Quote from garymc:
Even though there are several ways they could possibly break into your broker or clearing house servers, it would be difficult for them to get the your money sent to them, since there are humans in the loop when money is taken out of an account.
I think an easier way for hackers to deplete your account is to get your password(quite easy on some trading platforms since you can download a network sniffer and watch your password go by when you login. Try it.).
Now, they use their own money to buy some illiquid stock. Then they use all the money in your account to buy the same stock. The stock goes up a little, they sell, stocks falls and you have to get your money out of that illiquid position without running it to zero.
Now, most people do not have enough money to move even illiquid stocks too much, but hackers that get access to several accounts could really move a tiny market. When all those people check and see their new positions and try to get out you wouldn't have much left.
The SPIC insurance doesn't protect against this. You would be left holding the bag.