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

withdrawFees strict balance check can be permanently bricked by force-feeding ETH

smurfetc

Description

  • Normally, withdrawFees() should let the protocol withdraw its accumulated fees.

  • However, it requires address(this).balance == uint256(totalFees). Because the contract's ETH balance can be increased without going through enterRaffle (e.g. via selfdestruct force-feeding), an attacker can make address(this).balance strictly greater than totalFees permanently, so the require always reverts and the fees can never be withdrawn.

function withdrawFees() external {
require(address(this).balance == uint256(totalFees), "PuppyRaffle: There are currently players active!"); // line 158
uint256 feesToWithdraw = totalFees;
totalFees = 0;
(bool success,) = feeAddress.call{value: feesToWithdraw}("");
require(success, "PuppyRaffle: Failed to withdraw fees");
}

Risk

Likelihood: Medium

  • Any account can deploy a tiny contract and selfdestruct it to send 1 wei to PuppyRaffle; the attack is cheap, permanent, and needs no special privileges.

Impact: Medium

  • Permanent freeze (griefing) of all protocol fees — withdrawFees() reverts forever even though totalFees is correct.

Proof of Concept

contract ForceFeeder {
constructor(address payable target) payable {
selfdestruct(target); // sends this contract's ETH to PuppyRaffle, bypassing enterRaffle
}
}
// After `new ForceFeeder{value: 1}(payable(address(raffle)))`,
// address(raffle).balance > totalFees, so withdrawFees() reverts on every call.

Recommended Mitigation

Do not compare against address(this).balance. Track and withdraw the accounted amount directly:

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 8 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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