Slim Framework CSRF Protection

This repository contains a Slim Framework CSRF protection PSR-15 middleware. CSRF protection applies to all unsafe HTTP requests (POST, PUT, DELETE, PATCH).

You can fetch the latest CSRF token's name and value from the Request object with its getAttribute() method. By default, the CSRF token's name is stored in the csrf_name attribute, and the CSRF token's value is stored in the csrf_value attribute.

Install

Via Composer

$ composer require slim/csrf

Requires Slim 4.0.0 or newer.

Usage

In most cases you want to register Slim\Csrf for all routes, however, as it is middleware, you can also register it for a subset of routes.

Register for all routes

use DI\Container;
use Slim\Csrf\Guard;
use Slim\Factory\AppFactory;

require __DIR__ . '/vendor/autoload.php';

// Start PHP session
session_start();

// Create Container
$container = new Container();
AppFactory::setContainer($container);

// Create App
$app = AppFactory::create();
$responseFactory = $app->getResponseFactory();

// Register Middleware On Container
$container->set('csrf', function () use ($responseFactory) {
    return new Guard($responseFactory);
});

// Register Middleware To Be Executed On All Routes
$app->add('csrf');

$app->get('/foo', function ($request, $response, $args) {
    // CSRF token name and value
    $csrf = $this->get('csrf');
    $nameKey = $csrf->getTokenNameKey();
    $valueKey = $csrf->getTokenValueKey();
    $name = $request->getAttribute($nameKey);
    $value = $request->getAttribute($valueKey);

    /*
       Render HTML form which POSTs to /bar with two hidden input fields for the
       name and value:
       <input type="hidden" name="<?= $nameKey ?>" value="<?= $name ?>">
       <input type="hidden" name="<?= $valueKey ?>" value="<?= $value ?>">
     */
});

$app->post('/bar', function ($request, $response, $args) {
    // CSRF protection successful if you reached
    // this far.
});

$app->run();

Register per route

use DI\Container;
use Slim\Csrf\Guard;
use Slim\Factory\AppFactory;

require __DIR__ . '/vendor/autoload.php';

// Start PHP session
session_start();

// Create Container
$container = new Container();
AppFactory::setContainer($container);

// Create App
$app = AppFactory::create();
$responseFactory = $app->getResponseFactory();

// Register Middleware On Container
$container->set('csrf', function () use ($responseFactory) {
    return new Guard($responseFactory);
});

$app->get('/api/route',function ($request, $response, $args) {
    $csrf = $this->get('csrf');
    $nameKey = $csrf->getTokenNameKey();
    $valueKey = $csrf->getTokenValueKey();
    $name = $request->getAttribute($nameKey);
    $value = $request->getAttribute($valueKey);

    $tokenArray = [
        $nameKey => $name,
        $valueKey => $value
    ];
    
    return $response->write(json_encode($tokenArray));
})->add('csrf');

$app->post('/api/myEndPoint',function ($request, $response, $args) {
    //Do my Things Securely!
})->add('csrf');

$app->run();

Manual usage

If you are willing to use Slim\Csrf\Guard outside a Slim\App or not as a middleware, be careful to validate the storage:

use Slim\Csrf\Guard;
use Slim\Psr7\Factory\ResponseFactory;

// Start PHP session
session_start();

// Create Middleware
$responseFactory = new ResponseFactory(); // Note that you will need to import
$guard = new Guard($responseFactory);

// Generate new tokens
$csrfNameKey = $guard->getTokenNameKey();
$csrfValueKey = $guard->getTokenValueKey();
$keyPair = $guard->generateToken();

// Validate retrieved tokens
$guard->validateToken($_POST[$csrfNameKey], $_POST[$csrfValueKey]);

Token persistence

By default, Slim\Csrf\Guard will generate a fresh name/value pair after each request. This is an important security measure for certain situations. However, in many cases this is unnecessary, and a single token throughout the user's session will suffice. By using per-session requests it becomes easier, for example, to process AJAX requests without having to retrieve a new CSRF token (by reloading the page or making a separate request) after each request. See issue #49.

To use persistent tokens, set the sixth parameter of the constructor to true. No matter what, the token will be regenerated after a failed CSRF check. In this case, you will probably want to detect this condition and instruct your users to reload the page in their legitimate browser tab (or automatically reload on the next failed request).

Accessing the token pair in templates (Twig, etc)

In many situations, you will want to access the token pair without needing to go through the request object. In these cases, you can use getTokenName() and getTokenValue() directly on the Guard middleware instance. This can be useful, for example in a Twig extension:

use Slim\Csrf\Guard;

class CsrfExtension extends \Twig\Extension\AbstractExtension implements \Twig\Extension\GlobalsInterface
{
    /**
     * @var Guard
     */
    protected $csrf;
    
    public function __construct(Guard $csrf)
    {
        $this->csrf = $csrf;
    }

    public function getGlobals()
    {
        // CSRF token name and value
        $csrfNameKey = $this->csrf->getTokenNameKey();
        $csrfValueKey = $this->csrf->getTokenValueKey();
        $csrfName = $this->csrf->getTokenName();
        $csrfValue = $this->csrf->getTokenValue();
        
        return [
            'csrf'   => [
                'keys' => [
                    'name'  => $csrfNameKey,
                    'value' => $csrfValueKey
                ],
                'name'  => $csrfName,
                'value' => $csrfValue
            ]
        ];
    }
}

Once you have registered your extension, you may access the token pair in any template:

<input type="hidden" name="{{csrf.keys.name}}" value="{{csrf.name}}">
<input type="hidden" name="{{csrf.keys.value}}" value="{{csrf.value}}">

Handling validation failure

By default, Slim\Csrf\Guard will return a Response with a 400 status code and a simple plain text error message.

To override this, provide a callable as the third parameter to the constructor or via setFailureHandler(). This callable has the same signature as middleware: function($request, $handler) and must return a Response.

For example:

use Slim\Csrf\Guard;
use Slim\Psr7\Factory\ResponseFactory;

$responseFactory = new ResponseFactory();
$guard = new Guard($responseFactory);
$guard->setFailureHandler(function (ServerRequestInterface $request, RequestHandlerInterface $handler) {
    $request = $request->withAttribute("csrf_status", false);
    return $handler->handle($request);
});

In this example, an attribute is set on the request object that can then be checked in subsequent middleware or the route callable using:

if (false === $request->getAttribute('csrf_status')) {
    // display suitable error here
} else {
    // successfully passed CSRF check
}

Testing

$ phpunit

Contributing

Please see CONTRIBUTING for details.

Security

If you discover any security related issues, please email [email protected] instead of using the issue tracker.

Credits

• Josh Lockhart
• All Contributors
• Initial inspiration from OWASP

License

The MIT License (MIT). Please see License File for more information.