Submitted by ladboy233, also found by CodingNameKiki, adriro, StErMi, Tricko, rbserver, 0xmrhoodie, 0x4non, and ElKu
https://github.com/rabbitholegg/quest-protocol/blob/8c4c1f71221570b14a0479c216583342bd652d8d/contracts/Quest.sol#L113
Let us look closely into the Quest.sol#claim function
After the NFT is used to claim, the _setClaimed(token) is called to mark the NFT as used to prevent double claiming.
The NFT is also tradeable in the secondary marketplace. I would like to make a reasonable assumption that user wants to buy the NFT because they can use the NFT to claim the reward, which means after the reward is claimed, the NFT lose value.
Consider the case below:
User A can intentionally front-run User Bs buy transaction by monitoring the mempool in polygon using the service
https://www.blocknative.com/blog/polygon-mempool
Or it could be just two users submit transactions at the same time and User As claim transaction happens to execute first.
Disable NFT transfer and trade once the NFT is used to claim the reward.
waynehoover (RabbitHole) acknowledged
