Duplicate NFTs Can Be Minted if `payableToken` Has a Callback Attached to it

[code423n4]

Lines of code

https://github.com/code-423n4/2022-03-joyn/blob/main/core-contracts/contracts/CoreCollection.sol#L139-L167 https://github.com/code-423n4/2022-03-joyn/blob/main/core-contracts/contracts/ERC721Payable.sol#L50-L56

Vulnerability details

Impact

The mintToken() function is called to mint unique tokens from an ERC721 collection. This function will either require users to provide a merkle proof to claim an airdropped token or pay a fee in the form of a payableToken. However, because the payableToken is paid before a token is minted, it may be possible to reenter the mintToken() function if there is a callback attached before or after the token transfer. Because totalSupply() has not been updated for the new token, a user is able to bypass the totalSupply() + amount <= maxSupply check. As a result, if the user mints the last token, they can reenter and mint duplicate NFTs as the way tokenId is generated will wrap around to the start again.

Proof of Concept

For the sake of this example, let's say startingIndex = 0 and maxSupply = 100. tokenId is minted according to ((startingIndex + totalSupply()) % maxSupply) + 1. If we see that a user mints a token where totalSupply() = maxSupply - 1 = 99 and they reenter the function, then the next token to mint will actually be of index 1 as totalSupply() % maxSupply = 0. Calculating the first tokenId, we get ((0 + 0) % maxSupply) + 1 = 1 which is a duplicate of our example.

Recommended Mitigation Steps

Consider adding reentrancy protections to prevent users from abusing this behaviour. It may also be useful to follow the checks-effects pattern such that all external/state changing calls are made at the end.

[sofianeOuafir]

This is an issue we intend to investigate and fix if indeed it is an issue

[deluca-mike]

This is a valid high risk issue. Also, for reference, the checks-effects-interactions (CEI) pattern suggests you, in this order:

• perform checks that something can be done
• perform the effects (update storage and emit events)
• interact with other functions/contracts (since you may not be sure they will call out and re-enter)