Submission Details
Severity:
'snek_raffle.vy::fulfillRandomWords' does not correctly calculate the rarity of the winners NFT
Summary
The rarity for the 3 different possible snek NFT's is not being calculated correctly.
Vulnerability Details
The rarity of the winners NFT is supposed to be weighted with a 70% chance for a Brown Snek, 25% chance for a Jungle Snek, and 5% chance for a Cosmic Snek. But the rarity is being calculated as an equal 33% chance to get any of the 3 different sneks.
rarity: uint256 = random_words[0] % 3
self.tokenIdToRarity[ERC721._total_supply()] = rarity
Impact
Each of the 3 different NFT varieties have an equal chance of being won.
Tools Used
--Manual Review
Recommendations
Change the way the rarity is calculated so that the percentage chance of getting the 3 different NFT's is correct.
@internal
def fulfillRandomWords(request_id: uint256, random_words: uint256[MAX_ARRAY_SIZE]):
index_of_winner: uint256 = random_words[0] % len(self.players)
recent_winner: address = self.players[index_of_winner]
self.recent_winner = recent_winner
self.players = []
self.raffle_state = RaffleState.OPEN
self.last_timestamp = block.timestamp
- rarity: uint256 = random_words[0] % 3
- self.tokenIdToRarity[ERC721._total_supply()] = rarity
+ rarity: uint256 = random_words[0] % 100
+ if (rarity < 5) {
+ self.tokenIdToRarity[ERC721._total_supply()] = 2
+ }
+ else if (rarity < 25)
+ {
+ self.tokenIdToRarity[ERC721._total_supply()] = 1
+ }
+ else {
+ self.tokenIdToRarity[ERC721._total_supply()] = 0
+ }
log WinnerPicked(recent_winner)
ERC721._mint(recent_winner, ERC721._total_supply())
send(recent_winner, self.balance)
Updates
Lead Judging Commences
inallhonesty over 1 year ago
Submission Judgement Published Assigned finding tags:
Rarity is 1/3 instead of what the docs say
Rewards breakdown
nSLOC
136This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
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