That's how the entire financial world works. The trades on the stock market are only a highly organized way to agree to a deal. The money and assets change hand three days later.
The financial world does not rely on iron-clad cryptographic security. It relies on a very well developed system of layers of trust.
Exploited in what way?
Entered the network, manipulated data, created misinformation, use social engineering tricks and tried to control stock pricing. When you have a platform only accessible to the elite, it makes the possibilities extremely tempting.
You would have more luck impersonating voices on the phone!