First Flight #2: Puppy Raffle

Summary

As on-chain data is never truly random, but only pseudo-random, the caller of PuppyRaffle::selectWinner could call this function maliciously call this function when the data is right to let him win.

Vulnerability Details

The expression uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) is deterministic and can be calculated in advance (for example in a seperate contract) by the PuppyRaffle::selectWinner caller. This will lead to the winner being known before calling the function.

Impact

A player aware of this vulnerability can call PuppyRaffle::selectWinner, so that he would always win.

Tools Used

• VSCode

• Foundry

Recommendations

Implement Chainlink VRF2