First Flight #2: Puppy Raffle

Summary

The variables related to block can be checked if they are the same block, so the winnerIndex is predictable.

Vulnerability Details

The formula for the winnerIndex is as follows
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
Since block.timestamp and block.difficulty can be referenced for the same block, they are all predictable variables. In other words, players can predict the winner in advance, so they can add new players according to their winnerIndex (their subaddresses) and so on.

Impact

You can commit injustices that make you a winner.

Tools Used

Manual

Recommendations

Use an oracle to apply a random index to the winner