整数溢出

2018年4月，BeautyChain（BEC）代币上发生了一起涉及整数溢出漏洞的重大事件。该漏洞使攻击者能够凭空生成大量的BEC代币，导致了巨额财务损失，并削弱了该代币的价值。

有关BeautyChain事件的更详细信息，您可以访问以下资源：

该漏洞的概览可以在Tenable的 CVE-2018-10299 页面上找到。
Detailed discussions of the event and its implications on smart contract security are available on PeckShield's blog and in a Medium article.

These sources provide a comprehensive look at how such vulnerabilities can be exploited and the profound impacts they can have on digital assets.

Integer overflow vulnerability (Arithmetic Over/Under Flows) is a classic issue in programming that became less common in Solidity starting from version 0.8 due to the integration of the SafeMath library.

以太坊虚拟机（EVM）为整数设置了固定大小，因此它们只能表示特定范围内的数字。 For example, a uint8 can only represent numbers within the range of [0,255]. 将值 257分配给一个uint8类型变量会导致溢出，从而将值更改为1;同样，将-1赋给 uint8类型的变量会导致下溢，将值更改为 255。

Attackers can exploit this vulnerability: imagine a hacker with a balance of $0 spends $1, their balance could suddenly turn to $2^256 - 1.

易受攻击的合约示例

This example is a simple token contract inspired by the Ethernaut contracts. It features 2 state variables: accounts records each address's balance, supplyCap represents the total token supply.

它包括 3个函数:

constructor(): 初始化总代币供应量。
send(): 用于转移代币的函数。
getBalance(): 用于检查余额的函数。

The vulnerability in the send() function occurs because the check require(accounts[msg.sender] - _amount >= 0); always passes due to integer overflow, allowing users to transfer unlimited tokens.

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.12;

contract Asset {
  mapping(address => uint) accounts;
  uint public supplyCap;

  constructor(uint _startingSupply) {
    accounts[msg.sender] = supplyCap = _startingSupply;
  }

  function send(address _recipient, uint _amount) public returns (bool) {
    require(accounts[msg.sender] - _amount >= 0);
    accounts[msg.sender] -= _amount;
    accounts[_recipient] += _amount;
    return true;
  }

  function getBalance(address _owner) public view returns (uint balance) {
    return accounts[_owner];
  }
}

预防策略

在Solidity 0.8.0之前，在合约中使用SafeMath 库，可以在整数溢出的情况下抛出错误。
在Solidity 0.8.0之后, 内置了SafeMath,使得这类问题变得罕见。然而,开发人员有时候会使用unchecked块来节省gas ，暂时禁用整数溢出检查。 For additional information on this technique and its implications for gas optimization, refer to the Gas Optimization - Unchecked guide. Always ensure there is no potential for integer overflow vulnerabilities when using this approach.

易受攻击的合约示例​

预防策略​

易受攻击的合约示例

预防策略