代币变泡沫，以太坊Hexagon溢出漏洞比狗庄还过分

背景

目前发现受影响合约地址：

成因分析

问题出现在_transfer函数当中，当调用transfer转币时，会调用_transfer函数：

/* Send tokens */
function transfer(address _to, uint256 _value) public returns (bool success) {
    _transfer(msg.sender, _to, _value);
    return true;
}

_value可控，burnPerTransaction为常量，当_value + burnPerTransaction溢出时为0，可以导致绕过验证。

/* INTERNAL */
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 + burnPerTransaction);
    /* Check for overflows */
    require (balanceOf[_to] + _value > balanceOf[_to]);
    /* Subtract from the sender */
    balanceOf[_from] -= _value + burnPerTransaction;  
    /* Add the same to the recipient */
    balanceOf[_to] += _value;
    /* Apply transaction fee */
    balanceOf[0x0] += burnPerTransaction;
    /* Update current supply */
    currentSupply -= burnPerTransaction;
    /* Notify network */
    Burn(_from, burnPerTransaction);
    /* Notify network */
    Transfer(_from, _to, _value);
}

string public constant name = "Hexagon";
string public constant symbol = "HXG";
uint8 public constant decimals = 4;
uint8 public constant burnPerTransaction = 2;

漏洞利用

合约中 burnPerTransaction = 2 ，
所以当转账_value为0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe时，
_value + burnPerTransaction =0 ，即可成功攻击，为balanceOf[_to]增加大量代币。

require (balanceOf[_from] >= _value + burnPerTransaction);   
// require (balanceOf[_from] >= 0);   

[....]

require (balanceOf[_to] + _value > balanceOf[_to]);   
balanceOf[_from] -= _value + burnPerTransaction;  
// balanceOf[_from]-=0
balanceOf[_to] += _value;    
//balanceOf[_to]+=fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe

可参考下图Transcations，目前生成的代币已销毁：

总结

建议使用 SafeMath 来处理计算操作，避免溢出。同时，以太坊智能合约有很多开源合约，使用参考开源合约前，应对代码进行安全审计。