First Flight #2: Puppy Raffle

Summary

selectWinner relies on parameters that can be manipulated by the players/miners.

Vulnerability Details

The line 129:

uint256 winnerIndex = uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;,

uses block metadata for choosing the random winner, which can be manipulated to some extent:

1. block.timestamp, which can be manipulated by the miners.

2. block.difficulty, which can be predicted to some extent and, moreover, it adjusts only every 2016 blocks in Ethereum, and it remains constant for many blocks at a stretch.

3. msg.sender, which is constant and might give a small sense of the pattern

Impact

Bad players can manipulate the results of the game

Recommendations

Avoid using weak randomizing parameters like block.timestamp and block.metadata. For a more robust randomness solution in Solidity, consider using something like Chainlink VRF (Verifiable Random Function). Chainlink VRF provides provably-random numbers that are verifiable on-chain and are resistant to manipulation by any party, including miners.