ASP.NET MVC - CSRF on a GET request

Question

We have a ASP.NET MVC application. All the POST requests (form submits) have been protected from CSRF by using @Html.AntiForgeryToken and ValidateAntiForgery

Answer 1

In short, what you've just described is not an example of an XSRF attack...

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What is an XSRF attack?

Both CSRF and XSRF are used to describe what's called a Cross Site Request Forgery. It's where a malicious website takes advantage of your authenticated state on another website, to perform fraudulent cross-site requests.

Example: Online banking.

The Bank

Imagine that you're authenticated on your bank's website, and that your banks website contains a form to create new transactions, all pretty straight forward...

<!-- Your bank -->
<form action="/send_moneh" method="POST">
    <input type="text" name="amount" />
    <input type="text" name="accountNumber" />
    <input type="submit" value="Send Money" />
</form>

The Malicious website

Now let's think of the Malicious website you're also visiting, imagine that it also contains a form, one that is hidden and the values of which are pre-populated...

<!-- Malicious website -->
<form action="http://yourbank.com/send_moneh" method="POST">
    <input type="hidden" name="amount" value="100.00"/>
    <input type="hidden" name="accountNumber" value="123456" />
</form>

When the form on the malicious website is submitted, an HTTP request will be sent straight from you to your bank, and because you're authenticated on your bank's website, the transaction could be accepted.

Essentially, an attacker is using your own authentication against you by forging requests and using you as the messenger to deliver that request.

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How do prevent it?

You use an anti-forgery token, this token is a string containing a random value, the token is placed in your cookies, in addition to your HTML forms.

When you receive a request, you validate that the form contains an anti-forgery token and that it matches the one stored in your cookies. A malicious site can not see the tokens your website sets on a client, and without this information, XSRF attacks are stopped in their tracks.

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How do I implement it in ASP.NET MVC?

On your controller Action that will be handling the request, add the attribute [ValidateAntiForgeryToken], and in the HTML form add (@Html.AntiForgeryToken()).

public class ExampleController : Controller
{
    [ValidateAntiForgeryToken]
    [HttpPost]
    public ActionResult Test(Foo fooModel)
    { 
        // do your thing...
        return this.View();
    }
}


<form action="/Example/test" method="POST">
    @Html.AntiForgeryToken()
    <input type="text" name="bar" />
    <input type="submit" value="Submit" />
</form>

That's it!

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Tips/Pointers/Advice

Anti-Forgery Tokens don't make a lot of sense when performing GET requests, in fact, they don't make sense to have them anywhere that you're not modifying and persisting data, as any GET request will be returned to your user, not the attacker.

If you're Creating, Updating or Deleting data... make sure that you're using it then.

Answer 2

Ordinarily safe methods do not have to be protected against CSRF because they do not make changes to the application, and even if they're returning sensitive information this will be protected by the Same Origin Policy in the browser.

If your site is implemented as per standards, your GET requests should be safe and therefore do not need protection.

However, there is a specific case where a "Cross-Site DoS"^* attack could be executed. Say your reporting page takes 10 seconds to execute, with 100% CPU usage on your database server, and 80% CPU usage on your web server.

Users of your website know never to go to https://<baseurl>/Analysis/GetReport during office hours because it kills the server and gives other uses a bad user experience.

However, Chuck wants to knock your <baseurl> website offline because he doesn't like you or your company.

On the busy forum he posts to often, http://forum.walkertexasranger.example.com, he sets his signature to the following:

<img src="https://yoursite.example.org/Analysis/GetReport" width=0 height=0 />

He also knows that your company employees frequent the forum, often while also logged into yoursite.example.org.

Every time one of Chuck's posts are read by your employees, authentication cookies are sent to https://yoursite.example.org/Analysis/GetReport, so your site processes the request and generates the report, and your system goes offline because CPU is eaten by these constant requests.

So even though the request is a GET request and doesn't make any permanent changes to your system (aka "safe"), it is infact bringing down your system every time it is ran. Therefore, it would be better to protect this with some CSRF prevention methods. The easiest way would be to convert this so that the report can only be generated via a POST request and therefore the request can be validated via the AntiForgeryToken.

^{*XSDoS, or Cross-Site Denial of Service, is a phrase coined by me, so don't go Googling for it.}