MonoX攻击事件相关信息
在Ethereum和Polygon网络都发生了,攻击手段相同,以Ethereum为例进行分析:
-
攻击者地址:MonoX Finance Exploiter | Address 0xecbe385f78041895c311070f344b55bfaa953258 | Etherscan
-
攻击合约:Contract Address 0xf079d7911c13369e7fd85607970036d2883afcfd | Etherscan
-
攻击交易:Ethereum Transaction Hash (Txhash) Details | Etherscan
-
漏洞合约:Monoswap | Address 0x66e7d7839333f502df355f5bd87aea24bac2ee63 | Etherscan
Monox代码分析及攻击流程讲解
Monox介绍:
与Uniswap不同,其使用的是单边代币池模型,其使用vCash稳定币与AMM提供的代币创建虚拟的交易对
。Monox创建的是代币-vCash交易对,添加流动性的时候,只需添加代币,进行任意代币兑换,兑换方式为:代币A -- vCash -- 代币B
。
攻击原理及过程:
极大地提高Monoswap中Mono代币的价格,后将拥有的Mono代币通过Monoswap换取代币。
具体步骤,查看phalcon上攻击交易的调用序列进行分析
- 前置阶段
- 首先调用
WETH
的deposit()
函数,向WETH中存入0.1WETH - 随后调用
approve()
函数,向Monoswap进行授权,以便后续代币兑换正常进行(在foundry中写测试函数时,很容易遗忘approve这点) - 随后调用Monoswap的
swapExactTokenForToken()
函数,将0.1个WETH换成一定数量的Mono(该函数如何实现,可见漏洞合约Monoswap) - 调用Monoswap的
pools()
函数,具体后续介绍,获得Mono代币在Monoswap中的pid - 根据pid调用Monoxpool中的
totalSupplyOf()
函数,查询Mono-vCash池子中作为LP流动性证明的Mono总量。
- 移除用户流动性
在Monox的官方界面可以看到给Mono代币提供代币流动的用户地址,这里从交易序列中可以很明显发现一个漏洞,别的用户的流动性,攻击者竟然可以任意移除
在Monoswap源码中可以很明显发现,并没有流动性所有者进行相应的校验
function _removeLiquidity (address _token, uint256 liquidity,address to) view public returns(uint256 poolValue, uint256 liquidityIn, uint256 vcashOut, uint256 tokenOut) {require (liquidity>0, "MonoX:BAD_AMOUNT");uint256 tokenBalanceVcashValue;uint256 vcashCredit;uint256 vcashDebt;PoolInfo memory pool = pools[_token];IMonoXPool monoXPoolLocal = monoXPool;uint256 lastAdded = monoXPoolLocal.liquidityLastAddedOf(pool.pid, msg.sender);require((lastAdded + (pool.status == PoolStatus.OFFICIAL ? 4 hours : pool.status == PoolStatus.LISTED ? 24 hours : 0)) <= block.timestamp, "MonoX:WRONG_TIME"); // Users are not allowed to remove liquidity right after addingaddress topLPHolder = monoXPoolLocal.topLPHolderOf(pool.pid);require(pool.status != PoolStatus.LISTED || msg.sender != topLPHolder || pool.createdAt + 90 days < block.timestamp, "MonoX:TOP_HOLDER & WRONG_TIME"); // largest LP holder is not allowed to remove LP within 90 days after pool creation(poolValue, tokenBalanceVcashValue, vcashCredit, vcashDebt) = getPool(_token);uint256 _totalSupply = monoXPool.totalSupplyOf(pool.pid);liquidityIn = monoXPool.balanceOf(to, pool.pid)>liquidity?liquidity:monoXPool.balanceOf(to, pool.pid);uint256 tokenReserve = IERC20(_token).balanceOf(address(monoXPool));if(tokenReserve < pool.tokenBalance){tokenBalanceVcashValue = tokenReserve.mul(pool.price)/1e18;}if(vcashDebt>0){tokenReserve = (tokenBalanceVcashValue.sub(vcashDebt)).mul(1e18).div(pool.price);}// if vcashCredit==0, vcashOut will be 0 as wellvcashOut = liquidityIn.mul(vcashCredit).div(_totalSupply);tokenOut = liquidityIn.mul(tokenReserve).div(_totalSupply);}
攻击者发现三个主要提供流动性的用户,先调用Monoxpool的balanceOf()
函数查看地址在Monoswap中的Mono数量,后调用移除流动性函数,使得池子中的Mono为0.
