First Flight #2: Puppy Raffle

Summary

The use of block.timestamp and block.difficulty for generating hash can be the cause of bad randomness.

Vulnerability Details

     function selectWinner() external {
    require(block.timestamp >= raffleStartTime + raffleDuration, "PuppyRaffle: Raffle not over");
    require(players.length >= 4, "PuppyRaffle: Need at least 4 players");
 // @audit-issue : randomness vulnerability
    uint256 winnerIndex =
        uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
    address winner = players[winnerIndex];
    uint256 totalAmountCollected = players.length * entranceFee;
    uint256 prizePool = (totalAmountCollected * 80) / 100;
    uint256 fee = (totalAmountCollected * 20) / 100;
    totalFees = totalFees + uint64(fee);

    uint256 tokenId = totalSupply();

    // We use a different RNG calculate from the winnerIndex to determine rarity
    // @audit-issue : randomness vulnerability
    uint256 rarity = uint256(keccak256(abi.encodePacked(msg.sender, block.difficulty))) % 100;
    if (rarity <= COMMON_RARITY) {
        tokenIdToRarity[tokenId] = COMMON_RARITY;
    } else if (rarity <= COMMON_RARITY + RARE_RARITY) {
        tokenIdToRarity[tokenId] = RARE_RARITY;
    } else {
        tokenIdToRarity[tokenId] = LEGENDARY_RARITY;
    }

    delete players;
    raffleStartTime = block.timestamp;
    previousWinner = winner;
    (bool success,) = winner.call{value: prizePool}("");
    require(success, "PuppyRaffle: Failed to send prize pool to winner");
    _safeMint(winner, tokenId);
}

Impact

The block timestamp is the time at which a block was added to the blockchain, and it is considered to be unpredictable because it is determined by the miner who adds the block. However, the block timestamp is not truly random because it can be influenced by an attacker who controls a miner or has the ability to manipulate the time on their own machine.

Tools Used

Manual Review

Recommendations

use secure sources of randomness, such as hardware RNGs or decentralized randomness beacons, to ensure that their contracts are resistant to this type of attack.