First Flight #11: Snek-Raffle

Summary

NFTs of different rarity have the same chance of getting minted due to a bad logic implementation:
rarity: uint256 = random_words[0] % 3

Vulnerability Details

from this line of code: rarity: uint256 = random_words[0] % 3
We can deduce that the chances of getting each of the 3 types of NFTs are equal (33.333...% each), unlike what the documentation stated:
70% of chance of getting a common NFT
25% of chance of getting a rare NFT
5% of chance of getting a legendary NFT
The winner will have equal chances of getting these NFTs

Impact

The protocol will have to mint more rare and ledengary NFTs than they expected, and less common NFTs.

Tools Used

Manual Analysis

Recommendations

Make the following changes:

• rarity: uint256 = random_words[0] % 3

• percentage: uint256 = random_words[0] % 100

• rarity: uint256 = 0

• if percentage < LEGEND_RARITY:

• rarity = 2
  

• elif percentage < RARE_RARITY:

• rarity = 1