Blockchain Smart Contract Security¶

Common Vulnerabilities¶

1. Reentrancy¶

The most famous vulnerability (DAO hack).

// VULNERABLE
function withdraw() public {
    uint amount = balances[msg.sender];
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success);
    balances[msg.sender] = 0;  // Too late!
}

// SAFE: Checks-Effects-Interactions pattern
function withdraw() public {
    uint amount = balances[msg.sender];
    balances[msg.sender] = 0;  // Update first
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success);
}

2. Integer Overflow/Underflow¶

// Solidity 0.8+ has built-in checks
// For older versions, use SafeMath

// VULNERABLE (pre-0.8)
function transfer(uint amount) public {
    balances[msg.sender] -= amount;  // Can underflow!
}

3. Access Control¶

// VULNERABLE
function setOwner(address newOwner) public {
    owner = newOwner;  // Anyone can call!
}

// SAFE
modifier onlyOwner() {
    require(msg.sender == owner, "Not owner");
    _;
}

function setOwner(address newOwner) public onlyOwner {
    owner = newOwner;
}

4. Front-Running¶

// VULNERABLE: Price can be front-run
function buyToken() public payable {
    uint amount = msg.value * price;
    token.transfer(msg.sender, amount);
}

// SAFE: Commit-reveal scheme
function commitBuy(bytes32 commitment) public {
    commitments[msg.sender] = commitment;
}

function revealBuy(uint amount, bytes32 secret) public {
    require(keccak256(abi.encode(amount, secret)) == commitments[msg.sender]);
    // Process buy
}

Best Practices¶

1. Use OpenZeppelin¶

import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract MyContract is Ownable, ReentrancyGuard {
    function withdraw() public nonReentrant {
        // Safe from reentrancy
    }
}

2. Gas Optimization¶

// Bad: Multiple storage reads
function sum() public view returns (uint) {
    return values[0] + values[1] + values[2];
}

// Good: Cache in memory
function sum() public view returns (uint) {
    uint[3] memory v = values;
    return v[0] + v[1] + v[2];
}

3. Event Logging¶

event Transfer(address indexed from, address indexed to, uint amount);

function transfer(address to, uint amount) public {
    // ... logic
    emit Transfer(msg.sender, to, amount);
}

Security Checklist¶

• [ ] Reentrancy guards on external calls
• [ ] Integer overflow checks (or Solidity 0.8+)
• [ ] Access control on sensitive functions
• [ ] Input validation
• [ ] Event emission for state changes
• [ ] Gas limit considerations
• [ ] External contract interaction safety
• [ ] Upgrade mechanism security (if applicable)

Testing Tools¶