diff --git a/program/lib/Roundcube/html.php b/program/lib/Roundcube/html.php
index 3e6e47a56..a36711281 100644
--- a/program/lib/Roundcube/html.php
+++ b/program/lib/Roundcube/html.php
@@ -358,7 +358,7 @@ class html_inputfield extends html
     protected $tagname = 'input';
     protected $type = 'text';
     protected $allowed = array(
-        'type','name','value','size','tabindex','autocapitalize',
+        'type','name','value','size','tabindex','autocapitalize','required',
         'autocomplete','checked','onchange','onclick','disabled','readonly',
         'spellcheck','results','maxlength','src','multiple','accept',
         'placeholder','autofocus',