berndartmueller - `SoftVault` accrued interest is not refunded and stuck forever

[github-actions[bot]]

berndartmueller

high

SoftVault accrued interest is not refunded and stuck forever

Summary

Withdrawing isolated collateral tokens does not refund the accrued interest from Blueberry's lending market and remains stuck forever in the BlueBerryBank contract.

Vulnerability Detail

Isolated collateral is lent out to Blueberry's lending market (a Compound fork) via the SoftVault contract. These deposits accrue interest over time.

When a user withdraws isolated collateral using the BlueBerryBank.withdrawLend function within a spell, the withdrawn amount (wAmount) is capped at the initially deposited amount (pos.underlyingAmount).

Thus, if the withdrawn amount wAmount is greater than pos.underlyingAmount, the delta, which is the accrued interest, remains in the BlueBerryBank contract and is not refunded to the user.

Impact

Accrued cToken interest is unrecoverable and stuck forever in the BlueBerryBank contract.

Code Snippet

BlueBerryBank.sol#L693-L695

669: function withdrawLend(address token, uint256 shareAmount)
670:     external
671:     override
672:     inExec
673:     poke(token)
674: {
675:     Position storage pos = positions[POSITION_ID];
676:     Bank storage bank = banks[token];
677:     if (token != pos.underlyingToken) revert INVALID_UTOKEN(token);
678:     if (shareAmount == type(uint256).max) {
679:         shareAmount = pos.underlyingVaultShare;
680:     }
681:
682:     uint256 wAmount;
683:     if (address(ISoftVault(bank.softVault).uToken()) == token) {
684:         ISoftVault(bank.softVault).approve(
685:             bank.softVault,
686:             type(uint256).max
687:         );
688:         wAmount = ISoftVault(bank.softVault).withdraw(shareAmount);
689:     } else {
690:         wAmount = IHardVault(bank.hardVault).withdraw(token, shareAmount);
691:     }
692:
693:     wAmount = wAmount > pos.underlyingAmount // @audit-info Accrued Compound cToken interest is stuck from here on due to capping the `wAmount`. Interest  is unrecoverable and stuck forever
694:         ? pos.underlyingAmount
695:         : wAmount;
696:
697:     pos.underlyingVaultShare -= shareAmount;
698:     pos.underlyingAmount -= wAmount;
699:     bank.totalLend -= wAmount;
600:
701:     wAmount = doCutWithdrawFee(token, wAmount);
702:
703:     IERC20Upgradeable(token).safeTransfer(msg.sender, wAmount);
704: }

Tool used

Manual Review

Recommendation

Consider either keeping the accrued cToken interest as part of protocol revenue or refunding it to the user.

Duplicate of #109

[Gornutz]

duplicate of 109