First Flight #2: Puppy Raffle

Summary

PuppyRaffle.selectWinner() uses a weak PRNG so the miner could potentially manipulate the block timestamp or block difficulty to influence the outcome of the raffle

Vulnerability Details

The function selectWinner Function uses keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty)) to generate a random number. This is not truly random and can be manipulated by miners. A miner could potentially manipulate the block timestamp or block difficulty to influence the outcome of the raffle.

Impact

Weak PRNG due to a modulo on block.timestamp, now or blockhash can be influenced by miners to some extent so they should be avoided. The function also sends ether to an external address (winner.call{value: prizePool}("")) and then continues to execute further logic. This could potentially open up a reentrancy attack if the recipient is a contract that has a fallback function that calls back into the selectWinner function.

Tools Used

Slither, Manual Review

Recommendations

Do not use block.timestamp, now or blockhash as a source of randomness