Flow

Summary

The `_void` function in SablierFlow.sol contains a race condition vulnerability where the total debt calculation and state updates are not atomic. This could allow a recipient to withdraw more tokens than intended during the voiding process.

Vulnerability Details

https://github.com/Cyfrin/2024-10-sablier/blob/8a2eac7a916080f2022527408b004578b21c51d0/src/SablierFlow.sol#L728-L769

The debt calculation (`_uncoveredDebtOf`) and state updates are not atomic. So between the debt calculation and the stream being marked as voided, a recipient could do the following:

• Notice the void transaction in the mempool

• Front-run with a withdrawal

• Get more tokens than they should receive based on the final debt calculation

The vulnerability could

1. Stream has 100 tokens balance

2. Sender initiates void transaction

3. Recipient monitors mempool

4. When void transaction is seen:

   • Recipient front-runs with withdrawal of current withdrawable amount

   • Void transaction completes with incorrect debt calculationoid transaction completes with incorrect debt calculation

   • More tokens are withdrawn than should be possible as a result

Impact

Allows unauthorized withdrawal of additional tokens and could drain stream balance beyond intended amount. This exploit requires only mempool monitoring, can be automated and no special permissions needed beyond being stream recipient.

Tools Used

manual code review

Recommendations

Make Void workflow Atomic

As well consider adding a Withdrawal Lock During Void