README

Yii CSRF Protection Library

The package provides PSR-15 middleware for CSRF protection:

• It supports two algorithms out of the box:
  • Synchronizer CSRF token with customizable token generation and storage. By default, it uses random data and session.
  • HMAC based token with customizable identity generation. Uses session by default.
• It has ability to apply masking to CSRF token string to make BREACH attack impossible.

Requirements

• PHP 7.4 or higher.

Installation

The package could be installed with Composer:

composer require yiisoft/csrf

General usage

In order to enable CSRF protection you need to add CsrfMiddleware to your main middleware stack. In Yii it is done by configuring config/web/application.php:

return [
    Yiisoft\Yii\Http\Application::class => [
        '__construct()' => [
            'dispatcher' => DynamicReference::to(static function (Injector $injector) {
                return ($injector->make(MiddlewareDispatcher::class))
                    ->withMiddlewares(
                        [
                            ErrorCatcher::class,
                            SessionMiddleware::class,
                            CsrfMiddleware::class, // <-- add this
                            Router::class,
                        ]
                    );
            }),
        ],
    ],
];

By default, CSRF token is obtained from _csrf request body parameter or X-CSRF-Token header.

You can access currently valid token as a string using CsrfTokenInterface:

/** @var Yiisoft\Csrf\CsrfTokenInterface $csrfToken */
$csrf = $csrfToken->getValue();

If the token does not pass validation, the response 422 Unprocessable Entity will be returned. You can change this behavior by implementing your own request handler:

use Psr\Http\Message\ResponseFactoryInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use Yiisoft\Csrf\CsrfMiddleware;

/**
 * @var Psr\Http\Message\ResponseFactoryInterface $responseFactory
 * @var Yiisoft\Csrf\CsrfTokenInterface $csrfToken
 */
 
$failureHandler = new class ($responseFactory) implements RequestHandlerInterface {
    private ResponseFactoryInterface $responseFactory;
    
    public function __construct(ResponseFactoryInterface $responseFactory)
    {
        $this->responseFactory = $responseFactory;
    }

    public function handle(ServerRequestInterface $request): ResponseInterface
    {
        $response = $this->responseFactory->createResponse(400);
        $response
            ->getBody()
            ->write('Bad request.');
        return $response;
    }
};

$middleware = new CsrfMiddleware($responseFactory, $csrfToken, $failureHandler);

CSRF Tokens

In case Yii framework is used along with config plugin, the package is configured automatically to use synchronizer token and masked decorator. You can change that depending on your needs.

Synchronizer CSRF token

Synchronizer CSRF token is a stateful CSRF token that is a unique random string. It is saved in persistent storage available only to the currently logged-in user. The same token is added to a form. When the form is submitted, token that came from the form is compared against the token stored.

SynchronizerCsrfToken requires implementation of the following interfaces:

• CsrfTokenGeneratorInterface for generating a new CSRF token;
• CsrfTokenStorageInterface for persisting a token between requests.

Package provides RandomCsrfTokenGenerator that generates a random token and SessionCsrfTokenStorage that persists a token between requests in a user session.

To learn more about the synchronizer token pattern, check OWASP CSRF cheat sheet.

HMAC based token

HMAC based token is a stateless CSRF token that does not require any storage. The token is a hash from session ID and a timestamp used to prevent replay attacks. The token is added to a form. When the form is submitted, we re-generate the token from the current session ID and a timestamp from the original token. If two hashes match, we check that the timestamp is less than the token lifetime.

HmacCsrfToken requires implementation of CsrfTokenIdentityGeneratorInterface for generating an identity. The package provides SessionCsrfTokenIdentityGenerator that is using session ID thus making the session a token scope.

Parameters set via the HmacCsrfToken constructor are:

• $secretKey — shared secret key used to generate the hash;
• $algorithm — hash algorithm for message authentication. sha256, sha384 or sha512 are recommended;
• $lifetime — number of seconds that the token is valid for.

To learn more about HMAC based token pattern check OWASP CSRF cheat sheet.

Stub CSRF token

The StubCsrfToken simply stores and returns a token string. It does not perform any additional validation. This implementation can be useful when mocking CSRF token behavior during unit testing or when providing placeholder functionality in temporary solutions.

Masked CSRF token

MaskedCsrfToken is a decorator for CsrfTokenInterface that applies masking to a token string. It makes BREACH attack impossible, so it is safe to use token in HTML to be later passed to the next request either as a hidden form field or via JavaScript async request.

It is recommended to always use this decorator.

Documentation

• Internals

If you need help or have a question, the Yii Forum is a good place for that. You may also check out other Yii Community Resources.

License

The Yii CSRF Protection Library is free software. It is released under the terms of the BSD License. Please see LICENSE for more information.

Maintained by Yii Software.

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