diff --git a/ajax/sessionController.php b/ajax/sessionController.php
index b3db8bb8..285ac262 100644
--- a/ajax/sessionController.php
+++ b/ajax/sessionController.php
@@ -15,12 +15,15 @@ namespace OCA\Documents;
 class SessionController extends Controller{
 	
 	public static function joinAsGuest($args){
-		$uid = self::preDispatchGuest();
-		$uid = substr(@$_POST['name'], 0, 16) .' '. $uid;
-		$token = @$args['token'];
+		$postfix = self::preDispatchGuest();
+		
+		$uid = Helper::getArrayValueByKey($_POST, 'name');
+		$guestUid = substr($uid, 0, 16) .' '. $postfix;
+		
 		try {
-			$file = File::getByShareToken($token);
-			self::join($uid, $file);
+			$token = Helper::getArrayValueByKey($args, 'token');
+			$fileId = File::getIdByShareToken($token);
+			self::join($guestUid, $fileId);
 		} catch (\Exception $e){
 			Helper::warnLog('Starting a session failed. Reason: ' . $e->getMessage());
 			\OCP\JSON::error();
@@ -33,13 +36,15 @@ class SessionController extends Controller{
 		$fileId = intval(@$args['file_id']);
 		
 		try {
-			$file = new File($fileId);
-		
-			if ($file->getPermissions() & \OCP\PERMISSION_UPDATE) {
-				self::join($uid, $file);	
+			$view = \OC\Files\Filesystem::getView();
+			$path = $view->getPath($fileId);
+			
+			if ($view->isUpdatable($path)) {
+				self::join($uid, $fileId);	
 			} else {
+				$info = $view->getFileInfo();
 				\OCP\JSON::success(array(
-					'permissions' => $file->getPermissions(),
+					'permissions' => $info['permissions'],
 					'id' => $fileId
 				));
 			}
@@ -51,8 +56,8 @@ class SessionController extends Controller{
 		}
 	}
 	
-	protected static function join($uid, $file){
-		$session = Db_Session::start($uid, $file);
+	protected static function join($uid, $fileId){
+		$session = Db_Session::start($uid, $fileId);
 		\OCP\JSON::success($session);
 		exit();
 	}
diff --git a/lib/db/session.php b/lib/db/session.php
index f892859f..40c99f43 100644
--- a/lib/db/session.php
+++ b/lib/db/session.php
@@ -35,7 +35,8 @@ class Db_Session extends \OCA\Documents\Db {
 	 * @return array
 	 * @throws \Exception
 	 */
-	public static function start($uid, File $file){
+	public static function start($uid, $fileId){
+		$file = new File($fileId);
 		list($ownerView, $path) = $file->getOwnerViewAndPath();
 		
 		// Create a directory to store genesis
diff --git a/lib/file.php b/lib/file.php
index 349b5223..0f132beb 100644
--- a/lib/file.php
+++ b/lib/file.php
@@ -47,6 +47,23 @@ class File {
 		}
 	}
 	
+	public static function getIdByShareToken($token){
+		$linkItem = \OCP\Share::getShareByToken($token, false);
+		if (is_array($linkItem) && isset($linkItem['uid_owner'])) {
+			// seems to be a valid share
+			$rootLinkItem = \OCP\Share::resolveReShare($linkItem);
+			$fileOwner = $rootLinkItem['uid_owner'];
+		} else {
+			throw new \Exception('This file was probably unshared');
+		}
+		
+		if (!isset($rootLinkItem['path']) && isset($rootLinkItem['file_target'])){
+			$rootLinkItem['path'] = $rootLinkItem['file_target'];
+		}
+		
+		return $rootLinkItem['file_source'];
+	}
+	
 	public static function getByShareToken($token){
 		$linkItem = \OCP\Share::getShareByToken($token, false);
 		if (is_array($linkItem) && isset($linkItem['uid_owner'])) {