ETH:[BlockSec DeFi 攻擊分析系列之二] 傾囊相送：Sushiswap 手續費被盜

去中心化金融(DeFi)作為區塊鏈生態當紅項目形態，其安全尤為重要。從去年至今，發生了幾十起安全事件

BlockSec作為長期關注DeFi安全的研究團隊(https://blocksecteam

_toWETH函數：

function_toWETH(addresstoken)internalreturns(uint256){??//對于SUSHI，直接Sushimaker全部SUSHI轉給Bar??if(token==sushi){????uintamount=IERC20(token)

看出什么亮點了嗎？

SushiMaker里的轉賬邏輯都是：transfer(balanceOf(this))

我們可以分兩個階段來看convert的調用過程：第一個階段：在?convert函數中SushiMaker擁有的是?SLP，它通過burnbalanceOf(this)實現將全部SLP換成兩個底層代幣第二個階段：SushiMaker獲得了burn來的底層代幣，再拿burn得到的token去不同的交易池換wETH

你發現哪里有問題了嗎？我們來看第二階段的代碼_toWETH：注意兌換的邏輯為：

uintamount=IERC20(token).balanceOf(address(this));

所以這個balanceOf(address(this))真的只有在第一階段burn來的底層代幣嗎？Nononono~想一想，如果這里的token并不是底層代幣，而是一個SLP，會發生什么？

IERC20(SLP).balanceOf(address(this))是SushiMaker擁有的所有SLP，既包含剛剛burn出來的，也包含原本SushiMaker就有的

如果底層代幣是SLP，那第一階段burn的是什么？答案是?SLP的SLP！

還記得嗎？攻擊者的目標是將SushiMaker誘騙到其創建的惡意交易池中交易，這個SLP1/WETH池不恰好是攻擊者創建的，當攻擊者調用convert(SLP1,wETH)時，SushiMaker由于上面這一漏洞會將其全部的SLP1都到SLP1/WETH池中換取WETH——正中下懷

SLP/WETH池一般都是不存在的，或是深度很淺的。攻擊者其實也可以通過建立SLP1/SLP2來實現攻擊，但是這樣的話，還需要單獨為SLP1、SLP2建立SLP1/WETH，SLP2/WETH，不如直接一步到位

1.4RealWorld

管你看沒看懂，繼續看就完了下面我們來看看攻擊者在真實世界中到底做了什么吧？

攻擊者地址：0x1925e832C22522E0d9947eE4677120b2f28E4cD4(https://etherscan.io/address/0x1925e832c22522e0d9947ee4677120b2f28e4cd4)

一組攻擊包括很多筆交易，這里以對?MKR/WETH?池的攻擊為例：SwapExactETHforTokens:?0xa8c4edd85727dApprove：0xf1fdd4cf4d8aaAddLiquidityETH:0x7340edca1a17fApprove:0x41a33f0c91b7cAddLiquidityETH:0x896f412f15a7aTransfer:0x0e8a76bf7295dConvert:0x4947b4f075f8eSwapExactETHForTokens:0x5f37bb3b97341RemoteLiquidityETH:0xdff10159275e0SwapExactTokensForETH:0xb8889bbdeb478

攻擊者的一個完整的攻擊周期：

序章：攻擊準備

Step1：通過swapExactETHForTokens用一些ETH換取MKR，作為攻擊的啟動資金

Step2：將剛剛換到的MKR和ETH一起，向Sushi的MKR/wETH交易池中添加流動性，獲得該池的SLP(MKR/wETH)

Step3：將剛剛換到的SLP和ETH一起，向Sushi的SLP/wETH交易池中添加流動性，獲得新池的SLP(SLP/wETH)(后簡稱SLP')

Step4：將新生成的SLP'轉給SushiMaker

這步有什么意義呢？因為攻擊者后續會調用convert(SLP,wETH)，將SushiMaker.sol合約中存放的SLP'燃燒掉，換出SLP,wETH。如果SushiMaker.sol中沒有SLP'就沒法進行下去了

但是，實際復現過程中，我發現有沒有這步其實都影響不大。因為上一步addLiquidity(SLP,wETH)是，一方面會mint出SLP'給攻擊者，另一方面還會mint出一些SLP'給SushiMaker.sol，所以SushiMaker.sol中其實還是有一點SLP'的→現在攻擊者手里有SLP'，其實他有兩個不同的決策：

