The funds we loan to users for margin trading come from our own operating account. Customer funds cannot be lent for margin trading. See our "Financial Security" section on the Security Practices page. The funds we loan to users are protected from default by the system of Margin Call Level and Margin Liquidation Level. The basic idea is that we will forcibly liquidate a user's positions well before the point that the user couldn't repay the loan from the remaining value of their positions together with their account balances. The system is designed so that it's very unlikely that a user could get into a situation where they'd have to add funds to their account in order to repay a loan, and hence very unlikely that a user could default on a loan. For more information, see Leverage and Margin in the Trading Guide.