First Flight #2: Puppy Raffle

Summary

The selectWinner function in the PuppyRaffle smart contract relies on predictable and manipulable on-chain data such as block.timestamp, block.difficulty, and msg.sender for random number generation, leading to an insecure RNG vulnerability. Attackers could exploit this weakness to influence the outcome of the raffle and unfairly increase their chances of winning or receiving a rarer NFT.

Vulnerability Details

msg.sender: The address of the caller. An attacker can manipulate this by creating and using multiple addresses.
block.timestamp: The timestamp of the current block. Miners have some leeway to manipulate this value.
block.difficulty: The difficulty of the current block. While this is harder to manipulate, it is still predictable.

Impact

Manipulation of Results: Attackers can influence the outcome of the raffle by manipulating the on-chain data used for RNG, such as the msg.sender, block.timestamp, and block.difficulty. This allows them to increase their chances of winning or receiving a rarer NFT.

Tools Used

Foundry, manual

Recommendations

Use of Secure RNG Services: Implement secure and reliable random number generation services. You can use established RNG services like Chainlink VRF (Verifiable Random Function), which provides cryptographically secure random numbers that are verifiable on-chain.

Remove Dependency on On-Chain Data: Avoid using on-chain data such as block.timestamp, block.difficulty, and msg.sender for RNG, as these can be manipulated by miners or participants to influence the outcome.