ZongZi Exploit Analysis

Intro

Lost : $223K

Attack txs : https://phalcon.blocksec.com/explorer/tx/bsc/0x247f4b3dbde9d8ab95c9766588d80f8dae835129225775ebd05a6dd2c69cd79f

Time:2024-03-25

Detail

先标记下hacker 的合约

line4 zongzi 在pancake 里的 wbnb有 1500000000000000000000

闪电贷借wbnb 1904347826086956521739

把wbnb全部转到另一个合约，把它mark成hacker_contract2,真正的攻击过程都在这里

接下来就是一系列的兑换

op type	wbnb	zongzi
wbnb-zongzi	100000000000000000	224422567899502736268038175462
zongzi-wbnb	81203796500469234	224422567899502736268038175462
wbnb-zongzi	1904247826086956521739	1871659197312641953023279714603473
call 3 func	ㅤ	858377721424360720681539559296354
zongzi-wbnb	861167498663157296581	858377721424360720681539559296354

进来wbnb：1904347826086956521739

出去wbnb：2158209956815657765815

白白的多了，问题就处在那三个函数调用上面


    function burnToHolder(uint256 amount,address _invitation) external {
        require(amount >= 0, "TeaFactory: insufficient funds");

        address sender = _msgSender();
        if(Invitation[sender] == address(0) && _invitation != address(0) && _invitation != sender){
            Invitation[sender] = _invitation;
            InvitationList[_invitation].add(sender);//add hacker addr
        }
        if (!userList.contains(sender)) {
            userList.add(sender);
        }
        address[] memory path = new address[](2);
        path[0] = address(_burnToken);
        path[1] = uniswapRouter.WETH();
        uint256 deserved = 0;
        deserved = uniswapRouter.getAmountsOut(amount, path)[path.length - 1];
        require(payable(address(_burnToken)).balance>=deserved,'not enough balance');
        _burnToken.zongziToholder(sender, amount, deserved);//burntoken is zongzi
        _BurnTokenToDead(sender,amount);
        burnFeeRewards(sender,deserved);
    }

问题出在这行上面，错误的用getAmountsOut计算了金额。忽略了swap会存在slippage。直接用

getAmountsOut计算出的deserved会比正常swap大。


        deserved = uniswapRouter.getAmountsOut(amount, path)[path.length - 1];

最后转给bnb到hacker cotract2


    function receiveRewards(address payable to) external {
        address addr = msg.sender;
        uint256 balance = balanceOf(addr);
        uint256 amount = balance.sub(burnAmount[addr]); //.sub(Rewards[addr]);
        require(amount > 0 );
        Rewards[addr] = Rewards[addr].add(amount);
        historyRewards[addr] = historyRewards[addr].add(amount);
        to.transfer(amount.mul(10**9)); // pay hacker 1296.961254356 bnb
        _transfer(addr, address(this), balance);
        burnAmount[addr]=0;
        totalReceive = totalReceive.add(amount);
        emit ReceiveReward(addr, amount, totalReceive);
    }

End

错误的计算swap金额是此次被黑的元凶。还是要懂uniswap的原理再开发，千万不要用getAmountsOut计算