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1.將SLP'轉給SushiMaker，這樣SushiMaker可以換出更多的SLP和wETH，下一步回到這個交易池將自己所有的SLP換成wETH2.不轉SLP'給SushiMaker，因為之前創建SLP/wETH這個池的時候會mint一點點SLP'給SushiMaker，所以還是可以通過調用convert將SushiMaker存的SLP都換成wETH

第一種方式中，使SushiMaker擁有的SLP'更多，會換出更多的SLP和wETH，使得池中的滑點更大，SushiMaker能虧的SLP就更多，但是如果SushiMaker中本身存的SLP就不多，SLP/wETH深度也不大時，其實結果差不太多，都是幾乎將SushiMaker搬空。總而言之，還是第一種方式更優，可以看出攻擊者其實是有精心考慮過的

高潮：攻擊SushiMaker

Step5：調用convert(SLP,wETH)，這一步中，SushiMaker會先將SLP'burn掉，換成SLP和wETH，再將自己所有的SLP都轉成wETH，問題在于?SLP/wETH這個池是由攻擊者創建的，池中的深度非常小，滑點極高，SushiMaker通過?swap將SLP換成wETH這步會血虧

尾聲：獲利

Step6：上一步中，SushiMaker在SLP/wETH這個池中血虧了大量的SLP，池中wETH巨值錢，此時攻擊者只需要調用swap用一點點wETH就可以幾乎將整個池子搬空

