First Flight #2: Puppy Raffle

Summary

The PRNG used to calculate winnerIndex is not strong enough.

Vulnerability Details

winnerIndex is calculated as such:
uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;

• msg.sender, block.timestamp, and block.difficulty are all public information.

• players.length can be influenced by an attacker by entering or refunding a raffle.

• Under these conditions: the attacker could craft a large number of raffle entries. Once the block.timestamp reaches the condition to call selectWinner(), the attacker can find the set of players.length values that would make winnerIndex the same as one of his entries computationally. Then he just needs to either add more entries or remove entries to adjust to the correct value of players.length he needs.

• Not necessary, but he can also monitor the mempool and frontrun any attempts by third party to call selectWinner to be more precise in his attack.

Impact

Attacker can predict the next values for the PRNG and place himself as the winning index.

Tools Used

Manual Review.

Recommendations

Use Chainlink's PRNG which is unpredictable.