- 添加流动性
攻击者自己添加极少的Mono代币到Monoswap中,获得927个LP,为后续拉升Mono的价格做准备
- 拉高Mono代币在Monoswap中的价格
攻击交易中,重复了55次上述行为
先是调用Monoswap中的pools()
函数,从中我们可以看出solidity中这种mapping映射的获得,是通过调用函数的形式活动,可以看一下该函数返回的函数类型:
mapping (address => PoolInfo) public pools;struct PoolInfo {uint256 pid;uint256 lastPoolValue;address token;PoolStatus status;uint112 vcashDebt;uint112 vcashCredit;uint112 tokenBalance;uint256 price; // over 1e18uint256 createdAt; // timestamp}
这里重点关注的是我们可以通过调用该函数获得该代币在Monoswap中的tokenBalance
余额和price
当前价格,攻击交易这里主要想获得池子中的tokenBalance余额。
随后查看攻击者先前用0.1个WETH兑换的Mono代币的余额,即还剩多少个
随后最关键的步骤调用Monoswap的swapExactTokenForToken()
函数,这个函数的功能与uniswap很像,顾名思义,将精准数量的代币兑换成一定数量的另一种代币,这里我们能够很明显发现,参数tokenIn
和tokenOut
都是Mono,这就是攻击手段!
所以肯定是该函数中存在漏洞,导致Mono代币价格的拉高。进入函数中看一下。
function swapExactTokenForToken(address tokenIn,address tokenOut,uint amountIn,uint amountOutMin,address to,uint deadline) external virtual ensure(deadline) returns (uint amountOut) {amountOut = swapIn(tokenIn, tokenOut, msg.sender, to, amountIn);require(amountOut >= amountOutMin, 'MonoX:INSUFF_OUTPUT');}function swapIn (address tokenIn, address tokenOut, address from, address to,uint256 amountIn) internal lockToken(tokenIn) returns(uint256 amountOut) {address monoXPoolLocal = address(monoXPool);amountIn = transferAndCheck(from,monoXPoolLocal,tokenIn,amountIn); // uint256 halfFeesInTokenIn = amountIn.mul(fees)/2e5;uint256 tokenInPrice;uint256 tokenOutPrice;uint256 tradeVcashValue;(tokenInPrice, tokenOutPrice, amountOut, tradeVcashValue) = getAmountOut(tokenIn, tokenOut, amountIn);uint256 oneSideFeesInVcash = tokenInPrice.mul(amountIn.mul(fees)/2e5)/1e18;// trading inif(tokenIn==address(vCash)){vCash.burn(monoXPoolLocal, amountIn);// all fees go to the other sideoneSideFeesInVcash = oneSideFeesInVcash.mul(2);}else{_updateTokenInfo(tokenIn, tokenInPrice, 0, tradeVcashValue.add(oneSideFeesInVcash), 0);}// trading outif(tokenOut==address(vCash)){vCash.mint(to, amountOut);}else{if (to != monoXPoolLocal) {IMonoXPool(monoXPoolLocal).safeTransferERC20Token(tokenOut, to, amountOut);}_updateTokenInfo(tokenOut, tokenOutPrice, tradeVcashValue.add(oneSideFeesInVcash), 0, to == monoXPoolLocal ? amountOut : 0);}if(pools[tokenIn].vcashDebt > 0 && pools[tokenIn].status == PoolStatus.OFFICIAL){_internalRebalance(tokenIn);}emit Swap(to, tokenIn, tokenOut, amountIn, amountOut, tradeVcashValue);}
swapIn函数较复杂,我们可以从后往前看,看到它有个_updateTokenInfo()
函数,更新token的信息,看一下源码
function _updateTokenInfo (address _token, uint256 _price,uint256 _vcashIn, uint256 _vcashOut, uint256 _ETHDebt) internal {uint256 _balance = IERC20(_token).balanceOf(address(monoXPool));_balance = _balance.sub(_ETHDebt);require(pools[_token].status!=PoolStatus.PAUSED,"MonoX:PAUSED");require(_balance <= uint112(-1));(uint initialPoolValue, , ,) = getPool(_token);pools[_token].tokenBalance = uint112(_balance);pools[_token].price = _price;// record last trade's block number in mapping: lastTradedBlocklastTradedBlock[_token] = block.number;_updateVcashBalance(_token, _vcashIn, _vcashOut);(uint poolValue, , ,) = getPool(_token);require(initialPoolValue <= poolValue || poolValue >= poolSizeMinLimit,"MonoX:MIN_POOL_SIZE");}
从代码中我们可以看出,将Monoswap池子中代币的数量和价格更新,其中代币的价格就是函数参数的tokenInPrice
和tokenOutPrice
,这两个参数都是通过getAmountOut()
函数计算得到,进入该函数,分析源码:
