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Reentrancy in ERC721

Description

Users can mint up to 20 NFTs at a time. The Masks contract manages the information of these NFTs, with a total supply limit of 20 NFTs as defined by MAX_NFT_SUPPLY.

Expected Usage

  • Users can call mintNFT() to mint NFTs, with a maximum of 20 NFTs per transaction.

Attack

External Call

The call to IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, data) in the _safeMint() function triggers the user's function within the Masks contract.

Cause of Attack

The Masks contract checks the number of minting NFTs at the beginning of the function. The totalSupply() uses _tokenOwners.length() managed by EnumerableMap.UintToAddressMap. _tokenOwners.length() is updated when tokenOwners.set(tokenId, to); is called in _mint(). Thus, totalSupply() is updated for each NFT just before sending.

Reentrant Target

  1. The attacker calls mintNFT(20).
  2. Suppose totalSupply() is N.
  3. The _mint() function updates _tokenOwners, making totalSupply() N+1.
  4. The _checkOnERC721Received function calls onERC721Received() in the attacker contract.
    • The attacker calls mintNFT(20) again.
    • At this moment, totalSupply() is N+1.
    • This allows minting 20 more NFTs since the check uses N+1+20, but it should check N+20+20.
    • Repeat similarly to mint more than 20 NFTs in one transaction, exceeding the contract's minting limit.

Prevention

  • ReentrancyGuard: Implement a ReentrancyGuard to prevent reentrant calls.
  • Use a single variable to track totalSupply.

By adopting them, reentrancy attacks in ERC721 contracts can be prevented, ensuring the integrity of the minting process and adherence to supply limits.

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