class CronAccessCheck

system.services.yml in drupal/core/modules/system/system.services.yml

drupal/core/modules/system/system.services.yml

access_check.cron in drupal/core/modules/system/system.services.yml

Drupal\system\Access\CronAccessCheck

class CronAccessCheck implements AccessCheckInterface {

  /**
   * Implements AccessCheckInterface::applies().
   */
  public function applies(Route $route) {
    return array_key_exists('_access_system_cron', $route
      ->getRequirements());
  }

  /**
   * Implements AccessCheckInterface::access().
   */
  public function access(Route $route, Request $request) {
    $key = $request->attributes
      ->get('key');
    if ($key != \Drupal::state()
      ->get('system.cron_key')) {
      watchdog('cron', 'Cron could not run because an invalid key was used.', array(), WATCHDOG_NOTICE);
      return FALSE;
    }
    elseif (config('system.maintenance')
      ->get('enabled')) {
      watchdog('cron', 'Cron could not run because the site is in maintenance mode.', array(), WATCHDOG_NOTICE);
      return FALSE;
    }
    return TRUE;
  }

}