【智能合约审计】————4、LoliCoin
合约地址：https://etherscan.io/address/0x326ce88b27c196d0387f5df7ae085c1203185cb4#code
pragma solidity ^0.4.18;//hps//>=0.4.0 合约无fallback在收到eth时会抛出异常并回退
//hps//指明编译器版本符合推荐做法
/**
* LoliCoin
* Copyright (c) 2017 MING-CHIEN LEE
*/
contract owned {
address public owner;
//hps //虽然这里构造函数处理正确，但是建议还是使用construct来命名构造函数为好
function owned() public {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
//hps //只有合约的拥有者才可以更换合约的拥有者，符合规范
function transferOwnership(address newOwner) onlyOwner public {
owner = newOwner;
}
}
interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) public; }
contract TokenERC20 {
// Public variables of the token
string public name;
string public symbol;
uint8 public decimals = 18;
// 18 decimals is the strongly suggested default, avoid changing it
uint256 public totalSupply;
// This creates an array with all balances
mapping (address => uint256) public balanceOf;
mapping (address => mapping (address => uint256)) public allowance;
// This generates a public event on the blockchain that will notify clients
event Transfer(address indexed from, address indexed to, uint256 value);
// This notifies clients about the amount burnt
event Burn(address indexed from, uint256 value);
/**
* Constrctor function
*
* Initializes contract with initial supply tokens to the creator of the contract
*/
function TokenERC20() public {
totalSupply = 1000000000 * 10 ** uint256(decimals); // Update total supply with the decimal amount
balanceOf[msg.sender] = totalSupply; // Give the creator all initial tokens
name = "LoliCoin"; // Set the name for display purposes
symbol = "LOLI"; // Set the symbol for display purposes
}
/**
* Internal transfer, only can be called by this contract
*/
function _transfer(address _from, address _to, uint _value) internal {
// Prevent transfer to 0x0 address. Use burn() instead
require(_to != 0x0);
// Check if the sender has enough
require(balanceOf[_from] >= _value);
// Check for overflows
require(balanceOf[_to] + _value > balanceOf[_to]);
// Save this for an assertion in the future
uint previousBalances = balanceOf[_from] + balanceOf[_to];
// Subtract from the sender
balanceOf[_from] -= _value; //hps //先减后加符合推荐做法
// Add the same to the recipient
balanceOf[_to] += _value;
Transfer(_from, _to, _value);
// Asserts are used to use static analysis to find bugs in your code. They should never fail
assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
}
/**
* Transfer tokens
*
* Send `_value` tokens to `_to` from your account
*
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transfer(address _to, uint256 _value) public {
_transfer(msg.sender, _to, _value);
}
/**
* Transfer tokens from other address
*
* Send `_value` tokens to `_to` in behalf of `_from`
*
* @param _from The address of the sender
* @param _to The address of the recipient
* @param _value the amount to send
*/
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
require(_value <= allowance[_from][msg.sender]); // Check allowance
allowance[_from][msg.sender] -= _value;
_transfer(_from, _to, _value);
return true;
}
/**
* Set allowance for other address
*
* Allows `_spender` to spend no more than `_value` tokens in your behalf
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
*/
function approve(address _spender, uint256 _value) public
returns (bool success) {
allowance[msg.sender][_spender] = _value;
return true;
}
/**
* Set allowance for other address and notify
*
* Allows `_spender` to spend no more than `_value` tokens in your behalf, and then ping the contract about it
*
* @param _spender The address authorized to spend
* @param _value the max amount they can spend
* @param _extraData some extra information to send to the approved contract
*/
function approveAndCall(address _spender, uint256 _value, bytes _extraData)
public
returns (bool success) {
tokenRecipient spender = tokenRecipient(_spender);
if (approve(_spender, _value)) {
spender.receiveApproval(msg.sender, _value, this, _extraData);
return true;
}
}
/**
* Destroy tokens
*
* Remove `_value` tokens from the system irreversibly
*
* @param _value the amount of money to burn
*/
function burn(uint256 _value) public returns (bool success) {
require(balanceOf[msg.sender] >= _value); // Check if the sender has enough
balanceOf[msg.sender] -= _value; // Subtract from the sender
totalSupply -= _value; // Updates totalSupply
Burn(msg.sender, _value);
return true;
}
/**
* Destroy tokens from other account
*
* Remove `_value` tokens from the system irreversibly on behalf of `_from`.
*
* @param _from the address of the sender
* @param _value the amount of money to burn
*/
function burnFrom(address _from, uint256 _value) public returns (bool success) {
require(balanceOf[_from] >= _value); // Check if the targeted balance is enough
require(_value <= allowance[_from][msg.sender]); // Check allowance
balanceOf[_from] -= _value; // Subtract from the targeted balance
allowance[_from][msg.sender] -= _value; // Subtract from the sender's allowance
totalSupply -= _value; // Update totalSupply
Burn(_from, _value);
return true;
}
}
/******************************************/
/* LOLICOIN STARTS HERE */
/******************************************/
contract LoliCoin is owned, TokenERC20 {
mapping (address => bool) public frozenAccount;
/* This generates a public event on the blockchain that will notify clients */
event FrozenFunds(address target, bool frozen);
/* Initializes contract with initial supply tokens to the creator of the contract */
function LoliCoin() TokenERC20() public {}
/* Internal transfer, only can be called by this contract */
function _transfer(address _from, address _to, uint _value) internal {
require (_to != 0x0); // Prevent transfer to 0x0 address. Use burn() instead
require (balanceOf[_from] > _value); // Check if the sender has enough
require (balanceOf[_to] + _value > balanceOf[_to]); // Check for overflows
require(!frozenAccount[_from]); // Check if sender is frozen
require(!frozenAccount[_to]); // Check if recipient is frozen
balanceOf[_from] -= _value; // Subtract from the sender
balanceOf[_to] += _value; // Add the same to the recipient
Transfer(_from, _to, _value);
}
/// @notice Create `mintedAmount` tokens and send it to `target`
/// @param target Address to receive the tokens
/// @param mintedAmount the amount of tokens it will receive
//hps //这里存在整数溢出，可导致合约的拥有者“随意铸币”
function mintToken(address target, uint256 mintedAmount) onlyOwner public {
balanceOf[target] += mintedAmount;
totalSupply += mintedAmount;
Transfer(0, this, mintedAmount);
Transfer(this, target, mintedAmount);
}
/// @notice `freeze? Prevent | Allow` `target` from sending & receiving tokens
/// @param target Address to be frozen
/// @param freeze either to freeze it or not
function freezeAccount(address target, bool freeze) onlyOwner public {
frozenAccount[target] = freeze;
FrozenFunds(target, freeze);
}
}存在问题的代码：
漏洞概述：合约拥有者在调用铸币函数给某个地址增加token时，可以利用溢出漏洞突破该函数只能增加token的限制，从而减少该地址的token数量，从而实现对任一账户余额的任意篡改（增加或减少）。
漏洞危害：合约拥有者可以绕过合约限制，任意篡改所有地址的token余额。
漏洞证明：
部署该合约
铸币前地址"0x14723a09acff6d2a60dcdf7aa4aff308fddc160c"的balance为0：
对地址"0x14723a09acff6d2a60dcdf7aa4aff308fddc160c"进行铸币：
0x8000000000000000000000000000000000000000000000000000000000000000
"0x14723a09acff6d2a60dcdf7aa4aff308fddc160c"
之后的账户balance为：
再次铸币，数量和之前一样
之后再次查看地址balance数量
可以发现balance回归为0了，合约的管理员可以对任意账户进行“任意铸币”操作。