Fixes bug when showing a link with a "target" attribute see #3997

main/inc/lib/security.lib.php

@@ -282,6 +282,8 @@ class Security {
             $config->set('HTML.TidyLevel', 'light');
             $config->set('Core.ConvertDocumentToFragment', false);
             $config->set('Core.RemoveProcessingInstructions', true);
+            //Shows _target attribute in anchors
+            $config->set('Attr.AllowedFrameTargets', array('_blank')); 
             if ($user_status == STUDENT) {
                 global $allowed_html_student;
                 $config->set('HTML.SafeEmbed', true);