First Flight #2: Puppy Raffle

Summary

The generated randomness is not random as it can be predicted, hence giving a huge edge to an attacker as they would be able to know the expected outcome.

Vulnerability Details

When computing the winning index and rarity, the following variables are used:

• msg.sender

• block.timestamp

• block.difficulty
  After PoS, block.difficulty is constantly 0. So it can be removed from the equation.
  block.timestamp changes at every block, but can easily be predicted based on the average block time of the chain.
  msg.sender can be any address, so a contract can be created to an expected address with create2 and use to invoke selectWinner with an address that gives a favorable outcome.

Impact

The winner can be pre-computed, so the game would be considered to be rigged and not provably fair.

Tools Used

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