Submission Details
Severity:
Deterministic game
Summary
The winner and rarity of a raffle is calculated in a deterministic way and not in a random one.
Vulnerability Details
The winner is calculated like this:
uint256 winnerIndex = uint256(
keccak256(
abi.encodePacked(
msg.sender, block.timestamp, block.difficulty)
)
) % players.length;
address winner = players[winnerIndex];
This whole calculation is deterministic, as the block.timestamp and the block.difficulty can be known before executing the function.
The same holds true for the calculation of the NFTs rarity:
uint256 rarity = uint256(
keccak256(abi.encodePacked(msg.sender, block.difficulty))
) % 100;
Impact
Players can calculate the result of calling the selectWinner function and therefore abuse this vulnerability to cheat.
Tools Used
Manual Review
Recommendations
Use chainlinks VRF oracle.
Updates
Lead Judging Commences
Hamiltonite about 2 years ago
Submission Judgement Published Assigned finding tags:
weak-randomness
Root cause: bad RNG Impact: manipulate winner
Rewards breakdown
nSLOC
143This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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