Submission Details
Severity:
Weak Randomness Used in Battles
Summary
Using block.timestamp, block.prevrandao, msg.sender or block.number as a source of randomness is commonly advised against, as the outcome can be manipulated.
The _battle() function in contract RapBattle is using block.timestamp, block.prevrandao and msg.sender as source of randomness.
Vulnerability Details
The code below shows how the, supposedly, random value is being calculated:
function _battle(uint256 _tokenId, uint256 _credBet) internal {
address _defender = defender;
require(defenderBet == _credBet, "RapBattle: Bet amounts do not match");
uint256 defenderRapperSkill = getRapperSkill(defenderTokenId);
uint256 challengerRapperSkill = getRapperSkill(_tokenId);
uint256 totalBattleSkill = defenderRapperSkill + challengerRapperSkill;
uint256 totalPrize = defenderBet + _credBet;
@> uint256 random =
@> uint256(keccak256(abi.encodePacked(block.timestamp, block.prevrandao, msg.sender))) % totalBattleSkill;
// Reset the defender
Impact
An attacker can precompute valid "random" numbers to win every battle.
Tools Used
Manual analysis.
Recommendations
Consider using a decentralized oracle for the generation of random numbers, such as Chainlinks VRF.
Rewards breakdown
nSLOC
201This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
Live
The contest is live. Earn rewards by submitting a finding.
Appeals
This is your time to appeal against judgements on your submissions.
Appeals Review
Appeals are being carefully reviewed by our judges.