Categories
.net-framework-version asp.net csrf webforms

preventing cross-site request forgery (csrf) attacks in asp.net web forms

I have created an ASP.Net Web Forms application using Visual Studio 2013 and I am using .NET Framework 4.5. I want to make sure my site is secure from Cross-Site Request Forgery (CSRF), I have found many articles talking about how this feature is implemented on MVC apps, but very few talking about Web Forms. On this StackOverflow question one comment states that

“This is an old question, but the latest Visual Studio 2012 ASP.NET
template for web forms includes anti-CSRF code baked into the master
page. If you don’t have the templates, here’s the code it
generates:…”

My master page does not contain the code mentioned in that answer. Is it really included in new applications? If not, what is the best way to add it?

ViewStateUserKey & Double Submit Cookie

Starting with Visual Studio 2012, Microsoft added built-in CSRF protection to new web forms application projects. To utilize this code, add a new ASP .NET Web Forms Application to your solution and view the Site.Master code behind page. This solution will apply CSRF protection to all content pages that inherit from the Site.Master page.

The following requirements must be met for this solution to work:

All web forms making data modifications must use the Site.Master page.
All requests making data modifications must use the ViewState.
The web site must be free from all Cross-Site Scripting (XSS) vulnerabilities. See how to fix Cross-Site Scripting (XSS) using Microsoft .Net Web Protection Library for details.

public partial class SiteMaster : MasterPage
{
private const string AntiXsrfTokenKey = "__AntiXsrfToken";
private const string AntiXsrfUserNameKey = "__AntiXsrfUserName";
private string _antiXsrfTokenValue;
protected void Page_Init(object sender, EventArgs e)
{
//First, check for the existence of the Anti-XSS cookie
var requestCookie = Request.Cookies[AntiXsrfTokenKey];
Guid requestCookieGuidValue;
//If the CSRF cookie is found, parse the token from the cookie.
//Then, set the global page variable and view state user
//key. The global variable will be used to validate that it matches
//in the view state form field in the Page.PreLoad method.
if (requestCookie != null
&& Guid.TryParse(requestCookie.Value, out requestCookieGuidValue))
{
//Set the global token variable so the cookie value can be
//validated against the value in the view state form field in
//the Page.PreLoad method.
_antiXsrfTokenValue = requestCookie.Value;
//Set the view state user key, which will be validated by the
//framework during each request
Page.ViewStateUserKey = _antiXsrfTokenValue;
}
//If the CSRF cookie is not found, then this is a new session.
else
{
//Generate a new Anti-XSRF token
_antiXsrfTokenValue = Guid.NewGuid().ToString("N");
//Set the view state user key, which will be validated by the
//framework during each request
Page.ViewStateUserKey = _antiXsrfTokenValue;
//Create the non-persistent CSRF cookie
var responseCookie = new HttpCookie(AntiXsrfTokenKey)
{
//Set the HttpOnly property to prevent the cookie from
//being accessed by client side script
HttpOnly = true,
//Add the Anti-XSRF token to the cookie value
Value = _antiXsrfTokenValue
};
//If we are using SSL, the cookie should be set to secure to
//prevent it from being sent over HTTP connections
if (FormsAuthentication.RequireSSL &&
Request.IsSecureConnection)
{
responseCookie.Secure = true;
}
//Add the CSRF cookie to the response
Response.Cookies.Set(responseCookie);
}
Page.PreLoad += master_Page_PreLoad;
}
protected void master_Page_PreLoad(object sender, EventArgs e)
{
//During the initial page load, add the Anti-XSRF token and user
//name to the ViewState
if (!IsPostBack)
{
//Set Anti-XSRF token
ViewState[AntiXsrfTokenKey] = Page.ViewStateUserKey;
//If a user name is assigned, set the user name
ViewState[AntiXsrfUserNameKey] =
Context.User.Identity.Name ?? String.Empty;
}
//During all subsequent post backs to the page, the token value from
//the cookie should be validated against the token in the view state
//form field. Additionally user name should be compared to the
//authenticated users name
else
{
//Validate the Anti-XSRF token
if ((string)ViewState[AntiXsrfTokenKey] != _antiXsrfTokenValue
|| (string)ViewState[AntiXsrfUserNameKey] !=
(Context.User.Identity.Name ?? String.Empty))
{
throw new InvalidOperationException("Validation of " +
"Anti-XSRF token failed.");
}
}
}
}

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