function getAmountOut(address tokenIn, address tokenOut, uint256 amountIn) public view returns (uint256 tokenInPrice, uint256 tokenOutPrice, uint256 amountOut, uint256 tradeVcashValue) {require(amountIn > 0, 'MonoX:INSUFF_INPUT');uint256 amountInWithFee = amountIn.mul(1e5-fees)/1e5;address vcashAddress = address(vCash);uint tokenInPoolPrice = pools[tokenIn].price;uint tokenInPoolTokenBalance = pools[tokenIn].tokenBalance;if(tokenIn==vcashAddress){tradeVcashValue = amountInWithFee;tokenInPrice = 1e18;}else{require (tokenPoolStatus[tokenIn]==1, "MonoX:NO_POOL");// PoolInfo memory tokenInPool = pools[tokenIn];PoolStatus tokenInPoolStatus = pools[tokenIn].status;require (tokenInPoolStatus != PoolStatus.UNLISTED, "MonoX:POOL_UNLST");tokenInPrice = _getNewPrice(tokenInPoolPrice, tokenInPoolTokenBalance, amountInWithFee, 0, TxType.SELL);tradeVcashValue = _getAvgPrice(tokenInPoolPrice, tokenInPrice).mul(amountInWithFee)/1e18;}if(tokenOut==vcashAddress){amountOut = tradeVcashValue;tokenOutPrice = 1e18;}else{require (tokenPoolStatus[tokenOut]==1, "MonoX:NO_POOL");// PoolInfo memory tokenOutPool = pools[tokenOut];PoolStatus tokenOutPoolStatus = pools[tokenOut].status;uint tokenOutPoolPrice = pools[tokenOut].price;uint tokenOutPoolTokenBalance = pools[tokenOut].tokenBalance;require (tokenOutPoolStatus != PoolStatus.UNLISTED, "MonoX:POOL_UNLST");amountOut = tradeVcashValue.add(tokenOutPoolTokenBalance.mul(tokenOutPoolPrice).div(1e18));amountOut = tradeVcashValue.mul(tokenOutPoolTokenBalance).div(amountOut);bool allowDirectSwap=directSwapAllowed(tokenInPoolPrice,tokenOutPoolPrice,tokenInPoolTokenBalance,tokenOutPoolTokenBalance,tokenOutPoolStatus,true);// assuming p1*p2 = k, equivalent to uniswap's x * y = kuint directSwapTokenOutPrice = allowDirectSwap?tokenInPoolPrice.mul(tokenOutPoolPrice).div(tokenInPrice):uint(-1);// prevent the attack where user can use a small pool to update price in a much larger pooltokenOutPrice = _getNewPrice(tokenOutPoolPrice, tokenOutPoolTokenBalance, amountOut, 0, TxType.BUY);tokenOutPrice = directSwapTokenOutPrice < tokenOutPrice?directSwapTokenOutPrice:tokenOutPrice;amountOut = tradeVcashValue.mul(1e18).div(_getAvgPrice(tokenOutPoolPrice, tokenOutPrice));}}
通过上述代码可以得到,tokenInPrice
和tokenOutPrice
参数的计算都是通过_getNewPrice()
函数,得到函数源码
function _getNewPrice (uint256 originalPrice, uint256 reserve, uint256 delta, uint256 deltaBlocks, TxType txType) pure internal returns(uint256 price) {if(txType==TxType.SELL) {// no risk of being div by 0price = originalPrice.mul(reserve)/(reserve.add(delta));}else{ // BUYprice = originalPrice.mul(reserve).div(reserve.sub(delta));}}
通过,我们可以发现tokenIn代币,其TxType为SELL,tokenOut代币其Txtype为BUY。
故可分析,tokenIn代表先进行价格更新计算,originalPrice和reserve都是池子中原来保存的参数,其不会发生变动,相较于originalPrice价格,tokenInPrice变低了。
分析_getAvgPrice()
函数,我们进一步可以分析得到trashVcashValue也变低了,其与toknInPrice呈相同趋势。
function _getAvgPrice (uint256 originalPrice, uint256 newPrice) pure internal returns(uint256 price) {price = originalPrice.add(newPrice.mul(4))/5;}
随后,getAmountOut()
函数正常执行,计算tokenOut代币的相关信息,分析_getNewPrice()
函数,肯定可以得到的一个结论是相比于originalPrice也就是池子中代币的价格,tokenOutPrice变高的。
这时可以不用管其它参数的变化,这里最大的问题,就是这种同种代币的兑换,在swapIn()
函数中,其先对tokenIn进行处理,更新代币相应的信息,但其后对tokenOut进行处理时,没有考虑前后兑换为同一种代币的情况,导致代币的价格被覆盖。
从上述分析中,可得到tokenOut的价格被抬升,tokenIn价格降低,但Mono的价格在兑换时,被覆盖,导致Mono价格异常增长。
对phalcon中兑换交易的参数分析可得,每次兑换的数量都是交易池中Mono的总量减去1,使得_getNewPrice()
函数计算tokenOutPrice时,能够快速提升价格,这里也就不能理解第3步中添加流动性的时候,添加很少的Mono,确保攻击者有足够的余额拉高mono的价格。
- 转移非法资产
攻击者先通过Monoswap查看池子中USDC的价格和余额,随后通过uniswap的USDC/WETH池接入WETH,乐观转账,在uniswapV2call()
函数中调用Monoswap的swapTokenForExactToken()
函数,将价格极高的Mono代币,换成一定数量的USDB,用以偿还uniswap闪电贷中的USDC(在uniswap闪电贷中,其可以通过还对应的pair代币),这样就将高价格的Mono代币转换成了对应的WETH(可以注意一下phalcon上这里的USDC数字,应该只是6位小数)。
随后的资产转移方式相同。