How to Implement CSRF Protection

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CSRF, or Cross-site request forgery, is a type of attack where a malicious actor tricks a user into performing actions on a web application without their knowledge or consent.

The attack is based on the trust that a web application has in a user's browser (e.g. on session cookies). Here's a real example of a CSRF attack: a malicious actor could create the following website:

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<html>
    <body>
        <form action="https://example.com/settings/update-email" method="POST">
            <input type="hidden" name="email" value="malicious-actor-address@some-domain.com"/>
        </form>
        <script>
            document.forms[0].submit();
        </script>

        <!-- some content here to distract the user -->
    </body>
</html>

If you visit this website (e.g. by clicking on some email link or some social network post) and you were already logged in on the https://example.com site, the malicious actor could change the email address associated to your account (effectively taking over your account) without you even being aware of it.

An effective way of preventing CSRF attacks is to use anti-CSRF tokens. These are unique tokens added to forms as hidden fields. The legit server validates them to ensure that the request originated from the expected source and not some other malicious website.

Installation

Symfony provides all the needed features to generate and validate the anti-CSRF tokens. Before using them, install this package in your project:

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$ composer require symfony/security-csrf

Then, enable/disable the CSRF protection with the csrf_protection option (see the CSRF configuration reference for more information):

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# config/packages/framework.yaml
framework:
    # ...
    csrf_protection: ~

The tokens used for CSRF protection are meant to be different for every user and they are stored in the session. That's why a session is started automatically as soon as you render a form with CSRF protection.

Moreover, this means that you cannot fully cache pages that include CSRF protected forms. As an alternative, you can:

  • Embed the form inside an uncached ESI fragment and cache the rest of the page contents;
  • Cache the entire page and load the form via an uncached AJAX request;
  • Cache the entire page and use hinclude.js to load the CSRF token with an uncached AJAX request and replace the form field value with it.

CSRF Protection in Symfony Forms

Symfony Forms include CSRF tokens by default and Symfony also checks them automatically for you. So, when using Symfony Forms, you don't have to do anything to be protected against CSRF attacks.

By default Symfony adds the CSRF token in a hidden field called _token, but this can be customized (1) globally for all forms and (2) on a form-by-form basis. Globally, you can configure it under the framework.form option:

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# config/packages/framework.yaml
framework:
    # ...
    form:
        csrf_protection:
            enabled: true
            field_name: 'custom_token_name'

On a form-by-form basis, you can configure the CSRF protection in the setDefaults() method of each form:

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// src/Form/TaskType.php
namespace App\Form;

// ...
use App\Entity\Task;
use Symfony\Component\OptionsResolver\OptionsResolver;

class TaskType extends AbstractType
{
    // ...

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'data_class'      => Task::class,
            // enable/disable CSRF protection for this form
            'csrf_protection' => true,
            // the name of the hidden HTML field that stores the token
            'csrf_field_name' => '_token',
            // an arbitrary string used to generate the value of the token
            // using a different string for each form improves its security
            'csrf_token_id'   => 'task_item',
        ]);
    }

    // ...
}

You can also customize the rendering of the CSRF form field creating a custom form theme and using csrf_token as the prefix of the field (e.g. define {% block csrf_token_widget %} ... {% endblock %} to customize the entire form field contents).

CSRF Protection in Login Form and Logout Action

Read the following:

Generating and Checking CSRF Tokens Manually

Although Symfony Forms provide automatic CSRF protection by default, you may need to generate and check CSRF tokens manually for example when using regular HTML forms not managed by the Symfony Form component.

Consider a HTML form created to allow deleting items. First, use the csrf_token() Twig function to generate a CSRF token in the template and store it as a hidden form field:

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<form action="{{ url('admin_post_delete', { id: post.id }) }}" method="post">
    {# the argument of csrf_token() is an arbitrary string used to generate the token #}
    <input type="hidden" name="token" value="{{ csrf_token('delete-item') }}">

    <button type="submit">Delete item</button>
</form>

Then, get the value of the CSRF token in the controller action and use the isCsrfTokenValid() method to check its validity:

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use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
// ...

public function delete(Request $request): Response
{
    $submittedToken = $request->getPayload()->get('token');

    // 'delete-item' is the same value used in the template to generate the token
    if ($this->isCsrfTokenValid('delete-item', $submittedToken)) {
        // ... do something, like deleting an object
    }
}

CSRF Tokens and Compression Side-Channel Attacks

BREACH and CRIME are security exploits against HTTPS when using HTTP compression. Attackers can leverage information leaked by compression to recover targeted parts of the plaintext. To mitigate these attacks, and prevent an attacker from guessing the CSRF tokens, a random mask is prepended to the token and used to scramble it.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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