solidity-security

What this skill does

# Solidity Security

Master smart contract security best practices, vulnerability prevention, and secure Solidity development patterns.

## When to Use This Skill

- Writing secure smart contracts
- Auditing existing contracts for vulnerabilities
- Implementing secure DeFi protocols
- Preventing reentrancy, overflow, and access control issues
- Optimizing gas usage while maintaining security
- Preparing contracts for professional audits
- Understanding common attack vectors

## Critical Vulnerabilities

### 1. Reentrancy
Attacker calls back into your contract before state is updated.

**Vulnerable Code:**
```solidity
// VULNERABLE TO REENTRANCY
contract VulnerableBank {
    mapping(address => uint256) public balances;

    function withdraw() public {
        uint256 amount = balances[msg.sender];

        // DANGER: External call before state update
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success);

        balances[msg.sender] = 0;  // Too late!
    }
}
```

**Secure Pattern (Checks-Effects-Interactions):**
```solidity
contract SecureBank {
    mapping(address => uint256) public balances;

    function withdraw() public {
        uint256 amount = balances[msg.sender];
        require(amount > 0, "Insufficient balance");

        // EFFECTS: Update state BEFORE external call
        balances[msg.sender] = 0;

        // INTERACTIONS: External call last
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}
```

**Alternative: ReentrancyGuard**
```solidity
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract SecureBank is ReentrancyGuard {
    mapping(address => uint256) public balances;

    function withdraw() public nonReentrant {
        uint256 amount = balances[msg.sender];
        require(amount > 0, "Insufficient balance");

        balances[msg.sender] = 0;

        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}
```

### 2. Integer Overflow/Underflow

**Vulnerable Code (Solidity < 0.8.0):**
```solidity
// VULNERABLE
contract VulnerableToken {
    mapping(address => uint256) public balances;

    function transfer(address to, uint256 amount) public {
        // No overflow check - can wrap around
        balances[msg.sender] -= amount;  // Can underflow!
        balances[to] += amount;          // Can overflow!
    }
}
```

**Secure Pattern (Solidity >= 0.8.0):**
```solidity
// Solidity 0.8+ has built-in overflow/underflow checks
contract SecureToken {
    mapping(address => uint256) public balances;

    function transfer(address to, uint256 amount) public {
        // Automatically reverts on overflow/underflow
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}
```

**For Solidity < 0.8.0, use SafeMath:**
```solidity
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

contract SecureToken {
    using SafeMath for uint256;
    mapping(address => uint256) public balances;

    function transfer(address to, uint256 amount) public {
        balances[msg.sender] = balances[msg.sender].sub(amount);
        balances[to] = balances[to].add(amount);
    }
}
```

### 3. Access Control

**Vulnerable Code:**
```solidity
// VULNERABLE: Anyone can call critical functions
contract VulnerableContract {
    address public owner;

    function withdraw(uint256 amount) public {
        // No access control!
        payable(msg.sender).transfer(amount);
    }
}
```

**Secure Pattern:**
```solidity
import "@openzeppelin/contracts/access/Ownable.sol";

contract SecureContract is Ownable {
    function withdraw(uint256 amount) public onlyOwner {
        payable(owner()).transfer(amount);
    }
}

// Or implement custom role-based access
contract RoleBasedContract {
    mapping(address => bool) public admins;

    modifier onlyAdmin() {
        require(admins[msg.sender], "Not an admin");
        _;
    }

    function criticalFunction() public onlyAdmin {
        // Protected function
    }
}
```

### 4. Front-Running

**Vulnerable:**
```solidity
// VULNERABLE TO FRONT-RUNNING
contract VulnerableDEX {
    function swap(uint256 amount, uint256 minOutput) public {
        // Attacker sees this in mempool and front-runs
        uint256 output = calculateOutput(amount);
        require(output >= minOutput, "Slippage too high");
        // Perform swap
    }
}
```

**Mitigation:**
```solidity
contract SecureDEX {
    mapping(bytes32 => bool) public usedCommitments;

    // Step 1: Commit to trade
    function commitTrade(bytes32 commitment) public {
        usedCommitments[commitment] = true;
    }

    // Step 2: Reveal trade (next block)
    function revealTrade(
        uint256 amount,
        uint256 minOutput,
        bytes32 secret
    ) public {
        bytes32 commitment = keccak256(abi.encodePacked(
            msg.sender, amount, minOutput, secret
        ));
        require(usedCommitments[commitment], "Invalid commitment");
        // Perform swap
    }
}
```

## Security Best Practices

### Checks-Effects-Interactions Pattern
```solidity
contract SecurePattern {
    mapping(address => uint256) public balances;

    function withdraw(uint256 amount) public {
        // 1. CHECKS: Validate conditions
        require(amount <= balances[msg.sender], "Insufficient balance");
        require(amount > 0, "Amount must be positive");

        // 2. EFFECTS: Update state
        balances[msg.sender] -= amount;

        // 3. INTERACTIONS: External calls last
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}
```

### Pull Over Push Pattern
```solidity
// Prefer this (pull)
contract SecurePayment {
    mapping(address => uint256) public pendingWithdrawals;

    function recordPayment(address recipient, uint256 amount) internal {
        pendingWithdrawals[recipient] += amount;
    }

    function withdraw() public {
        uint256 amount = pendingWithdrawals[msg.sender];
        require(amount > 0, "Nothing to withdraw");

        pendingWithdrawals[msg.sender] = 0;
        payable(msg.sender).transfer(amount);
    }
}

// Over this (push)
contract RiskyPayment {
    function distributePayments(address[] memory recipients, uint256[] memory amounts) public {
        for (uint i = 0; i < recipients.length; i++) {
            // If any transfer fails, entire batch fails
            payable(recipients[i]).transfer(amounts[i]);
        }
    }
}
```

### Input Validation
```solidity
contract SecureContract {
    function transfer(address to, uint256 amount) public {
        // Validate inputs
        require(to != address(0), "Invalid recipient");
        require(to != address(this), "Cannot send to contract");
        require(amount > 0, "Amount must be positive");
        require(amount <= balances[msg.sender], "Insufficient balance");

        // Proceed with transfer
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}
```

### Emergency Stop (Circuit Breaker)
```solidity
import "@openzeppelin/contracts/security/Pausable.sol";

contract EmergencyStop is Pausable, Ownable {
    function criticalFunction() public whenNotPaused {
        // Function logic
    }

    function emergencyStop() public onlyOwner {
        _pause();
    }

    function resume() public onlyOwner {
        _unpause();
    }
}
```

## Gas Optimization

### Use `uint256` Instead of Smaller Types
```solidity
// More gas efficient
contract GasEfficient {
    uint256 public value;  // Optimal

    function set(uint256 _value) public {
        value = _value;
    }
}

// Less efficient
contract GasInefficient {
    uint8 public value;  // Still uses 256-bit slot

    function set(uint8 _value) public {
        value = _value;  // Extra gas for type conversion
    }
}
```

### Pack Storage Variables
```solidity
// Gas efficient (3 variables in 1 slot)
contract PackedStorage {
    uint128 public