Security is based on the support features that the broker gives you as customer, and the deposit guarantee they offer. Some brokers offers up to 500.000 USD in deposit guarantee and others 0 USD. That is a huge different, that in the end could be very important for you. When you trade, it is also important to know how to get in contact with your broker. So support languages and support opportunities like Telephone, Trade desk and Live chat is also a criteria you can choose.