Smart Contract Security: 4. Integer Overflow

Jul 25, 2023#Web3 #Blockchain #NFT #DeFi #DAO #DApp #区块链 #區塊鏈 #加密货币 #DeGame #链游 #Crypto #比特币 #以太坊 #Ethereum #合约开发334

AI Translation

This post is translated from Chinese into English through AI.View Original

AI-generated summary

Integer overflow occurs when the result of an arithmetic operation exceeds the maximum value that can be stored in a variable. In Solidity, integers are default to 256 bits. If the result of an addition operation exceeds 256 bits, it will overflow. This can lead to unexpected results and can be exploited by hackers. To prevent integer overflow, it is recommended to use safe math libraries like SafeMath, implement overflow checks for critical variables, and avoid calling functions that are prone to overflow in loops. The provided code example demonstrates both bad and good practices for handling integer overflow.

What is integer overflow?

Integers in Solidity are default to 256 bits. When performing addition operations, if the result exceeds 256 bits, an overflow occurs. This can lead to unexpected results and can be exploited by hackers.

For example, User A deposits 10 tokens into a contract, and the contract uses an unchecked uint256 variable called balance to calculate the token balance. Then, a hacker passes in a very large value, causing an overflow in the calculation of balance, and the balance is reset to 0, allowing the hacker to extract tokens indefinitely.

Methods to prevent integer overflow:

Use secure math libraries to avoid direct calls to overflow-prone operations, such as Safemath.
Implement overflow checks for critical variables, and verify the new value before making changes.
Avoid calling functions that are prone to overflow in loops. Limit the number of times users can call them.

Code example:

// Bad
uint256 balance;

function add(uint256 value) external {
  balance += value; 
}

// Good
using SafeMath for uint256;

uint256 balance;

function add(uint256 value) external {
  balance = balance.add(value);
  require(balance >= value, "Overflow detected");
}