Nouns Builder contest - arcoun's results

Lines of code

https://github.com/code-423n4/2022-09-nouns-builder/blob/main/src/token/Token.sol#L101-L119 https://github.com/code-423n4/2022-09-nouns-builder/blob/main/src/token/Token.sol#L130-L136

Vulnerability details

Impact

When initializing founders in _addFounders, base token IDs, which should be numbers between 0 and 99, are distributed to founders, based on ownershipPct. Each founder should have as many base token ID than the ownershipPct value.

With specific distributions, some base tokens IDs may be higher than 99. In that case, the founder will be unable to get them because the _isForFounder method will calculate the base token ID using baseTokenId = _tokenId % 100 (https://github.com/code-423n4/2022-09-nouns-builder/blob/main/src/token/Token.sol#L179)

There are two issues in the code regarding this finding:

• The _getNextTokenId method does not check if the returned token is between 0 and 99. If all tokens between the initial _tokenId argument and 99 have already been distributed, the return value will be higher than 99.
• The code used to update the baseTokenId in the loop is (baseTokenId += schedule) % 100. The % 100 operation is executed but not applied to baseTokenId. The baseTokenId variable can still be higher than 99 after this instruction.

These issues may lead to an unfair distribution amongst founders.

Proof of Concept

• Initialize the founders will the following ownershipPct: [11%, 12%, 13%]
• The distribution of base token ID will be:
  • Founder-1: [0, 9, 18, 27, 36, 45, 54, 63, 72, 81, 90]
  • Founder-2: [1, 10, 19, 28, 37, 46, 55, 64, 73, 82, 91, 99]
  • Founder-3: [2, 11, 20, 29, 38, 47, 56, 65, 74, 83, 92, 100, 107]
• When tokens will later be minted, base token ID 100 and 107 will never be distributed to Founder-3 by _isForFounder.
• Founder-3 will only have 11 valid base token ID. Founder-3 will have less tokens than Founder-2 although he has a higher ownershipPct.

Recommended Mitigation Steps

The _getNextTokenId method should at least be modified to apply a mod 100 operation for each potential token ID.