuery to locate the hidden input generated by the Html.AntiForgeryToken() method and use it’s value.

Summary

Overall, this implementation is nothing groundbreaking.  It borrows heavily from the the SPA MVC template and from other examples online but it does allow ProxyApi to prevent CSRF attacks with minimal change to the code for developers.

The source code for this is available on GitHub, and the updated package is available for download via nuget.

Advertisements