flask-security/flask_security/templates/security/messages.html

{%- with messages = get_flashed_messages(with_categories=true) -%}
  {% if messages %}
    <ul class=flashes>
    {% for category, message in messages %}
      <li class="{{ category }}">{{ message }}</li>
    {% endfor %}
    </ul>
  {% endif %}
{%- endwith %}