function notionalCallback(
    address sender,
    address account,
    bytes calldata callbackData
) external returns (uint256) {
// @audit sender can be passed in by the attacker
require(sender == address(this), "Unauthorized callback");
// @audit using a malicious contract, this can be any token
address underlyingToken = CTokenInterface(cTokenBorrow).underlying();
bool success = IERC20(underlyingToken).transferFrom(account, address(this), cTokenRepayAmount);
require(success, "Transfer of repayment failed");

// Use the amount transferred to repay the borrow
// @audit using a malicious contract, this can be a no-op
uint code = CErc20Interface(cTokenBorrow).repayBorrowBehalf(account, cTokenRepayAmount);
