Request Verification

Request Verification in ASP.NET Razor Pages is a mechanism designed to prevent possible Cross Site Request Forgery attacks, also referred to by the acronyms XSRF and CSRF. During a CSRF attack, a malicious user will use the credentials of an authenticated user to perform some action on a web site to their benefit.

The canonical example used to illustrate this type of attack involves online banking. When you log into your bank account online, your browser receives an authentication cookie which is then passed back to the banking site each time you make a request, keeping you logged in. The authentication cookie will have a predetermined life. It may be session-based, which means it could be valid for a period of time after you have closed the online banking's browser tab without using the banking application's log out feature and not having closed your browser.

While this cookie is still valid, you find yourself on another site that initiates a form post to your banking site that makes a transfer from your account to another. This form post is authenticated by your cookie and the banking site honours the transaction because it failed to verify where the request came from.

The prevention mechanism provided by the ASP.NET framework for this type of attack involves verifying that any POST request made to a Razor page originates from a form on the same site.

This is achieved by injecting a hidden form field named __RequestVerificationToken at the end of every form with an encrypted value, and passing the same value to a cookie which is sent with the form request. The presence of both of these items and their values are validated when ASP.NET Core processes a POST request. If verification fails, the framework returns an HTTP status code of 400, signifying a Bad Request.

Opting Out

This behaviour is baked into the Razor Pages framework. However, it is possible to turn off antiforgery token validation. This can be done globally in the ConfigureServices method in Startup:

services.AddMvc().AddRazorPagesOptions(o =>
{
    o.Conventions.ConfigureFilter(new IgnoreAntiforgeryTokenAttribute());
});

Or you can bypass the checks by adding the IgnoreAntiforgeryTokenAttribute to the relevant PageModel class (not a handler method):

[IgnoreAntiforgeryToken(Order = 1001)]
public class IndexModel : PageModel
{
    public void OnPost()
    {

    }
}

The ValidateAntiForgeryToken attribute which is applied by default has an order of 1000, therefore the IgnoreAntiforgeryToken attribute needs a higher order number to be activated.

Alternatively, you can turn off token validation globally as above, and then selectively apply token validation on a case by case basis by decorating the appropriate PageModel class with the ValidateAntiForgeryToken attribute (applying the same logic to the Order value as previously):

[ValidateAntiForgeryToken(Order = 1000)]
public class IndexModel : PageModel
{
    public void OnPost()
    {

    }
}

The ValidateAntiForgeryToken attribute has an upper case "F" in its name, whereas the IgnoreAntiforgeryToken has a lower case "f". This is by design.

Choosing to opt out of Antiforgery validation using these methods does not prevent the generation of the hidden field or the cookie. All it does is to skip the verification process.

If, in addition to disabling request verification, you want to prevent the hidden form field being rendered, pass false to the antiforgery attribute in the form tag helper:

<form asp-antiforgery="false" method="post">

Note: this action on its own does not disable request verification.

Generally, there is no good reason to disable request verification. If you want your site to accept POST requests from external domains, the recommended solution is to use a Web API controller instead.

Configuration

Various options relating to the AntiForgery feature are configurable via the AntiforgeryOptions class:

Option Description Default Value
Cookie Provides access to configuring aspects of the cookie If a value for the cookie name is not specified, a unique value will be generated prefixed with .AspNetCore.Antiforgery.
FormFieldName The name used for the hidden form field __RequestVerificationToken
HeaderName The name used for the request header RequestVerificationToken
SuppressXFrameOptionsHeader If set to true, the X-Frame-Options header will not be set. By default, it is set with the value SAMEORIGIN 'false'

You can configure these options in the ConfigureServices method in Startup. The following example changes the header name from RequestVerificationToken to XSRF-TOKEN:

services.AddAntiforgery(o => o.HeaderName = "XSRF-TOKEN");

AJAX Post Requests And JSON

It is easy to forget about the anti-forgery token when crafting a POST request made with AJAX. If you omit the value from the request, the server will return a 400 Bad Request result.

For most requests, you have a choice whether to include the value as a form field or a request header, but if you want to post your data as JSON via AJAX, you have to send the token as part of the headers because ASP.NET Core does not parse JSON looking for the verification token. Here's how you might achieve that using jQuery's $.ajax method:

var postSubmit = $.ajax({
    type: "POST",
    headers: { "RequestVerificationToken": $('input[name="__RequestVerificationToken"]').val() },
    url: "/yourformhandler",
    data: JSON.stringify({ ... }),
    contentType: "application/json"
}).done(function(response){
    //...
});

A form is not necessarily needed for an AJAX POST request. However, you need to generate an antiforgery token so that you can pass its value with the request. You can do by either including an empty form element with the method set to post:

<form method="post"></form>

or you can use the AntiForgeryToken HTML helper:

@Html.AntiForgeryToken()

Both of these approaches will result in the __RequestVerificationToken hidden field being added to the page. If you prefer not to have a form or even a hidden field, you can inject the IAntiforgery interface into the page itself, and use its GetAndStoreTokens method to generate the token:

@page
@model LearnRazorPages.Pages.IndexModel
@inject IAntiforgery antiforgery
@{
    var token = antiforgery.GetAndStoreTokens(HttpContext).RequestToken;
}
...
<script>
$(function(){
    ...
    $post("/yourformhandler", {name1: param1, __RequestVerificationToken: token});
});
Last updated: 13/07/2019 06:56:03

© 2019 - Learn Razor Pages.
All rights reserved.