【以太坊开发】BeautyChain (BEC) 溢出漏洞分析

前言
2018年4月23日中午11点30分左右，BEC代币被Hacker攻击。
Hacker利用数据溢出的漏洞攻击与美图合作的公司美链 BEC 的智能合约，成功地向两个地址转出了天量级别的 BEC 代币，导致市场上海量 BEC 被抛售，该数字货币价值几近归零，给 BEC 市场交易带来了毁灭性打击。
下面我们来分析一下此次受攻击的漏洞，作为前车之鉴。

一、Token地址：

https://etherscan.io/address/0xc5d105e63711398af9bbff092d4b6769c82f793d

二、BEC智能合约源码

我们不需要看完以下所有代码，重点关注第三节要分析的内容。

pragma solidity ^0.4.16;

/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 */
library SafeMath {
  function mul(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }

  function div(uint256 a, uint256 b) internal constant returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return c;
  }

  function sub(uint256 a, uint256 b) internal constant returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  function add(uint256 a, uint256 b) internal constant returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}

/**
 * @title ERC20Basic
 * @dev Simpler version of ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/179
 */
contract ERC20Basic {
  uint256 public totalSupply;
  function balanceOf(address who) public constant returns (uint256);
  function transfer(address to, uint256 value) public returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);
}

/**
 * @title Basic token
 * @dev Basic version of StandardToken, with no allowances.
 */
contract BasicToken is ERC20Basic {
  using SafeMath for uint256;

  mapping(address => uint256) balances;

  /**
  * @dev transfer token for a specified address
  * @param _to The address to transfer to.
  * @param _value The amount to be transferred.
  */
  function transfer(address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(_value > 0 && _value <= balances[msg.sender]);

    // SafeMath.sub will throw if there is not enough balance.
    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    Transfer(msg.sender, _to, _value);
    return true;
  }

  /**
  * @dev Gets the balance of the specified address.
  * @param _owner The address to query the the balance of.
  * @return An uint256 representing the amount owned by the passed address.
  */
  function balanceOf(address _owner) public constant returns (uint256 balance) {
    return balances[_owner];
  }
}

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
contract ERC20 is ERC20Basic {
  function allowance(address owner, address spender) public constant returns (uint256);
  function transferFrom(address from, address to, uint256 value) public returns (bool);
  function approve(address spender, uint256 value) public returns (bool);
  event Approval(address indexed owner, address indexed spender, uint256 value);
}


/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * @dev https://github.com/ethereum/EIPs/issues/20
 * @dev Based on code by FirstBlood: https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 */
contract StandardToken is ERC20, BasicToken {

  mapping (address => mapping (address => uint256)) internal allowed;


  /**
   * @dev Transfer tokens from one address to another
   * @param _from address The address which you want to send tokens from
   * @param _to address The address which you want to transfer to
   * @param _value uint256 the amount of tokens to be transferred
   */
  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(_value > 0 && _value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    Transfer(_from, _to, _value);
    return true;
  }

  /**
   * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
   *
   * Beware that changing an allowance with this method brings the risk that someone may use both the old
   * and the new allowance by unfortunate transaction ordering. One possible solution to mitigate this
   * race condition is to first reduce the spender's allowance to 0 and set the desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   * @param _spender The address which will spend the funds.
   * @param _value The amount of tokens to be spent.
   */
  function approve(address _spender, uint256 _value) public returns (bool) {
    allowed[msg.sender][_spender] = _value;
    Approval(msg.sender, _spender, _value);
    return true;
  }

  /**
   * @dev Function to check the amount of tokens that an owner allowed to a spender.
   * @param _owner address The address which owns the funds.
   * @param _spender address The address which will spend the funds.
   * @return A uint256 specifying the amount of tokens still available for the spender.
   */
  function allowance(address _owner, address _spender) public constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
  }
}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
  address public owner;


  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);


  /**
   * @dev The Ownable constructor sets the original `owner` of the contract to the sender
   * account.
   */
  function Ownable() {
    owner = msg.sender;
  }


  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }


  /**
   * @dev Allows the current owner to transfer control of the contract to a newOwner.
   * @param newOwner The address to transfer ownership to.
   */
  function transferOwnership(address newOwner) onlyOwner public {
    require(newOwner != address(0));
    OwnershipTransferred(owner, newOwner);
    owner = newOwner;
  }

}

/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract Pausable is Ownable {
  event Pause();
  event Unpause();

  bool public paused = false;


  /**
   * @dev Modifier to make a function callable only when the contract is not paused.
   */
  modifier whenNotPaused() {
    require(!paused);
    _;
  }

  /**
   * @dev Modifier to make a function callable only when the contract is paused.
   */
  modifier whenPaused() {
    require(paused);
    _;
  }

  /**
   * @dev called by the owner to pause, triggers stopped state
   */
  function pause() onlyOwner whenNotPaused public {
    paused = true;
    Pause();
  }

  /**
   * @dev called by the owner to unpause, returns to normal state
   */
  function unpause() onlyOwner whenPaused public {
    paused = false;
    Unpause();
  }
}

/**
 * @title Pausable token
 *
 * @dev StandardToken modified with pausable transfers.
 **/

contract PausableToken is StandardToken, Pausable {

  function transfer(address _to, uint256 _value) public whenNotPaused returns (bool) {
    return super.transfer(_to, _value);
  }

  function transferFrom(address _from, address _to, uint256 _value) public whenNotPaused returns (bool) {
    return super.transferFrom(_from, _to, _value);
  }

  function approve(address _spender, uint256 _value) public whenNotPaused returns (bool) {
    return super.approve(_spender, _value);
  }
  
  function batchTransfer(address[] _receivers, uint256 _value) public whenNotPaused returns (bool) {
    uint cnt = _receivers.length;
    uint256 amount = uint256(cnt) * _value;
    require(cnt > 0 && cnt <= 20);
    require(_value > 0 && balances[msg.sender] >= amount);

    balances[msg.sender] = balances[msg.sender].sub(amount);
    for (uint i = 0; i < cnt; i++) {
        balances[_receivers[i]] = balances[_receivers[i]].add(_value);
        Transfer(msg.sender, _receivers[i], _value);
    }
    return true;
  }
}

