// ❌ WRONG — assumes 18 decimals. Transfers 1 TRILLION USDC.
uint256 oneToken = 1e18;

// ✅ CORRECT — check decimals
uint256 oneToken = 10 ** IERC20Metadata(token).decimals();

// Converting USDC amount to 18-decimal internal accounting
uint256 normalized = usdcAmount * 1e12; // 6 + 12 = 18 decimals

// ❌ WRONG — this equals 0
uint256 fivePercent = 5 / 100;

// ✅ CORRECT — basis points (1 bp = 0.01%)
uint256 FEE_BPS = 500; // 5% = 500 basis points
uint256 fee = (amount * FEE_BPS) / 10_000;

// ❌ WRONG — loses precision
uint256 result = a / b * c;

// ✅ CORRECT — multiply first
uint256 result = (a * c) / b;

// ❌ VULNERABLE — state updated after external call
function withdraw() external {
    uint256 bal = balances[msg.sender];
    (bool success,) = msg.sender.call{value: bal}(""); // ← attacker re-enters here
    require(success);
    balances[msg.sender] = 0; // Too late — attacker already withdrew again
}

// ✅ SAFE — Checks-Effects-Interactions pattern + reentrancy guard
import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";

function withdraw() external nonReentrant {
    uint256 bal = balances[msg.sender];
    require(bal > 0, "Nothing to withdraw");
    
    balances[msg.sender] = 0;  // Effect BEFORE interaction
    
    (bool success,) = msg.sender.call{value: bal}("");
    require(success, "Transfer failed");
}

// ❌ WRONG — breaks with USDT and other non-standard tokens
token.transfer(to, amount);
token.approve(spender, amount);

// ✅ CORRECT — handles all token implementations
import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
using SafeERC20 for IERC20;

token.safeTransfer(to, amount);
token.safeApprove(spender, amount);

// ❌ DANGEROUS — manipulable in one transaction
function getPrice() internal view returns (uint256) {
    (uint112 reserve0, uint112 reserve1,) = uniswapPair.getReserves();
    return (reserve1 * 1e18) / reserve0; // Spot price — easily manipulated
}

// ✅ SAFE — Chainlink with staleness + sanity checks
function getPrice() internal view returns (uint256) {
    (, int256 price,, uint256 updatedAt,) = priceFeed.latestRoundData();
    require(block.timestamp - updatedAt < 3600, "Stale price");
    require(price > 0, "Invalid price");
    return uint256(price);
}

function convertToShares(uint256 assets) public view returns (uint256) {
    return assets.mulDiv(
        totalSupply() + 1e3,    // Virtual shares
        totalAssets() + 1        // Virtual assets
    );
}

// ❌ DANGEROUS — if this contract is exploited, attacker drains your entire balance
token.approve(someContract, type(uint256).max);

// ✅ SAFE — approve only what's needed
token.approve(someContract, exactAmountNeeded);

// ✅ ACCEPTABLE — approve a small multiple for repeated interactions
token.approve(someContract, amountPerTx * 5); // 5 transactions worth

import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";

// ❌ WRONG — anyone can drain the contract
function emergencyWithdraw() external {
    token.transfer(msg.sender, token.balanceOf(address(this)));
}

// ✅ CORRECT — only owner
function emergencyWithdraw() external onlyOwner {
    token.transfer(owner(), token.balanceOf(address(this)));
}

function deposit(uint256 amount, address recipient) external {
    require(amount > 0, "Zero amount");
    require(recipient != address(0), "Zero address");
    require(amount <= maxDeposit, "Exceeds max");
    
    // Now proceed
}

1. You submit: swap 10 ETH → USDC on Uniswap (slippage 1%)
2. Attacker sees your tx in the mempool
3. Attacker frontruns: buys USDC before you → price rises
4. Your swap executes at a worse price (but within your 1% slippage)
5. Attacker backruns: sells USDC after you → profits from the price difference
6. You got fewer USDC than the true market price

// ✅ Set explicit minimum output — don't set amountOutMinimum to 0
ISwapRouter.ExactInputSingleParams memory params = ISwapRouter
    .ExactInputSingleParams({
        tokenIn: WETH,
        tokenOut: USDC,
        fee: 3000,
        recipient: msg.sender,
        amountIn: 1 ether,
        amountOutMinimum: 1900e6, // ← Minimum acceptable USDC (protects against sandwich)
        sqrtPriceLimitX96: 0
    });

// Implementation contract (the logic)
import {UUPSUpgradeable} from "@openzeppelin/contracts-upgradeable/proxy/utils/UUPSUpgradeable.sol";
import {Initializable} from "@openzeppelin/contracts-upgradeable/proxy/utils/Initializable.sol";
import {OwnableUpgradeable} from "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";

contract MyContractV1 is Initializable, UUPSUpgradeable, OwnableUpgradeable {
    uint256 public value;

    /// @custom:oz-upgrades-unsafe-allow constructor
    constructor() {
        _disableInitializers(); // Prevent implementation from being initialized
    }

    function initialize(address owner) public initializer {
        __Ownable_init(owner);
        __UUPSUpgradeable_init();
        value = 42;
    }

    function _authorizeUpgrade(address) internal override onlyOwner {}
}

// ❌ WRONG — reordering storage breaks everything
// V1: uint256 a; uint256 b;
// V2: uint256 b; uint256 a;  ← Swapped! 'a' now reads 'b's value

// ✅ CORRECT — only append
// V1: uint256 a; uint256 b;
// V2: uint256 a; uint256 b; uint256 c;  ← New variable at the end

// EIP-712 domain separator — prevents replay across contracts and chains
bytes32 public constant DOMAIN_TYPEHASH = keccak256(
    "EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"
);

bytes32 public constant PERMIT_TYPEHASH = keccak256(
    "Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)"
);

function permit(
    address owner, address spender, uint256 value,
    uint256 deadline, uint8 v, bytes32 r, bytes32 s
) external {
    require(block.timestamp <= deadline, "Permit expired");

    bytes32 structHash = keccak256(abi.encode(
        PERMIT_TYPEHASH, owner, spender, value, nonces[owner]++, deadline
    ));
    bytes32 digest = keccak256(abi.encodePacked(
        "\x19\x01", DOMAIN_SEPARATOR(), structHash
    ));

    address recovered = ecrecover(digest, v, r, s);
    require(recovered == owner, "Invalid signature");

    _approve(owner, spender, value);
}

// ❌ CRITICAL VULNERABILITY — delegatecall to user-supplied address
function execute(address target, bytes calldata data) external {
    target.delegatecall(data); // Attacker can overwrite ANY storage slot
}

// ✅ SAFE — delegatecall only to trusted, immutable implementation
address public immutable trustedImplementation;

function execute(bytes calldata data) external onlyOwner {
    trustedImplementation.delegatecall(data);
}

# Static analysis
slither .                     # Detects common vulnerabilities
mythril analyze Contract.sol  # Symbolic execution

# Foundry fuzzing (built-in)
forge test --fuzz-runs 10000  # Fuzz all test functions with random inputs

# Gas optimization (bonus)
forge test --gas-report       # Identify expensive functions