SMF 1.1.1 is vulnerabil too.
solution ->
In file PersonalMessage.php change 1417 and 1418 lines adding htmlspecialchars()
like this ->
$_REQUEST['to'] = empty($_POST['to']) ? (empty($_GET['to']) ? '' :
$_GET['to']) : htmlspecialchars(stripslashes($_POST['to']));
$_REQUEST['bcc'] = empty($_POST['bcc']) ? (empty($_GET['bcc']) ? '' :
$_GET['bcc']) : htmlspecialchars(stripslashes($_POST['bcc']));