Flow

Summary

By using recipient as broker, sender and recipient can short protocol fee by the max of 10%

Vulnerability Details

If a stream is created, protocol should get fee on every withdrawal made by recipient of the stream.

However, there is the option of depositing through a broker which gives both parties the oppurtunity to avoind fee payment.

Examine a scenerio;

1. Alice creates a stream with Bob as the recipient

2. ratePerSecon is such thst after a year debt will be 1,000,000_USDC

3. Alice decides to pay debt after a year

4. Under normal circumstance, if protocol fee is 1% of debt, it amounts to 10,000_USDC

5. Now, Alice calls depositViaBroker() with Bob address as the broker address with 10% fee

6. Only 900,000_USDC is then deposited and when Bod attempts to withdraw, only 9,000_USDC is deducted as fee

7. Note; this is profitable for the recipient since Bob still gets additonal 100k USDC which includes what would have been protocol fee.

8. Sender then voids stream, an action that cancels the remaining debt.

9. The only party that lost here is the protocol

Tools Used

Manual review.

Recommendations

Deduct what is the protocol fee from what is being sent to the broker as well.