diff --git a/auth/mnet/auth.php b/auth/mnet/auth.php
index 807d91f75cf..9c463d66e9f 100644
--- a/auth/mnet/auth.php
+++ b/auth/mnet/auth.php
@@ -890,8 +890,6 @@ class auth_plugin_mnet extends auth_plugin_base {
         global $CFG, $DB;
         $remoteclient = get_mnet_remote_client();
 
-        $CFG->usesid = true;
-
         // We don't want to output anything to the client machine
         $start = ob_start();
 
diff --git a/lib/sessionlib.php b/lib/sessionlib.php
index f52808249b7..73cbf2a1fb6 100644
--- a/lib/sessionlib.php
+++ b/lib/sessionlib.php
@@ -106,7 +106,7 @@ abstract class session_stub implements moodle_session {
 
         if (NO_MOODLE_COOKIES) {
             // session not used at all
-            $CFG->usesid = 0;
+            $CFG->usesid = false;
 
             $_SESSION = array();
             $_SESSION['SESSION'] = new stdClass();
@@ -118,12 +118,9 @@ abstract class session_stub implements moodle_session {
 
             $newsession = empty($_COOKIE['MoodleSession'.$CFG->sessioncookie]);
 
-            if (!empty($CFG->usesid) && $newsession) {
-                sid_start_ob();
-            } else {
-                $CFG->usesid = 0;
-                ini_set('session.use_trans_sid', '0');
-            }
+            // cookieless mode is prevented for security reasons
+            $CFG->usesid = false;
+            ini_set('session.use_trans_sid', '0');
 
             session_name('MoodleSession'.$CFG->sessioncookie);
             session_set_cookie_params(0, $CFG->sessioncookiepath, $CFG->sessioncookiedomain, $CFG->cookiesecure, $CFG->cookiehttponly);