Securing your HTML Form in ASP.NET MVC

By now, developers should really understand how to build a form and properly secure it.  But this still seems to allude some.  It’s rather embarrassing to fail security assessments for certain secuirty flaws that can be easily avoided.

In this blog, a refresher on the basics will be covered for securing your post as well as writing an extension to add to your MVC Infrastrucutre to be used by all of your team members.

It does not cover data validation.  I am really focusing on the form and the action of that form.  In addition, this is also from the perspective of a business web application that is only accessed via an authenticated user.

Let’s get started…

Poorly Secured Form and Action

Let’s take a simple Edit many developers write everyday and show the numerous flaws with this.  Here is the Razor View:

<h2>Edit</h2>
@using (Html.BeginForm())
{
     ...  Something here with a submit button...
}

Then the controller action would look something similar to this:

public ActionResult Edit(YourModel yourModel)
{
     if (ModelState.IsValid)
     {
          SaveYourModel(yourModel);
          return RedirectToAction("Index");
     }
     return View(client);
}

There are numerous concerns with the above form.

  1. When changing sensitive data, it should only happen via the proper HTTP Action.  So setup your ActionMethodSelectorAttributes properly.
  2. What about Cross-Site Request Forgery?  The above easily allows a hacker to submit data so prevent that as best as possible.
  3. Check the referrer.  Make sure the referrer is your site.
  4. Who is the originator?  Usually, this is the site that served the form.  So check the refferer.
  5. Next, who can change this data.  If only authenticated users, then which ones?

Crash course on ActionMethodSelectorAttribute

What is an ActionMethodSelectorAttribute?  It is an attribute on your controller’s action that is used to influence the selection of an action method.  This is easy to see the benefit of using them.

You can control the routing in MVC to actions decorated with HTTPGet.

[HttpGet()]
public ActionResult Edit(int id)
{
     YourModel yourModel = GetYourModel(id)
     return View(yourModel);
}

Or control the routing for inserting, updating and deleting of data using HTTPPost.

[HttpPost()]
public ActionResult Edit(YourModel yourModel)
{
     if (ModelState.IsValid)
     {
          SaveYourModel(yourModel);
          return RedirectToAction("Index");
     }
     return View(client);
}

When a user navigates to the Edit page for from a link, double clicking a row in a grid, or by any other means, that action should be called via a HTTPGet; therefore the first Action is called due to the HTTPGet attribute on that method.  Once on that page, the user then modifies the data and clicks a submit button in a form performing a POST thus calling the second Action decorated with HTTPPost.

For a complete list of action methods, click the following link for ActionMethodSelectorAttributes.

When do you want to use an ActionMethodSelectorAttribute?  In my opinion, pretty much on every Action in your controller.

Rules to live by:

1.  If you are selecting data to display in your Action method, then your Action should be decorated with the HTTPGet Attribute.

2.  If you are creating, updating or deleting data via your Action method, then your Action should be decorated with the HTTPPost Attribute.

Oh, and by the way, using HTTPPost helps prevent some attacks via CSRF (see next section).  Without that ActionMethodSelectorAttribute, it’s easy for a hacker to create links that a user can click and unknowingly modify their data.  

Without the HTTPPost a well crafted link (obviously depending on your model and your action etc…) can be used to update sensitive info.  Even worse, if your id is just an identity column in SQL Server and if you did not properly secure your form, a hacker could call this link over and over again for each identity id in your database, thus updating your data to what ever he wants.

http://localhost:2572/InsecureDOR/EditSubmit?ClientId=3&FirstName=Maverick&MiddleInitial=A&LastName=Smith

Pretty scary if your site really contains sensitive information like account information, employee data or critical business processes that if disrupted by a hacker causes serious damage to your company’s reputation.

So use HTTPPost and you will prevent the above hacks via get from working.

Cross-Site Request Forgery (CSRF)

Cross-Site Request Forgery (CSRF) is the #8 security flaw on the OWASP Top 10 for 2013. Follow the link for great information about CSRF.  For nitty grittey detail, read “Part 5: Cross-Site Request Forgery (CSRF), 1 Nov 2010 in Troy Hunt’s OWASP Top 10 book. It’s free. This will explain in great detail how an attacker can take advantage of your site. Luckily for us, we don’t have to implement the “Synchronizer Token Pattern” as Tony explains. Instead, we just need to add the following in our form:

@Html.AntiForgeryToken()

Then on the Controller’s Action, the following ActionFilter must be added.

[HttpPost()]
[ValidateAntiForgeryToken()]
public ActionResult Edit(YourModel yourModel)

This handles most CSRF attacks.

Note:  The user must accept cookies.  This only works with POST Requests.  It does not work with GET Requests.

Without a valideAntiForgeryToken, the server will threw exceptions like the following:

AntiForgeryException

AntiForgeryExceptionMissing

Can this be circumvented?  Why, yes….  But, a hacker has to actually get the AntiForgeryToken, then craft a POST using the correct form values.  Gettign the AntiForgeryToken is possible if your site  is vulnerable to XSS or your users are on older browser’s that allow cross-domain access.  Therefore, dropping support for older browsers in web applications is critical when the data is sensitve.  But that might be easier said than done.

The next step is to check the referrer.

Checking the Referrer

The next issue that I see with the above code is that the post could originate from another site.   There are expections, but for most business web application, a post originates from your site.

In order to check the referrer, you need to write a Attribute that inherits from AuthorizeAttribute:

public class IsPostedFromThisSiteAttribute : AuthorizeAttribute
{
  public override void OnAuthorization(AuthorizationContext filterContext)  {
    if (filterContext.HttpContext != null)
    {
      if (filterContext.HttpContext.Request.UrlReferrer == null)
        throw new System.Web.HttpException("IsPostedFromThisSite has invalid post - Missing UrlReferrer!"); 
       if (filterContext.HttpContext.Request.UrlReferrer.Host != filterContext.HttpContext.Request.Url.Host)
          throw new System.Web.HttpException("IsPostedFromThisSite has invalid post - Form was not submitted from this site!");
    }
    base.OnAuthorization(filterContext);
 }
}

Now, use this Attribute on your posts by applying it to the Action:

 [AcceptVerbs(HttpVerbs.Post)]
 [IsPostedFromThisSite]
 public ActionResult MyAction()

Checking the Origin

See the Origin on the following site:

https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet

Authenticated or Authorized

Next check the user to see if the user is authenticated and authorized to perform the action.

Finally

Build an HTML Extension to make this even easier to follow in your Business Application.  It is so easy to do…

Conclusion

These techniques help tremendously when securing your site, protecting your data and for passing security assessments.  So don’t forget to use them in all your forms and protect your reputation, your companies reputation and your bottom line.

More coming soon…

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