NFT Security

// VULNERABLE: Predictable token IDs allow front-running
contract VulnerableNFT is ERC721 {
    uint256 public nextTokenId;
    
    function mint() public payable {
        _mint(msg.sender, nextTokenId);  // Predictable!
        nextTokenId++;
    }
}

// VULNERABLE: Metadata can be changed post-mint
contract ChangeableNFT is ERC721 {
    mapping(uint256 => string) private _tokenURIs;
    
    function setTokenURI(uint256 tokenId, string memory uri) public {
        // No ownership check!
        _tokenURIs[tokenId] = uri;
    }
}

// VULNERABLE: Reentrancy in mint
contract ReentrableNFT is ERC721 {
    function mint(uint256 quantity) public payable {
        for (uint i = 0; i < quantity; i++) &#123;
            _safeMint(msg.sender, nextId++);
            // safeMint calls onERC721Received - reentrancy!
        &#125;
    &#125;
&#125;

// Signature replay attacks
// - Listing signatures can be replayed if not properly invalidated
// - Cross-chain replay (same sig works on multiple chains)

// OpenSea Wyvern exploit (2022)
// - Users had old listings they forgot about
// - Attacker bought NFTs at old (low) prices

// Approval attacks
// - Marketplace contracts need setApprovalForAll
// - If marketplace is compromised, all approved NFTs at risk

// Testing checklist:
// 1. Check signature validation
// 2. Look for replay protection (nonces, deadlines)
// 3. Verify ownership checks
// 4. Test metadata mutability
// 5. Check royalty enforcement
// 6. Test listing cancellation