CSRF Protection

• Introduction
• Preventing CSRF Requests
  • Excluding URIs
• X-CSRF-Token
• X-XSRF-Token

Introduction

Cross-site request forgeries are a type of malicious exploit whereby unauthorized commands are performed on behalf of an authenticated user. Thankfully, Framework makes it easy to protect your application from cross-site request forgery (CSRF) attacks.

An Explanation of the Vulnerability

In case you’re not familiar with cross-site request forgeries, let’s discuss an example of how this vulnerability can be exploited. Imagine your application has a /user/email route that accepts a POST request to change the authenticated user’s email address. Most likely, this route expects an email input field to contain the email address the user would like to begin using.

Without CSRF protection, a malicious website could create an HTML form that points to your application’s /user/email route and submits the malicious user’s own email address:

<form action="https://your-application.com/user/email" method="POST">
    <input type="email" value="malicious-email@example.com">
</form>

<script>
    document.forms[0].submit();
</script>

If the malicious website automatically submits the form when the page is loaded, the malicious user only needs to lure an unsuspecting user of your application to visit their website and their email address will be changed in your application.

To prevent this vulnerability, we need to inspect every incoming POST, PUT, PATCH, or DELETE request for a secret session value that the malicious application is unable to access.

Preventing CSRF Requests

Framework automatically generates a CSRF “token” for each active user session managed by the application. This token is used to verify that the authenticated user is the person actually making the requests to the application. Since this token is stored in the user’s session and changes each time the session is regenerated, a malicious application is unable to access it.

The current session’s CSRF token can be accessed via the request’s session or via the csrf_token helper function:

// routes/web.php
$router->get('/token', '\App\Controllers\TokenController@show');

// App/Controllers/TokenController.php
namespace App\Controllers;

use MacropaySolutions\Kernel\Http\Request;

class TokenController
{
    public function show(Request $request)
    {
        // Retrieve via the request session
        $token = $request->session()->token();

        // Retrieve via the global helper
        $token = csrf_token();

        // ...
    }
}

Anytime you define a “POST”, “PUT”, “PATCH”, or “DELETE” HTML form in your application, you should include a hidden CSRF _token field in the form so that the CSRF protection middleware can validate the request. For convenience, you may use the @csrf Template directive to generate the hidden token input field:

<form method="POST" action="/profile">
    @csrf

    <!-- Equivalent to... -->
    <input type="hidden" name="_token" value="{{ csrf_token() }}" />
</form>

The App\Http\Middleware\VerifyCsrfToken middleware will automatically verify that the token in the request input matches the token stored in the session. When these two tokens match, we know that the authenticated user is the one initiating the request.

Excluding URIs From CSRF Protection

Sometimes you may wish to exclude a set of URIs from CSRF protection. For example, if you are utilizing a third-party webhook system, you will need to exclude your webhook handler route from CSRF protection since the third party will not know what CSRF token to send to your routes.

You may exclude the routes by adding their URIs to the $except property of the VerifyCsrfToken middleware:

<?php

namespace App\Http\Middleware;

use MacropaySolutions\Kernel\Session\Middleware\VerifyCsrfToken as Middleware;

class VerifyCsrfToken extends Middleware
{
    /**
     * The URIs that should be excluded from CSRF verification.
     *
     * @var array
     */
    protected $except = [
        'webhook/*',
        'http://example.com/foo/bar',
        'http://example.com/foo/*',
    ];
}

[!NOTE]
For convenience, the CSRF middleware is automatically disabled for all routes when running tests.

X-CSRF-TOKEN

In addition to checking for the CSRF token as a POST parameter, the App\Http\Middleware\VerifyCsrfToken middleware will also check for the X-CSRF-TOKEN request header. You could, for example, store the token in an HTML meta tag:

<meta name="csrf-token" content="{{ csrf_token() }}">

Then, you can instruct a library like jQuery to automatically add the token to all request headers. This provides simple, convenient CSRF protection for your AJAX based applications using legacy JavaScript technology:

$.ajaxSetup({
    headers: {
        'X-CSRF-TOKEN': $('meta[name="csrf-token"]').attr('content')
    }
});

X-XSRF-TOKEN

Framework stores the current CSRF token in an encrypted XSRF-TOKEN cookie that is included with each response generated by the framework. You can use the cookie value to set the X-XSRF-TOKEN request header.

This cookie is primarily sent as a developer convenience since some JavaScript frameworks and libraries, like Angular and Axios, automatically place its value in the X-XSRF-TOKEN header on same-origin requests.

[!NOTE]
By default, the resources/js/bootstrap.js file includes the Axios HTTP library which will automatically send the X-XSRF-TOKEN header for you.