Finding description and impact

The contract may hold an increasing balance of unusable ETH over time, reducing efficiency and potentially leading to disputes over unrefunded funds.

Proof of Concept

if (msg.value > (amountXIn + fee + 1)) {
    (bool success, ) = payable(msg.sender).call{value: msg.value - amountXIn - fee - 1}("");
    require(success, "ETH transfer failed");
}

The conditional check msg.value > (amountXIn + fee + 1) requires the excess ETH to exceed (amountXIn + fee + 1) before issuing a refund. If the msg.value provided is exactly equal to (amountXIn + fee + 1), no refund occurs—even though the extra 1 wei remains in the contract.
In scenarios where users provide slightly more ETH than required and the condition fails, the small amounts left behind accumulate in the contract. Over many transactions, this can result in a growing unclaimed ETH balance.

Example

Suppose:

• msg.value = 1.000001 ETH
• amountXIn = 1 ETH
• fee = 0.000001 ETH

Here, msg.value - amountXIn - fee = 1 wei. Since the condition checks for msg.value > (amountXIn + fee + 1), the 1 wei refund won't execute, and it remains stuck in the contract.

Recommended mitigation steps

Try this:

uint256 refundAmount = msg.value - amountXIn - fee;
if (refundAmount > 0) {
    (bool success, ) = payable(msg.sender).call{value: refundAmount}("");
    require(success, "ETH transfer failed");
}