First Flight #10: One Shot

Summary

In RapBattle.sol, the winner is determined by 'uint256 random' variable, which is not 100% random as it can be manipulated.

Vulnerability Details

uint256 random = uint256(keccak256(abi.encodePacked(block.timestamp, block.prevrandao, msg.sender))) % totalBattleSkill;
The random variable depends on the hash of three elements: block.timestamp, block.prevrandao, msg.sender. The value of these elements can be predictable (block.timestamp is the block number to be mined and block.prevrandao is related to the difficulty of mining the block), therefore the hash also would.

Impact

Predicting the outcome from the hash means predicting who the winner will be, which allows to manipulate any of the variables to change the random number and the winner of the battle. This clearly allows miners to influence the battle and make the other player win in an unfair way.

Tools Used

Manual review.

Recommendations

In general, random numbers obtained with the hash of predictable variables are not secure for protocols. The main recommendations to mitigate this issue is to use an off-chain service as an Oracle, such as Chainlink VRF which can truly provide a 100% reliable random number.
https://docs.chain.link/vrf