Source: wshobson/agents Original Plugin: blockchain-web3

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 a;  // Slot 0
    uint64 public b;   // Slot 0
    uint64 public c;   // Slot 0
    uint256 public d;  // Slot 1
}

// Gas inefficient (each variable in separate slot)
contract UnpackedStorage {
    uint256 public a;  // Slot 0
    uint256 public b;  // Slot 1
    uint256 public c;  // Slot 2
    uint256 public d;  // Slot 3
}

Use `calldata` Instead of `memory` for Function Arguments

SOLIDITY

contract GasOptimized {
    // More gas efficient
    function processData(uint256[] calldata data) public pure returns (uint256) {
        return data[0];
    }

    // Less efficient
    function processDataMemory(uint256[] memory data) public pure returns (uint256) {
        return data[0];
    }
}

Use Events for Data Storage (When Appropriate)

SOLIDITY

contract EventStorage {
    // Emitting events is cheaper than storage
    event DataStored(address indexed user, uint256 indexed id, bytes data);

    function storeData(uint256 id, bytes calldata data) public {
        emit DataStored(msg.sender, id, data);
        // Don't store in contract storage unless needed
    }
}

Common Vulnerabilities Checklist

SOLIDITY

// Security Checklist Contract
contract SecurityChecklist {
    /**
     * [ ] Reentrancy protection (ReentrancyGuard or CEI pattern)
     * [ ] Integer overflow/underflow (Solidity 0.8+ or SafeMath)
     * [ ] Access control (Ownable, roles, modifiers)
     * [ ] Input validation (require statements)
     * [ ] Front-running mitigation (commit-reveal if applicable)
     * [ ] Gas optimization (packed storage, calldata)
     * [ ] Emergency stop mechanism (Pausable)
     * [ ] Pull over push pattern for payments
     * [ ] No delegatecall to untrusted contracts
     * [ ] No tx.origin for authentication (use msg.sender)
     * [ ] Proper event emission
     * [ ] External calls at end of function
     * [ ] Check return values of external calls
     * [ ] No hardcoded addresses
     * [ ] Upgrade mechanism (if proxy pattern)
     */
}

Testing for Security

JAVASCRIPT

// Hardhat test example
const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("Security Tests", function () {
    it("Should prevent reentrancy attack", async function () {
        const [attacker] = await ethers.getSigners();

        const VictimBank = await ethers.getContractFactory("SecureBank");
        const bank = await VictimBank.deploy();

        const Attacker = await ethers.getContractFactory("ReentrancyAttacker");
        const attackerContract = await Attacker.deploy(bank.address);

        // Deposit funds
        await bank.deposit({value: ethers.utils.parseEther("10")});

        // Attempt reentrancy attack
        await expect(
            attackerContract.attack({value: ethers.utils.parseEther("1")})
        ).to.be.revertedWith("ReentrancyGuard: reentrant call");
    });

    it("Should prevent integer overflow", async function () {
        const Token = await ethers.getContractFactory("SecureToken");
        const token = await Token.deploy();

        // Attempt overflow
        await expect(
            token.transfer(attacker.address, ethers.constants.MaxUint256)
        ).to.be.reverted;
    });

    it("Should enforce access control", async function () {
        const [owner, attacker] = await ethers.getSigners();

        const Contract = await ethers.getContractFactory("SecureContract");
        const contract = await Contract.deploy();

        // Attempt unauthorized withdrawal
        await expect(
            contract.connect(attacker).withdraw(100)
        ).to.be.revertedWith("Ownable: caller is not the owner");
    });
});

Audit Preparation

SOLIDITY

contract WellDocumentedContract {
    /**
     * @title Well Documented Contract
     * @dev Example of proper documentation for audits
     * @notice This contract handles user deposits and withdrawals
     */

    /// @notice Mapping of user balances
    mapping(address => uint256) public balances;

    /**
     * @dev Deposits ETH into the contract
     * @notice Anyone can deposit funds
     */
    function deposit() public payable {
        require(msg.value > 0, "Must send ETH");
        balances[msg.sender] += msg.value;
    }

    /**
     * @dev Withdraws user's balance
     * @notice Follows CEI pattern to prevent reentrancy
     * @param amount Amount to withdraw in wei
     */
    function withdraw(uint256 amount) public {
        // CHECKS
        require(amount <= balances[msg.sender], "Insufficient balance");

        // EFFECTS
        balances[msg.sender] -= amount;

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

Resources

references/reentrancy.md: Comprehensive reentrancy prevention
references/access-control.md: Role-based access patterns
references/overflow-underflow.md: SafeMath and integer safety
references/gas-optimization.md: Gas saving techniques
references/vulnerability-patterns.md: Common vulnerability catalog
assets/solidity-contracts-templates.sol: Secure contract templates
assets/security-checklist.md: Pre-audit checklist
scripts/analyze-contract.sh: Static analysis tools

Tools for Security Analysis

Slither: Static analysis tool
Mythril: Security analysis tool
Echidna: Fuzzing tool
Manticore: Symbolic execution
Securify: Automated security scanner

Common Pitfalls

Using tx.origin for Authentication: Use msg.sender instead
Unchecked External Calls: Always check return values
Delegatecall to Untrusted Contracts: Can hijack your contract
Floating Pragma: Pin to specific Solidity version
Missing Events: Emit events for state changes
Excessive Gas in Loops: Can hit block gas limit
No Upgrade Path: Consider proxy patterns if upgrades needed