【智能合约审计】————2、GEMCHAIN

合约地址：https://etherscan.io/address/0xfb340423dfac531b801d7586c98fe31e12a32f31#code

pragma solidity ^0.4.16;
//指定编译器版本，符合推荐做法

interface tokenRecipient { function receiveApproval(address _from, uint256 _value, address _token, bytes _extraData) public; }

contract GEMCHAIN {
    // 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;
    
    mapping(address=>bool) public frozenAccount;
    uint256 public rate = 30000 ;//1 ether=how many tokens
    uint256 public amount; 
    
    address public owner;
    bool public fundOnContract=true;    
    bool public contractStart=true;  
    bool public exchangeStart=true;

    // 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);

    /**
     * Constrctor function
     *
     * Initializes contract with initial supply tokens to the creator of the contract
     */
     
    modifier  onlyOwner{
        if(msg.sender != owner){
            revert();
        }else{
            _;
        }
    }

    function transferOwner(address newOwner)  public onlyOwner{
        owner = newOwner;
    }
     

     
    function GEMCHAIN() public payable{
        decimals=18;
        totalSupply = 10000000000 * (10 ** uint256(decimals));  // Update total supply with the decimal amount
        balanceOf[msg.sender] = totalSupply;                // Give the creator all initial tokens
        name = "GEMCHAIN";                                   // Set the name for display purposes
        symbol = "GEM";                               // Set the symbol for display purposes
        owner = msg.sender;
        rate=30000;
        fundOnContract=true;
        contractStart=true;
        exchangeStart=true;
    }

    /**
     * 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;   //先减后加符合推荐规范
        // 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 {
        if(!contractStart){
            revert();
        }
        _transfer(msg.sender, _to, _value);
    }

    /**
     * Transfer tokens from other address
     *
     * Send `_value` tokens to `_to` on 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) {
        if(!contractStart){
            revert();
        }
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        require(_value > 0);     // 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 on 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) {
        if(!contractStart){
            revert();
        }
        require(balanceOf[msg.sender] >= _value);
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    /**
     * Set allowance for other address and notify
     *
     * Allows `_spender` to spend no more than `_value` tokens on 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) {
        if(!contractStart){
            revert();
        }
        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) {
        if(!contractStart){
            revert();
        }
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough
        require(_value > 0);
        balanceOf[msg.sender] -= _value;            // Subtract from the sender
        totalSupply -= _value;                      // Updates totalSupply
        Transfer(msg.sender, 0, _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 onlyOwner returns (bool success) {
        require(balanceOf[_from] >= _value);                // Check if the targeted balance is enough
        require(_value> 0); 
        balanceOf[_from] -= _value;                         // Subtract from the targeted balance
        totalSupply -= _value;                              // Update totalSupply
        Transfer(_from, 0, _value);
        return true;
    }
    
    function () public payable{
        if(!contractStart){
            revert();
        }
        if(frozenAccount[msg.sender]){
            revert();
        }
        if(rate <= 0){
            revert();
        }
        amount = uint256(msg.value * rate);
        
        if(balanceOf[msg.sender]+amount0){
            if(exchangeStart){
                balanceOf[owner] -=amount ;
                balanceOf[msg.sender] +=amount;
                Transfer(owner, msg.sender, amount); //token event
            }
            if(!fundOnContract){
                owner.transfer(msg.value);
            }
        //}
    }

    function transferFund(address target,uint256 _value) public onlyOwner{
        if(frozenAccount[target]){
            revert();
        }
        if(_value<=0){
            revert();
        }
        if(_value>this.balance){
            revert();
        }
        if(target != 0){
            target.transfer(_value);
        }
    }
    
    
    function setFundOnContract(bool _fundOnContract)  public onlyOwner{
            fundOnContract = _fundOnContract;
    }
    
    function setContractStart(bool _contractStart)  public onlyOwner{
            contractStart = _contractStart;
    }
    
    function freezeAccount(address target,bool _bool)  public onlyOwner{
        if(target != 0){
            frozenAccount[target] = _bool;
        }
    }
    function setRate(uint thisRate) public onlyOwner{
       if(thisRate>0){
         rate = thisRate;
        }
    }
    //此处存在整数溢出，导致合约的owner可以随意铸币，管理员调用铸币函数给某个地址增加token的时候，
    //利用溢出漏洞可以突破该函数的只能增加token的限制，实际减少该地址的token数量，从而实现对任一
    //账户余额的任意篡改（增加或减少）。
    
    function mintToken(address target, uint256 mintedAmount) public onlyOwner {
        balanceOf[target] += mintedAmount;
        totalSupply += mintedAmount;
        Transfer(0, owner, mintedAmount);
        Transfer(owner, target, mintedAmount);
    }
    function ownerKill(address target) public onlyOwner {
        selfdestruct(target);
    }
    function withdraw(address target) public onlyOwner {
        target.transfer(this.balance);
    }
    function getBalance() public constant returns(uint) {
        return this.balance;
    }
    
    
    function setExchangeStart(bool _exchangeStart)  public onlyOwner{
            exchangeStart = _exchangeStart;
    }
}

在mintToken函数处存在整数溢出可导致“随意铸币”

管理员调用铸币函数给某个地址增加token的时候，利用溢出漏洞可以突破该函数的只能增加token的限制，实际减少该地址的token数量，从而实现对任一账户余额的任意篡改（增加或减少）。

下面使用Remix进行演示

首先部署该智能合约

之后进行铸币

再次铸币提交可以导致溢出