TwigSandboxPolicy.php

<?php

namespace Drupal\Core\Template;

use Drupal\Core\Site\Settings;
use Twig\Sandbox\SecurityError;
use Twig\Sandbox\SecurityPolicyInterface;

/**
 * Default sandbox policy for Twig templates.
 *
 * Twig's sandbox extension is usually used to evaluate untrusted code by
 * limiting access to potentially unsafe properties or methods. Since we do not
 * use ViewModels when passing objects to Twig templates, we limit what those
 * objects can do by only loading certain classes, method names, and method
 * names with an allowed prefix. All object properties may be accessed.
 */
class TwigSandboxPolicy implements SecurityPolicyInterface {
    
    /**
     * An array of allowed methods in the form of methodName => TRUE.
     *
     * @var array
     */
    protected $allowed_methods;
    
    /**
     * Allowed method prefixes.
     *
     * Any method starting with one of these prefixes will be allowed.
     *
     * @var array
     */
    protected $allowed_prefixes;
    
    /**
     * An array of class names for which any method calls are allowed.
     *
     * @var array
     */
    protected $allowed_classes;
    
    /**
     * Constructs a new TwigSandboxPolicy object.
     */
    public function __construct() {
        // Allow settings.php to override our default allowed classes, methods, and
        // prefixes.
        $allowed_classes = Settings::get('twig_sandbox_allowed_classes', [
            // Allow any operations on the Attribute object as it is intended to be
            // changed from a Twig template, for example calling addClass().
'Drupal\\Core\\Template\\Attribute',
        ]);
        // Flip the array so we can check using isset().
        $this->allowed_classes = array_flip($allowed_classes);
        $allowed_methods = Settings::get('twig_sandbox_allowed_methods', [
            // Only allow idempotent methods.
'id',
            'label',
            'bundle',
            'get',
            '__toString',
            'toString',
        ]);
        // Flip the array so we can check using isset().
        $this->allowed_methods = array_flip($allowed_methods);
        $this->allowed_prefixes = Settings::get('twig_sandbox_allowed_prefixes', [
            'get',
            'has',
            'is',
        ]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function checkSecurity($tags, $filters, $functions) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function checkPropertyAllowed($obj, $property) {
    }
    
    /**
     * {@inheritdoc}
     */
    public function checkMethodAllowed($obj, $method) {
        foreach ($this->allowed_classes as $class => $key) {
            if ($obj instanceof $class) {
                return TRUE;
            }
        }
        // Return quickly for an exact match of the method name.
        if (isset($this->allowed_methods[$method])) {
            return TRUE;
        }
        // If the method name starts with an allowed prefix, allow it. Note:
        // strpos() is between 3x and 7x faster than preg_match() in this case.
        foreach ($this->allowed_prefixes as $prefix) {
            if (strpos($method, $prefix) === 0) {
                return TRUE;
            }
        }
        throw new SecurityError(sprintf('Calling "%s" method on a "%s" object is not allowed.', $method, get_class($obj)));
    }

}