CSRF Protection

A CSRF (Cross-Site Request Forgery) attack tricks a user into performing unwanted actions on your website without their knowledge. This can be done by sending a request to your website from another website the user is logged into. To prevent this, Leaf provides a powerful CSRF protection module that handles all the funny business for you.

How does CSRF work?

If you're not familiar with CSRF attacks, this amazing explanation from the Laravel documentation will help you understand how they work.

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.

Setting Up

You can install the CSRF module through the Leaf CLI or with composer.

Leaf CLI

leaf install csrf

Composer

composer require leafs/csrf

Preventing CSRF Requests

Once installed, you can enable CSRF protection in your app by passing CSRF config to your app instance. Since CSRF is a Leaf module, it comes with first-class support for Leaf apps.

app()->csrf();

// ... your app

This will automatically generate a CSRF token using a default secret key and a random hash. The token is saved in a session, and if no session exists, the CSRF module will create a session for your app and save the token in that session. This token is then used to verify that all incoming requests are from your app.

If the CSRF token is missing or invalid, the CSRF module will throw an exception, which you can catch and handle as you see fit.

Protecting your forms

To protect your forms from CSRF attacks, you can add the CSRF token to your forms. The CSRF module provides a beautiful form() method that generates a hidden input field with the CSRF token.

<form action="/submit" method="POST">
    <?php csrf()->form(); ?>

    <!-- your form information -->
    <input type="text" name="name">
    <button type="submit">Submit</button>
</form>

Once this form is submitted, the CSRF module will verify the token and allow the request to go through if the token is valid. Note that you don't need to do anything else to verify the token; the CSRF module handles everything for you.

X-CSRF-Token Header

In addition to forms, Leaf will also automatically check for the X-CSRF-Token header in your requests. If it finds the header, it will use the token in the header to verify the request. This is great for APIs or single-page applications that don't use PHP forms.

fetch('/submit', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'X-CSRF-Token': 'YOUR_CSRF_TOKEN',
  },
  body: JSON.stringify({ name: 'John Doe' }),
});

This will send a POST request to /submit with the CSRF token in the X-CSRF-Token header. The CSRF module will automatically verify the token and allow the request to go through if the token is valid.

Displaying the generated token

The CSRF module also provides a token() method that returns the CSRF token. You can use this method to display the token in your views or to send the token to your frontend. Be careful not to expose the token to the public, as it can be used to bypass CSRF protection.

$csrfToken = csrf()->token();

Changing default Methods

By default, the CSRF module protects all POST, PUT, PATCH, and DELETE requests. If you want to exclude certain methods or add more methods to the list, you can pass an array of methods to the csrf() method.

app()->csrf([
  'methods' => ['GET', 'POST'],
]);

This will tell the CSRF module to only protect GET and POST requests.

Excluding Routes from CSRF Protection

There are many situations where you would want to exclude certain routes from CSRF protection. For example, you might want to exclude your homepage or a webhook route from CSRF protection since the third-party service sending requests to your webhook will have no way of knowing your CSRF token.

You can exclude routes from CSRF protection by passing an array of routes to the csrf() method.

app()->csrf([
  'except' => ['/my-route', '/my-other-route'],
]);

Test Mode

Leaf automatically disables CSRF protection in test mode. This is to make it easier to test your app without having to worry about CSRF tokens. If you want to test CSRF protection, you can disable test mode by setting the APP_ENV environment variable to production.

Updating the Encryption Secret

Leaf uses a default secret key to encrypt the CSRF token. It is paired together with a random hash to create a unique token for your app. If you want to change the secret key, you can do so by passing a secret key to the csrf() method.

app()->csrf([
  'secret' => 'my-new-secret-key',
]);

It is not required to change the secret key, but it is recommended to do so if you want to add an extra layer of security to your app.

If you have an environment file, you can set the secret key there.

X_CSRF_SECRET=my-new-secret-key

Error Handling

By default, Leaf will output a built-in error page when a CSRF token is invalid. You can customize the messages shown on this page by updating your config object.

app()->csrf([
  'messages.tokenNotFound' => 'Token not found',
  'messages.tokenInvalid' => 'Invalid token.',
]);

This will update the messages shown when a token is not found or invalid. If you want to handle the error yourself, you can pass an error handler to the csrf method.

app()->csrf([
  'onError' => function($error) {
    if ($error === "tokenNotFound") {
      // handle token not found error
    } else {
      // handle invalid token error
    }
  }
]);

You can use this to handle the error in any way you want.