These limitations make the last traded rate insecure as a price oracle by which to value fCash. Given that we allow users to borrow against their fCash, we need to ensure that the price oracle used to value fCash can't be manipulated. If the price oracle can be manipulated, an attacker could artificially move interest rates to a level that would push accounts into a state of under-collateralization. The attacker could then liquidate these accounts at an artificial price.