```swapGivenInputAmount()``` will always revert

[code423n4]

Lines of code

https://github.com/code-423n4/2023-08-shell/blob/c61cf0e01bada04c3d6055acb81f61955ed600aa/src/proteus/EvolvingProteus.sol#L296

Vulnerability details

Impact

swapGivenInputAmount(...) will always revert leading to a Denial of service.

Proof of Concept

A call to swapGivenInputAmount(...) will always revert owing to the logic implementation in the function.

    function swapGivenInputAmount(
        uint256 xBalance,
        uint256 yBalance,
        uint256 inputAmount,
        SpecifiedToken inputToken
    ) external view returns (uint256 outputAmount) {
        ...

        int256 result = _swap(
            FEE_DOWN,
            int256(inputAmount),
            int256(xBalance),
            int256(yBalance),
            inputToken
        );
        // amount cannot be less than 0
        require(result < 0);

        ...
    }

The _swap(...) function returns a positive signed integer value cached in result. However, the function requires that result must be negative for this reason the check will not pass and this will lead to the function reverting.

Tools Used

Manual review.

Recommended Mitigation Steps

Change the less than sign to greater than as shown below

    function swapGivenInputAmount(
        uint256 xBalance,
        uint256 yBalance,
        uint256 inputAmount,
        SpecifiedToken inputToken
    ) external view returns (uint256 outputAmount) {
        ...

        int256 result = _swap(
            FEE_DOWN,
            int256(inputAmount),
            int256(xBalance),
            int256(yBalance),
            inputToken
        );
        // amount cannot be less than 0
        require(result > 0);

        ...
    }

Assessed type

DoS

[c4-pre-sort]

0xRobocop marked the issue as low quality report

[0xRobocop]

Invalid

result returned by _swap in swapGivenInputAmount should be negative.

[c4-pre-sort]

0xRobocop marked the issue as primary issue

[c4-judge]

JustDravee marked the issue as unsatisfactory: Invalid