How to Implement CSRF Protection ¶

CSRF Protection in Symfony Forms ¶

Forms created with the Symfony Form component include CSRF tokens by default and Symfony checks them automatically, so 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 on a form-by-form basis:

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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 Forms ¶

See Security for a login form that is protected from CSRF attacks. You can also configure the CSRF protection for the logout action.

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->request->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.