Erc 721

_safeMint and safeTransferFrom invoke onERC721Received() on the recipient if it's a contract. This is an external call after the token has been minted, creating a reentrancy vector.

Real exploit (HypeBears, Feb 2022): State updated after _safeMint allowed attacker to re-enter and bypass per-address minting limits.

// VULNERABLE: state update after _safeMint
function mintNFT() public {
    require(!addressMinted[msg.sender], "Already minted");
    _safeMint(msg.sender, tokenId);
    addressMinted[msg.sender] = true;  // too late
}

// SAFE: state update before _safeMint
function mintNFT() public {
    require(!addressMinted[msg.sender], "Already minted");
    addressMinted[msg.sender] = true;  // update first
    _safeMint(msg.sender, tokenId);
}

2. On-Chain SVG Stack-Too-Deep

When generating SVG on-chain, the tokenURI or generateSVG function easily hits Solidity's 16-local-variable stack limit. Split SVG generation into multiple helper functions (e.g., _svgBackground, _svgShapes, _svgText) rather than building the entire SVG in one function.

3. Marketplace Metadata attributes Array

The attributes array in NFT metadata JSON is not in the ERC-721 EIP but is the de facto standard used by OpenSea, Blur, and every marketplace. Without it, traits won't display:

{
  "name": "My NFT #1",
  "description": "...",
  "image": "data:image/svg+xml;base64,...",
  "attributes": [
    { "trait_type": "Color", "value": "Blue" },
    { "trait_type": "Rarity", "value": "Rare" }
  ]
}

4. Required Overrides with ERC721Enumerable

When combining ERC721 + ERC721Enumerable, both define _update and _increaseBalance. You must explicitly override them or the contract won't compile:

function _update(address to, uint256 tokenId, address auth) internal override(ERC721, ERC721Enumerable) returns (address) {
    return super._update(to, tokenId, auth);
}

function _increaseBalance(address account, uint128 amount) internal override(ERC721, ERC721Enumerable) {
    super._increaseBalance(account, amount);
}

function supportsInterface(bytes4 interfaceId) public view override(ERC721, ERC721Enumerable) returns (bool) {
    return super.supportsInterface(interfaceId);
}

5. IPFS Base URI Trailing Slash

OpenZeppelin's tokenURI() concatenates _baseURI() + tokenId.toString(). If the base URI is ipfs://QmCID without a trailing slash, token 42 becomes ipfs://QmCID42 instead of ipfs://QmCID/42.