Integer Bugs

Arithmetic

Integer overflow and underflow were common vulnerabilities before Solidity 0.8.0. Understanding these bugs is still important for auditing older contracts and unchecked blocks.

Overflow/Underflow Examples

integer-bugs.sol
solidity 
// Pre-Solidity 0.8.0 - No automatic checks

// OVERFLOW: uint8 max is 255
uint8 x = 255;
x = x + 1;  // x becomes 0!

// UNDERFLOW
uint8 y = 0;
y = y - 1;  // y becomes 255!

// Real-world impact: Token balance manipulation
contract VulnerableToken {
    mapping(address => uint256) balances;
    
    // VULNERABLE: Underflow on insufficient balance
    function transfer(address to, uint256 amount) public {
        balances[msg.sender] -= amount;  // Underflows if amount > balance
        balances[to] += amount;
    }
}

// Solidity 0.8.0+ - Automatic overflow checks
// But unchecked blocks bypass this:
unchecked {
    uint8 x = 255;
    x++;  // Still overflows to 0
}

Mitigation

integer-mitigation.sol
solidity 
// Pre-0.8.0: Use SafeMath library
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

contract SafeContract {
    using SafeMath for uint256;
    
    function transfer(address to, uint256 amount) public {
        balances[msg.sender] = balances[msg.sender].sub(amount);
        balances[to] = balances[to].add(amount);
    }
}

// Solidity 0.8.0+: Built-in checks
// Use unchecked blocks carefully when gas optimization needed

// Auditing checklist:
// 1. Check Solidity version
// 2. Search for unchecked blocks
// 3. Look for custom math libraries
// 4. Check multiplication before division
// 5. Validate user input ranges

Related Topics

Reentrancy

Access Control