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

withdrawFees() is permanently grief-able with a forced ETH transfer

nexus0ps

Description

The intended behavior: once a round ends, the accumulated fees should be withdrawable to feeAddress.

The bug: withdrawFees() requires the contract balance to exactly equal totalFees. A contract balance can be increased without going through enterRaffle by force-sending ETH (for example via selfdestruct, which transfers value regardless of any receive/fallback). One wei is enough to make the equality false forever.

function withdrawFees() external {
@> require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!");
...
}

Risk

Likelihood: Medium. Any address can force-send ETH at any time for the cost of 1 wei plus gas.

Impact: Medium. Permanent denial of service on fee withdrawal; accumulated fees are locked with no recovery path. (Pure griefing: the attacker gains nothing.)

Proof of Concept

Run forge test --match-test test_M3_selfdestruct_griefs_withdrawFees. After a normal round leaves balance == totalFees, a helper selfdestructs 1 wei into the raffle. withdrawFees() then reverts permanently.

function test_M3_selfdestruct_griefs_withdrawFees() public {
_enterFresh(4, 1);
vm.warp(raffle.raffleStartTime() + duration + 1);
raffle.selectWinner(); // balance == totalFees (withdraw would work)
​
ForceFunder f = (new ForceFunder){value: 1 wei}();
f.boom(payable(address(raffle))); // selfdestruct 1 wei into the raffle
​
vm.expectRevert("PuppyRaffle: There are currently players active!");
raffle.withdrawFees(); // now reverts forever
}

Recommended Mitigation

Do not compare address(this).balance to totalFees. Track owed fees in a dedicated accumulator and withdraw exactly that amount.

- 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");
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"); } ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!