Using deterministic data like block.timestamp/difficulty for randomness, randomness can be predictable
Summary
PuppyRaffle::selectWinner function is using deterministic data for generating random number and rarity of NFT, winner can be predictable.
Vulnerability Details
Protocol is using block.timestamp, block.difficulty, msg.sender for generating random number but these can be manipulated by miners, and if that happens winner can be changed according to them also rarity of nft will be predictable.
Impact
Potential winner can be looser and vice-versa. Malicious actor can also make favourable for him to mint the rarest nft.
Tools Used
Manual Review
Recommendations
Use a decentralised oracle service like chainlink VRF.
Docs: https://docs.chain.link/vrf
Lead Judging Commences
weak-randomness
Root cause: bad RNG Impact: manipulate winner
Rewards breakdown
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