0x2.附錄

2.1本次事件相關的攻擊交易：[??{????"YFI/WETH":{??????"0x18be4ac177c5ada3522e3cafd6c40d750d4288a3a7b22e140521124074fed2ae":"SwapExactETHForTokens",??????"0xc8653ef0c4658acf3d44059babea07c62578769cac10176e61310f7af04e1597":"Approve",??????"0x18ed3e5efeec3e475501786541a4f7602340e5aa6bc6cbf24b9c9aa1062449ac":"AddLiquidityETH",??????"0x42d553b614d0a64ea2cfe95bf2a99d337c07f1b53ac4d7987966fc39817975e6":"Approve",??????"0x041463e755e91548b9d6dd1e61519b08f101ff96a44df969b33ee57996619a88":"AddLiquidityETH",??????"0x8a2d7926e50123459b7814c0e19b5ac59f7ce221cb5fa82f79d2c0c37ef7ef1f":"Transfer",??????"0xaa001ac10841c784954b0028192f0ea232bf84e390b964af98f1c49074ec4beb":"Convert",??????"0x08bbbeaf7cbd2649738812cf720042eb7ebe1411be2f7cf3ede41411dcbf8bc0":"SwapExactETHForTokens",??????"0x3952f24ef1e5dce37463edcf0f3902316131fb823d79946b96f2a2397a9cd25d":"RemoveLiquidityETH",??????"0x47c200fefd8c007d972b65b97dadf429c3e6e91359b7b49aaa66ee0fc4a555f4":"SwapExactTokensForETH"????},????"profit":"0.2558139ether"??},??{????"USDT/WETH":{??????"0x123ec6c44fa3baa70c66f583f8ee6bc6b6d2b4d39a1119ab8490d2c1120d4647":"Convert",??????"0x08bbbeaf7cbd2649738812cf720042eb7ebe1411be2f7cf3ede41411dcbf8bc0":"SwapExactETHForTokens",??????"0x42973ea279d0bda4222e4689709f86bdb12117dce14420f84f3e9cc6fd42ede2":"RemoteLiquidityETH",??????"0x30eab0a184f9a1ed6cee309754bb6536bd443fea567776a72b72f3ab911e0113":"SwapExactTokensForETH"????},????"profit":"0.1835604ether"??},??{????"MKR/WETH":{??????"0xa8c4edd85727d3d25401f0cbca1136982edb833f77ff0a93178b222063eb57a4":"SwapExactETHforTokens",??????"0xf1fdd4cf4d8aa073ff01876333cfeadeab2b87423664d1daa4cf36ad5e415587":"Approve",??????"0x7340edca1a17fc9e5a6587d6a89c0ed01c5d1f1d20cb19738ab993f6ef7b8b4d":"AddLiquidityETH",??????"0x41a33f0c91b7cd17edf888356dfb110d7744f593e432b54ec2f21aae19076aa0":"Approve",??????"0x896f412f15a7abb959c3c43b8cd9206720fc37f4eabeae336ba101670003e9db":"AddLiquidityETH",??????"0x0e8a76bf7295d9316704fde7f953eced612daa9ca7a70322248eeb8a7f508656":"Transfer",??????"0x4947b4f075f8e9f89f6364158b3d05a92cd72b7920c44c2150a0d12ba009a7a0":"Convert",??????"0x5f37bb3b9734175b605f6e02700c4b710e5e01622f30d822bcca227d91363d77":"SwapExactETHForTokens",??????"0xdff10159275e064dad2c7c79585d1f3c4fd60b73d843715e0cff1a2292730e99":"RemoteLiquidityETH",??????"0xb8889bbdeb478809c3781ccb1c8a16f9fa90b6844d180818b6e73a16963a5ddc":"SwapExactTokensForETH"????},????"profit":"1.9761631ether"??},??{????"KP3R/WETH":{??????"0xe5a025658cd28a688442a57a9b441980feeed8f5e7892064efc68966eb4253eb":"SwapExactETHForTokens",??????"0xe77e7e9d636c2b733d226693d95d06005ae2b5aa8532957240310225aa73e051":"Approve",??????"0x5ce8451f9f39f2d32ffbd943d38723a8da6fff59ece8f62caa84c4c2a311ede3":"AddLiquidityETH",??????"0x171b741bb7cdcafebdbed1c503eb33babc70c2e2e45b2b55330df3bc6d5c9eda":"Approve",??????"0x0ad658ebab282a26d67da0a97083a41c83d0eb1ab6d19a4adb3676b57fc3851c":"AddLiquidityETH",??????"0x676dd29fade2b93e5ca4b0da9d2b5e4ede86f69d26a6201440e359892360b096":"Transfer",??????"0x3b37a792edbb785b223a2b4f0971834083acd52503e54d00989a365bbc533627":"Convert",??????"0x4df5506fdc7616ff65b4ae3b93360e180c7b6f5e976914c4af04648ad6559817":"SwapExactETHForTokens",??????"0x07e56eaa7935fb341654ac20223b0d32fab006b3f34e372aecf6bce05c3f903f":"RemoveLiquidityETH",??????"0x8f4d9d676e9a12d07dcd0c8225e207df48f8cb575cdaa6d81c4b7124c3500510":"Approve",??????"0xac3c837c249fb68ceebd896a72e1e4e8cf355caf3f5fcd0bdc7df7c101ea0596":"SwapExactTokensForETH"????},????"profit":"1.8226875ether"??},??{????"YFI/WETH":{??????"0x1b0eb70bc4d2407745df416acf6e2c312d25ee63c7e8e1291545d66e23fbf704":"PreAttack",??????"0x8f6caabd4ecfad30793db76f60a7e28832a18e2bcf006c0ee3652063c7386778":"Convert",??????"0xe6d28052b55ab520a211daebbc67ee3aece6ae356044e9352fc82910214a038a":"Swap"????},????"profit":"-0.031382ether"??},??{????"AXS/WETH":{??????"0x375f5befeb1af976ebb09cf5539b3a3b9746b87153cfa96ba862deb8828970f1":"PreAttack",??????"0xed8508a18c347c3829024f7acb94056a78cc63152e2246ebd42b59cd86d0c8f9":"Convert",??????"0xd08eb264e39cd5dca8edb3684dfd36afb905aea9c47a43110b6c5c45bdb2f7d1":"Swap"????},????"profit":"1.349482ether"??},??{????"CRD/WETH":{??????"0xc6c86de784359a3813b31b4be5806a23f82fdd8d5cd9bd3a7379d33a413eb920":"PreAttack",??????"0x7d1b36019fb287867419dffb281b5ad73b37d8dc21abca47f7b618875842ef0a":"Convert",??????"0x8c14595b205cd04d1e5062a3efe47d3e4dae946eabe744e2d8350d6ba951495a":"Swap"????},????"profit":"1.325357ether"??},??{????"USDC/WETH":"Failed"??},??{????"COVER/WETH":"Failed"??},??{????"YAX/WETH":{??????"0x063bc5df97c0ec87d6c309b54fbc4e3ed13b22f5a115fb1170015e4e9d23cd3d":"PreAttack",??????"0x9537551d0db7ed6ae372c050c73cae1272f4d7e7847ea908d6e4d440809a871f":"Convert",??????"0x9f6bdf1c9065f5f5e5c71f32058093b7ac06e140c3a33db2f21cc8a5ff8a5d21":"SwapExactETHforTokens",??????"0x9281c35277ea6594924600d70b73077a4307973c540edb5b4ed534c7e25f2423":"Approve",??????"0xe2f682d8b4801061d504025f11b3032648a3b94019396f7b12749b162f8404cf":"RemoteLiquidityETH",??????"0xf6ea6c9eb92ca9aed70e8b4b03a6e5565a3a13df71a0aa527fa55607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AOFEX平臺幣OT現報價16.21AQ，24H漲幅7.55%:數據顯示，AOFEX交易所平臺幣OT最新成交價為16.21AQ，24H漲幅7.55%，24H成交量為1406萬OT。

