Flow

Summary

During the ERC20 token transfer, malicious contracts could re-enter the withdrawal function

Vulnerability Details

https://github.com/Cyfrin/2024-10-sablier/blob/main/src/SablierFlow.sol#L772-#L880.
The potential reentrancy vulnerability exists due to the order of operations in the _withdraw function:

The function updates critical state variables (balance and aggregateBalance) before making the external call (token.safeTransfer).

Attack Sequence

Initial State:

Stream Balance: 100 tokens
Attacker calls withdraw for 50 tokens

Function Execution:

First call:

1. Checks pass (amount <= balance)

2. Updates balance (100 -> 50)

3. Calls safeTransfer

During safeTransfer:

4. Malicious contract re-enters _withdraw

5. Checks pass again (balance still shows 50)

6. Updates balance (50 -> 0)
7. Second transfer executes

Result:

• Attacker withdraws 100 tokens when only 50 were available

Impact

This vulnerability allows a user to carry out double withdrawal of funds which leads to Incorrect balance tracking and Protocol fee manipulation.

Tools Used

Manual code review

Recommendations

use the Checks-Effects-Interactions Pattern and Implement ReentrancyGuard.