/**
 * @title Bec Token
 *
 * @dev Implementation of Bec Token based on the basic standard token.
 */
contract BecToken is PausableToken {
    /**
    * Public variables of the token
    * The following variables are OPTIONAL vanities. One does not have to include them.
    * They allow one to customise the token contract & in no way influences the core functionality.
    * Some wallets/interfaces might not even bother to look at this information.
    */
    string public name = "BeautyChain";
    string public symbol = "BEC";
    string public version = '1.0.0';
    uint8 public decimals = 18;

    /**
     * @dev Function to check the amount of tokens that an owner allowed to a spender.
     */
    function BecToken() {
      totalSupply = 7000000000 * (10**(uint256(decimals)));
      balances[msg.sender] = totalSupply;    // Give the creator all initial tokens
    }

    function () {
        //if ether is sent to this address, send it back.
        revert();
    }
}

三、漏洞所在处

从上述源码中我们摘出batchTransfer这个函数，如下：

  function batchTransfer(address[] _receivers, uint256 _value) public whenNotPaused returns (bool) {
    uint cnt = _receivers.length;
    uint256 amount = uint256(cnt) * _value; // <==== 败在这个乘法运算
    require(cnt > 0 && cnt <= 20);
    require(_value > 0 && balances[msg.sender] >= amount);

    balances[msg.sender] = balances[msg.sender].sub(amount); // 下面知道用库函数，却粗心在上一步没用
    for (uint i = 0; i < cnt; i++) {
        balances[_receivers[i]] = balances[_receivers[i]].add(_value);
        Transfer(msg.sender, _receivers[i], _value);
    }
    return true;
  }

这个函数的作用就是批量转账，其中我们可以看到有这么一句uint256 amount = uint256(cnt) * _value;，问题就出在这里，复现一下：
1、给·_receivers参数传递两个或更多元素的数组，即_receivers.length>=2了，这里我们假定_receivers.length=2。
2、给_value传一个什么样的值呢，要让_value * 2 = 2 ** 256 + 1刚好溢出，得到这个10 进制数57896044618658097711785492504343953926634992332820282019728792003956564819968，但笔者在Remix测试了一下，直接传这个数是不行的，要转换成16进制，即0x8000000000000000000000000000000000000000000000000000000000000000（8后面63个0），可以用web3.fromDecimal('数字')做一下转换。
两个参数的值如下：

_value = 0x8000000000000000000000000000000000000000000000000000000000000000
_receivers.length=2

这样就超出了amount的值范围，从而导致溢出归0，即amount=0，进而绕过了异常检查语句：

 require(cnt > 0 && cnt <= 20);
require(_value > 0 && balances[msg.sender] >= amount);

从攻击的参数中可以找到：

3、再往下看for循环的加法运算，突破了前面的检查，到达for循环这里就如进入了无人看管的银库，想往自己账号上加多少钱就加多少，于是就有了下面这一幕：
BEC加钱现场：
https://etherscan.io/tx/0xad89ff16fd1ebe3a0a7cf4ed282302c06626c1af33221ebe0d3a470aba4a660f

四、单元测试

针对以上的溢出漏洞，我们再来做个简单的测试：

pragma solidity ^0.4.18;

contract BECTest {
    uint256 _value = 2**255;
    function mulTest() public constant returns (uint) {
        uint256 amount = 2 * _value;
        return amount;
    }
}

在Remix中运行，可以看到运行结果为0.

五、堵漏洞

  function batchTransfer(address[] _receivers, uint256 _value) public whenNotPaused returns (bool) {
    uint cnt = _receivers.length;
    uint256 amount = _value.mul(uint256(cnt)); // <===修改成这样即可
    require(cnt > 0 && cnt <= 20);
    require(_value > 0 && balances[msg.sender] >= amount);

    balances[msg.sender] = balances[msg.sender].sub(amount);
    for (uint i = 0; i < cnt; i++) {
        balances[_receivers[i]] = balances[_receivers[i]].add(_value);
        Transfer(msg.sender, _receivers[i], _value);
    }
    return true;
  }

或者

  function batchTransfer(address[] _receivers, uint256 _value) public whenNotPaused returns (bool) {
    uint cnt = _receivers.length;
    uint256 amount = uint256(cnt) * _value; 
    require(cnt > 0 && cnt <= 20);
    require(_value > 0 && balances[msg.sender] >= amount);
    assert(amount / _value == cnt); // <===加上这么一句即可

    balances[msg.sender] = balances[msg.sender].sub(amount);
    for (uint i = 0; i < cnt; i++) {
        balances[_receivers[i]] = balances[_receivers[i]].add(_value);
        Transfer(msg.sender, _receivers[i], _value);
    }
    return true;
  }

总结

1、BEC的整个合约源码读下来，设计模式很清晰，如图：

2、用两个字来概括导致BEC破产的原因就是粗心。合约编写者并不是没有安全意识，在源码中我们看到SafeMath已经导入了，只是在这关键的一步没有使用SafeMath中的乘法 mul方法，一失足成千古恨。
3、在Solidity语言中最大可以处理256位数字，最大值为2 ** 256-1，再往上就会溢出归0。
4、值得程序员警惕注意的是：在合约中做加减乘除运算时，一定要做溢出检查，使用SafeMath可以防止溢出问题。

参考：
https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol 安全运算库