If your exchange uses its own book's price, you are vulnerable. At BitMEX, we use a Composite Index derived from a basket of 16 major spot exchanges (like Coinbase/Kraken). To liquidate a BitMEX trader, you'd have to crash the entire global spot market simultaneously. More on risk management and other lessons we’ve learned from 11 years of delivering unrivalled stability and user safety: http://bitmex.com/blog/11-year-anniversary(BitMEX)