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

`TSwapPool::sellPoolTokens` calls `swapExactOutput` function instead of `swapExactInput` resulting in incorrect token deduction.

4lifemen

Summary

The sellPoolTokens function in the TSwapPool contract uses the swapExactOutput function instead of the swapExactInput function.

Vulnerability Details

The sellPoolTokens function is intended to allow users to sell a specified amount of pool tokens and receive the corresponding amount of WETH in return. However, the function calls swapExactOutput instead of swapExactInput. This results in the calculation of the required input amount (pool tokens) based on the desired output amount (WETH), which can lead to an incorrect amount of pool tokens being deducted from the user.

function sellPoolTokens(uint256 poolTokenAmount) external returns (uint256 wethAmount) {
return swapExactOutput(i_poolToken, i_wethToken, poolTokenAmount, uint64(block.timestamp));
}
Proof of Concept

Add this test in the TSwapPool.t.sol

function testSellPoolTokens() public {
// 1 weth to 2 pt
vm.startPrank(liquidityProvider);
weth.approve(address(pool), type(uint256).max);
poolToken.approve(address(pool), type(uint256).max);
pool.deposit(100e18, 100e18, 200e18, uint64(block.timestamp));
vm.stopPrank();
​
vm.startPrank(user);
weth.approve(address(pool), type(uint256).max);
poolToken.approve(address(pool), type(uint256).max);
​
uint256 amountToSell = 1e17;
// User current poolToken balance
uint256 initialPoolToken = poolToken.balanceOf(user);
​
// User sell pool token
pool.sellPoolTokens(amountToSell);
vm.stopPrank();
​
// User pool token balance after swap
uint256 finalPoolTokenBalance = poolToken.balanceOf(user);
​
uint256 actualPoolTokenDeducted = initialPoolToken - finalPoolTokenBalance;
​
// Incorrect pool token deducted from user
assert(actualPoolTokenDeducted > amountToSell);
}

Impact

When users attempt to sell a specific amount of pool tokens, the amount deducted from their balance is not what they expected. This discrepancy can lead to users selling more tokens than intended, undermining trust in the protocol's functionality.

Tools Used

  • Manual review

Recommendations

  1. Replace the swapExactOutput function with swapExactInput function.

  2. Add additional param minOutputAmount

+ function sellPoolTokens(uint256 poolTokenAmount, minOutputAmount) external returns (uint256 wethAmount) {
+ return swapExactInput(i_poolToken, poolTokenAmount, i_wethToken, minOutputAmount, uint64(block.timestamp));
- return swapExactOutput(i_poolToken, i_wethToken, poolTokenAmount, uint64(block.timestamp));
}
Updates

Appeal created

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

`sellPoolTokens` mismatches input and output tokens causing users to receive the incorrect amount of tokens

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