CurrentUser

\Bitrix\Main\Engine\CurrentUser class is responsible for current user.

You can get CurrentUser using two options:

1.     \Bitrix\Main\Engine\Controller::getCurrentUser()
   
        public function getAction(Folder $folder)
        {
        	$securityContext = $folder->getStorage()->getSecurityContext($this->getCurrentUser()->getId());
        	if(!$folder->canRead($securityContext))
        	{
        		$this->addError(new Error('Could not read the folder.'));
   
        		return null;
        	}
       		
        	return $folder->toArray();
        }

2. Integrate dependency via method parameters of AJAX action

        public function getAction(Folder $folder, CurrentUser $currentUser)
        {
        	$securityContext = $folder->getStorage()->getSecurityContext($currentUser->getId());
        	...
        }