Sig, if a hacker wants just the customers' data, it is way more efficient to hack Equifax and get 143 million persons data, instead of a broker with a few 10 thousands of traders. The point of hacking a broker is to have access to trading records. And hey, it doesn't have to be a stock trader, they can back a swing futures trader, just with much more leverage. Just like probably TST does with its best traders... Also if a broker can trade against its customers, so can a hacker...
One can be an excellent hacker, but you need a good trader to understand whom to back and with what leverage. Anyhow, if I was a hacker, that is how I would do it...