Flow

The calculateAmountsFromFee function in the Helpers library can encounter an underflow error when small values of totalAmount are input. This occurs because the calculated feeAmount, derived from the specified fee percentage, may round to a value equal to or exceeding totalAmount. When feeAmount is subtracted from totalAmount to calculate netAmount, this causes a revert due to underflow. This flaw can disrupt operations dependent on calculateAmountsFromFee, as seen in the _withdraw function, potentially impacting withdrawal functionality.

https://github.com/Cyfrin/2024-10-sablier/blob/8a2eac7a916080f2022527408b004578b21c51d0/src/libraries/Helpers.sol#L13
https://github.com/Cyfrin/2024-10-sablier/blob/8a2eac7a916080f2022527408b004578b21c51d0/src/SablierFlow.sol#L848

Impact:

If the underflow occurs, the _withdraw function may revert, denying users the ability to withdraw small amounts from their streams. This can impair user experience, especially in cases involving low stream balances, though it does not lead to a direct loss of funds.

Proof of Concept:

Used an existing test suite in the withdraw.t.sol under the concrete folder.

Here you can see that the protocolFeeAmount > withdrawAmount.

When the following function is executed using forge test --mt test_WhenCallerRecipient -vvvvv.

These are the following error logs:

Meaning that an underflow revert was reached.

Recommended Mitigation:

Introduce a check in calculateAmountsFromFee to verify that feeAmount does not exceed totalAmount. If feeAmount equals or surpasses totalAmount, netAmount should be set to zero, or a specific error should be returned, indicating the amount is too small for fee deduction.

So in the Helpers.sol place this changes in the calculateAmountsFromFee function in the Helpers lib.