🤖system prompt•7 months ago
solidity-security
Master smart contract security best practices to prevent common
security
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# 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
1. **Using `tx.origin` for Authentication**: Use `msg.sender` instead
2. **Unchecked External Calls**: Always check return values
3. **Delegatecall to Untrusted Contracts**: Can hijack your contract
4. **Floating Pragma**: Pin to specific Solidity version
5. **Missing Events**: Emit events for state changes
6. **Excessive Gas in Loops**: Can hit block gas limit
7. **No Upgrade Path**: Consider proxy patterns if upgrades needed
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