ERC721R Bug

Abstract

Stauts	Fixed
Type	Contract
Date	Apr 12, 2022
Source	@BenWAGMI
Direct Loss	None. It was reported in the early stage.
Project Repo	https://github.com/erc721r/ERC721R

What is NFT721R?

An NFT protocol enabling minters to return the minted NFT for a refund in a certain period.

Issue

In the typical case, the NFT dev calls this withdraw() function after refundEndTime to withdraw the Eth raised from minting. This step is OK.

function withdraw() external onlyOwner {
    require(block.timestamp > refundEntime), "Refund period not over");
    uint256 balance = address(this).balance;
    Address.sendValue(payable(owner)), blance);
}

But, check the refund function: Minter calls this func to return the NFTs he minted to the refundAddress(an address set and controlled by dev) then gets the corresponding amount of $ETH back from the NFT contract. But what if refundAddress is a minter holding one of the NFTs?

function refund(uint256 [] calldata tokenIds) external {
    require(refundGuaranteeActive(), "Refund expired");

    for (uint256 i = 0; i < tokenIds.length; i++) {
        uint256 tokenId = tokenIds[i];
        require(msg.sender == ownerOf(tokenId), "Not token owner");
        transferFrom(msg.sender, refundAddress, tokenId);
    }

    uint256 refundAmount = tokenIds.length * mintPrice;
    Address.sendValue(payable(msg.sender)), refundAmount);
}

A scam dev will set a refundAddress, then mint an NFT with this refundAddress. Next step, he calls refund(). Because the NFT will always return to refundAddress, he still possesses that NFT while collecting some amount of Eth. He can do it multiple times until all funds run out.