It is impossible to repay all debt.

[c4-bot-5]

Lines of code

https://github.com/code-423n4/2024-02-wise-lending/blob/main/contracts/MainHelper.sol#L127 https://github.com/code-423n4/2024-02-wise-lending/blob/main/contracts/MainHelper.sol#L44

Vulnerability details

Impact

User cannot repay all debt because at least 1 wei of borrowShare remains. When repaying all debt with AaveHub.sol#paybackExactAmountETH, 1 wei of borrowShare remains.

Proof of Concept

AaveHub.sol#paybackExactAmountETH function which repays debt is as follows.

    function paybackExactAmountETH(
        uint256 _nftId
    )
        external
        payable
        nonReentrant
        returns (uint256)
    {
        _checkPositionLocked(
            _nftId
        );

        address aaveWrappedETH = aaveTokenAddress[
            WETH_ADDRESS
        ];

498     uint256 userBorrowShares = WISE_LENDING.getPositionBorrowShares(
            _nftId,
            aaveWrappedETH
        );

        WISE_LENDING.syncManually(
            aaveWrappedETH
        );

507     uint256 maxPaybackAmount = WISE_LENDING.paybackAmount(
            aaveWrappedETH,
            userBorrowShares
        );

        (
            uint256 paybackAmount,
            uint256 ethRefundAmount

        ) = _getInfoPayback(
            msg.value,
            maxPaybackAmount
        );

        _wrapETH(
            paybackAmount
        );

        uint256 actualAmountDeposit = _wrapAaveReturnValueDeposit(
            WETH_ADDRESS,
            paybackAmount,
            address(this)
        );

531     uint256 borrowSharesReduction = WISE_LENDING.paybackExactAmount(
            _nftId,
            aaveWrappedETH,
            actualAmountDeposit
        );

        if (ethRefundAmount > 0) {
            _sendValue(
                msg.sender,
                ethRefundAmount
            );
        }

        emit IsPaybackAave(
            _nftId,
            block.timestamp
        );

        return borrowSharesReduction;
    }

On L498, we say that userBorrowShare is 10. And on 507 the equation which calculates paybackAmount from borrowShare is userBorrowShare * pseudoTotalBorrowAmount / totalBorrowShares + 1. Here if we say pseudoTotalBorrowAmount = 100, totalBorrowShares = 50, maxPaybackAmount becomes 20 + 1. And we say actualAmountDeposit = paybackAmount = maxPaybackAmount. And the equation which calculates borrowShares from paybackAmount is maxPaybackAmount * totalBorrowShares / pseudoTotalBorrowAmount - 1. So borrowSharesReduction becomes (20 + 1) * 50 / 100 - 1 = 10 - 1.

Therefore, userBorrowShares cannot be removed and 1 wei of borrowShare remains.

Tools Used

Manual Review

Recommended Mitigation Steps

We have to modify all processes of rounding up or down correctly. For example, mitigation steps for this problem are as follows.

1. We have to modify MainHelper.sol#paybackAmount function as follows.

   
   function paybackAmount(
       address _poolToken,
       uint256 _shares
   )
       public
       view
       returns (uint256)
   {
       uint256 product = _shares
           * borrowPoolData[_poolToken].pseudoTotalBorrowAmount;
   
       uint256 totalBorrowShares = borrowPoolData[_poolToken].totalBorrowShares;

• return product / totalBorrowShares + 1;
• returnn (product + totalBorrowShares - 1) / totalBorrowShares; }

  
  2. We have to modify `MainHelper.sol#_calculateShares` function as follows.
  ```solidity
  function _calculateShares(
      uint256 _product,
      uint256 _pseudo,
      bool _maxSharePrice
  )
      private
      pure
      returns (uint256)
  {
      return _maxSharePrice == true

• ? _product / _pseudo + 1
• : _product / _pseudo - 1;
• ? (_product + _pseudo - 1) / _pseudo
• : _product / _pseudo; }

Assessed type

Math

[c4-pre-sort]

GalloDaSballo marked the issue as duplicate of #27

[c4-pre-sort]

GalloDaSballo marked the issue as sufficient quality report

[trust1995]

A much weaker version of the selected report.

[c4-judge]

trust1995 marked the issue as partial-50