First Flight #18: T-Swap
First Flight #18
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Submission Details
Severity: high
Valid

In TSwapPool::_swap the extra tokens given to users after every swapCount breaks the protocol invariant of x * y = k

darshan

Summary

Description: The protocol follows a strict invariant of x * y = k. Where:

x: The balance of the pool token
y: The balance of WETH
k: The constant product of the two balances
This means, that whenever the balances change in the protocol, the ratio between the two amounts should remain constant, hence the k. However, this is broken due to the extra incentive in the _swap function. Meaning that over time the protocol funds will be drained.

Vulnerability Details

The follow block of code is responsible for the issue.

swap_count++;
if (swap_count >= SWAP_COUNT_MAX) {
swap_count = 0;
outputToken.safeTransfer(msg.sender, 1_000_000_000_000_000_000);
}

Impact

A user could maliciously drain the protocol of funds by doing a lot of swaps and collecting the extra incentive given out by the protocol.

Tools Used

Recommendations

Remove the extra incentive mechanism. If you want to keep this in, we should account for the change in the x * y = k protocol invariant. Or, we should set aside tokens in the same way we do with fees.

- swap_count++;
- // Fee-on-transfer
- if (swap_count >= SWAP_COUNT_MAX) {
- swap_count = 0;
- outputToken.safeTransfer(msg.sender, 1_000_000_000_000_000_000);
- }
Updates

Appeal created

inallhonesty Lead Judge about 1 year ago
Submission Judgement Published
Validated
Assigned finding tags:

In `TSwapPool::_swap` the extra tokens given to users after every swapCount breaks the protocol invariant of x * y = k

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