Author : Kai Song(exp-sky)、hearmen、salt、sekaiwu of Tencent Security Xuanwu Lab
“盗币”
十一月六日，我们观察到以太坊上出现了这样一份合约，经调查发现是某区块链安全厂商发布的一份让大家来“盗币”的合约。
pragma solidity ^0.4.21;
contract DVPgame {
ERC20 public token;
uint256[] map;
using SafeERC20 for ERC20;
using SafeMath for uint256;
constructor(address addr) payable{
token = ERC20(addr);
}
function (){
if(map.length>=uint256(msg.sender)){
require(map[uint256(msg.sender)]!=1);
}
if(token.balanceOf(this)==0){
//airdrop is over
selfdestruct(msg.sender);
}else{
token.safeTransfer(msg.sender,100);
if (map.length <= uint256(msg.sender)) {
map.length = uint256(msg.sender) + 1;
}
map[uint256(msg.sender)] = 1;
}
}
//Guess the value(param:x) of the keccak256 value modulo 10000 of the future block (param:blockNum)
function guess(uint256 x,uint256 blockNum) public payable {
require(msg.value == 0.001 ether || token.allowance(msg.sender,address(this))>=1*(10**18));
require(blockNum>block.number);
if(token.allowance(msg.sender,address(this))>0){
token.safeTransferFrom(msg.sender,address(this),1*(10**18));
}
if (map.length <= uint256(msg.sender)+x) {
map.length = uint256(msg.sender)+x + 1;
}
map[uint256(msg.sender)+x] = blockNum;
}
//Run a lottery
function lottery(uint256 x) public {
require(map[uint256(msg.sender)+x]!=0);
require(block.number > map[uint256(msg.sender)+x]);
require(block.blockhash(map[uint256(msg.sender)+x])!=0);
uint256 answer = uint256(keccak256(block.blockhash(map[uint256(msg.sender)+x])))%10000;
if (x == answer) {
token.safeTransfer(msg.sender,token.balanceOf(address(this)));
selfdestruct(msg.sender);
}
}
}
经过观察之后，我们在这个合约中，发现了我们之前研究的一个 EVM 存储的安全问题，即 EVM 存储中的 hash 碰撞问题。