RACE #16 Of The Secureum Bootcamp Epoch∞

This is a Write-Up of RACE-16, Quiz of the Secureum Bootcamp (opens in a new tab) for Ethereum Smart Contract Auditors. It was designed by recently joined Secureum Mentor Jon Stephens (opens in a new tab) and his company Veridise (opens in a new tab).

March 25, 2023 by patrickd

Code

// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;


import "@openzeppelin/contracts/interfaces/IERC3156FlashLender.sol";
import "@openzeppelin/contracts/interfaces/IERC20.sol";


contract FlashLoan is IERC3156FlashLender {
   bytes32 public constant CALLBACK_SUCCESS = keccak256("ERC3156FlashBorrower.onFlashLoan");
   uint256 public fee;


   /**
    * @param fee_ the fee that should be paid on a flashloan
    */
   constructor (
       uint256 fee_
   ) {
       fee = fee_;
   }


   /**
    * @dev The amount of currency available to be lent.
    * @param token The loan currency.
    * @return The amount of `token` that can be borrowed.
    */
   function maxFlashLoan(
       address token
   ) public view override returns (uint256) {
       return IERC20(token).balanceOf(address(this));
   }


   /**
    * @dev The fee to be charged for a given loan.
    * @param token The loan currency. Must match the address of this contract.
    * @param amount The amount of tokens lent.
    * @return The amount of `token` to be charged for the loan, on top of the returned principal.
    */
   function flashFee(
       address token,
       uint256 amount
   ) external view override returns (uint256) {
       return fee;
   }


   /**
    * @dev Loan `amount` tokens to `receiver`, and takes it back plus a `flashFee` after the ERC3156 callback.
    * @param receiver The contract receiving the tokens, needs to implement the `onFlashLoan(address user, uint256 amount, uint256 fee, bytes calldata)` interface.
    * @param token The loan currency. Must match the address of this contract.
    * @param amount The amount of tokens lent.
    * @param data A data parameter to be passed on to the `receiver` for any custom use.
    */
   function flashLoan(
       IERC3156FlashBorrower receiver,
       address token,
       uint256 amount,
       bytes calldata data
   ) external override returns (bool){
       uint256 oldAllowance = IERC20(token).allowance(address(this), address(receiver));
       uint256 oldBal = IERC20(token).balanceOf(address(this));
       require(amount <= oldBal, "Too many funds requested");
       IERC20(token).approve(address(receiver), oldAllowance + amount);

       require(
           receiver.onFlashLoan(msg.sender, token, amount, fee, data) == CALLBACK_SUCCESS,
           "Callback failed"
       );

       uint256 newBal = IERC20(token).balanceOf(address(this));
       if(newBal < oldBal + fee) {
           uint retAmt = oldBal + fee - newBal;
           require(IERC20(token).transferFrom(msg.sender, address(this), retAmt), "All funds not returned");
       }

       if (IERC20(token).allowance(address(this), address(receiver)) > oldAllowance) {
           IERC20(token).approve(address(receiver), oldAllowance);
       }

       return true;
   }
}

Question 1 of 8

Which of the following is an explanation of why flashLoan() could revert?

• A. The transaction reverts because a user requested to borrow more than maxFlashLoan()
• B. The transaction reverts because the receiver’s onFlashLoan() did not return CALLBACK_SUCCESS
• C. The transaction reverts because the user returned more than retAmt funds
• D. The transaction reverts because a user tried to spend more funds than their allowance in onFlashLoan()

Question 2 of 8

If the FlashLoan contract were safe, which of the following invariants should hold at the end of any given transaction for some ERC20 token t?

Note: old(expr) evaluates expr at the beginning of the transaction.

• A. t.balanceOf(address(this)) >= old(t.balanceOf(address(this)))
• B. t.balanceOf(address(this)) == old(t.balanceOf(address(this)))
• C. t.balanceOf(address(this)) > old(t.balanceOf(address(this)))
• D. t.balanceOf(address(this)) == old(t.balanceOf(address(this))) + fee

Question 3 of 8

Which of the following tokens would be unsafe for the above contract to loan as doing so could result in theft?

• A. ERC223
• B. ERC677
• C. ERC777
• D. ERC1155

Question 4 of 8

Which external call made by flashLoan() could result in theft if the token(s) identified in the previous question were to be used?

• A. onFlashLoan()
• B. balanceOf()
• C. transferFrom()
• D. approve()

Question 5 of 8

What is the purpose of the fee in the FlashLoan contract as is?

• A. To increase the size of available flashloans over time
• B. To pay the owner of the flashloan contract
• C. To pay those who staked their funds to be flashloaned
• D. It has no purpose

Question 6 of 8

Which of the following describes the behavior of maxFlashLoan for a standard ERC20 token over time?

• A. strictly-increasing
• B. non-decreasing
• C. constant
• D. None of the above

Question 7 of 8

For some arbitrary ERC20 token t, which of the following accurately describes the FlashLoan contract’s balance of t after a successful (i.e. non-reverting) call to flashLoan() (where t is the token requested for the flashloan):

• A. The FlashLoan contract's balance of token t will INCREASE OR STAY THE SAME.
• B. The FlashLoan contract's balance of token t will DECREASE OR STAY THE SAME.
• C. The FlashLoan contract's balance of token t will STAY THE SAME.
• D. None of the above.

Question 8 of 8

Which of the following are guaranteed to hold after a successful (i.e., non-reverting) execution of flashLoan(), assuming the token for which the flashloan is requested uses OpenZeppelin’s Standard ERC20 implementation?

• A. The receiver’s balance of “token” increases
• B. The funds that the FlashLoan contract has approved the receiver to spend has either stayed the same or decreased.
• C. The sum of all flashloans granted by the FlashLoan contract is less than the maxFlashLoan amount.
• D. The token balance of any contract/user other than the FlashLoan contract, the caller of the flashLoan(), and the “receiver” contract will remain the same as before the call to flashLoan().