AOFEX數字貨幣金融衍生品交易所，旨在為用戶提供優質服務和資產安全保障。[2021/3/27 19:22:35]

后續攻擊者又嘗試攻擊Sushiswap的其他相關仿盤項目，如LuaSwap：

https://etherscan.io/tx/0xeacfed6fb18563c18b9af5cb0d3a5b13e97cf02026ef9a4cb625cc33580d02a7

但可能獲利不多，只成功幾筆就不了了之了

2.2本次事件后續結果：

在**#11351530**塊，Sushi的管理員向攻擊者喊話：

可以看到，好像有些延遲，攻擊者的攻擊依然持續了一段時間：

最后一條Convert中：

SushiMaker地址從0x6684977bBED67變成了0x280ac711bb99d：

Old：0x6684977bBED67e101BB80Fc07fCcfba655c0a64F??functionconvert(addresstoken0,addresstoken1)public{????//Atleastwetrytomakefront-runninghardertodo.????//限制只能EOA來調用該函數????require(msg.sender==tx.origin,"donotconvertfromcontract");????//獲取相應SLPtoken合約的地址????IUniswapV2Pairpair=IUniswapV2Pair(factory.getPair(token0,token1));????//將SushiMaker中存有的全部SLP，轉給交易池????pair.transfer(address(pair),pair.balanceOf(address(this)));????//Burn掉這些SLP換成兩種token????pair.burn(address(this));????//分別將兩種token找到相應的和wETH的交易對，全部轉換為wETH????uint256wethAmount=_toWETH(token0)_toWETH(token1);????//將轉換的到的全部wETH找wETH/SUSHI交易對換成SUSHI_toSUSHI(wethAmount);??}??function_toETH(addresstoken){????...????uintamountIn=IERC20(token).balanceOf(address(this));??}New：0x280ac711bb99dE7C73FB70fb6DE29846D5e4207F??functionconvert(addresstoken0,addresstoken1)public{????//Atleastwetrytomakefront-runninghardertodo.????require(msg.sender==tx.origin,"donotconvertfromcontract");????IUniswapV2Pairpair=IUniswapV2Pair(factory.getPair(token0,token1));????pair.transfer(address(pair),pair.balanceOf(address(this)));????(uintamount0,uintamount1)=pair.burn(address(this));????uint256wethAmount=_toWETH(token0,amount0)_toWETH(token1,amount1);_toSUSHI(wethAmount);??}??function_toETH(addresstoken,uintamountIn){????...??}

可以看到區別在于：代碼第26行_toWETH限制了amount，這樣修改以后就不會將SushiMaker中存的全部SLP都去池中換成wETH，而只是換取burn出的一部分

但是這樣問題就解決了嗎？其實還沒有，時隔兩個月，同樣的地方，Sushiswap又中槍了見：amenda：SushiSwap攻擊事件(2021年1月27號)分析(https://zhuanlan.zhihu.com/p/372058217)

0x3.參考

以小博大，簡述SushiSwap攻擊事件始末：https://www.chainnews.com/zh-hant/articles/726256718885.htm

Astokenpricerisesandreputationmends,Sushiswapfoilsmidnightexploit:?https://cointelegraph.com/news/as-token-price-rises-and-reputation-mends-sushiswap-foils-midnight-exploit

ETC突破9.2美元關口 日內漲幅為27.11%:火幣全球站數據顯示，ETC短線上漲，突破9.2美元關口，現報9.2023美元，日內漲幅達到27.11%，行情波動較大，請做好風險控制。[2021/1/10 15:48:29]

AnevolutionofUniswapwithSUSHItokenomics：https://sushichef.medium.com/the-sushiswap-project-dd6eb80c6ba2

一文看懂Uniswap和Sushiswap：https://zhuanlan.zhihu.com/p/226085593

sokdecentralizedexchanges(dex)withautomatedmarketmaker(amm)protocols:?https://arxiv.org/abs/2103.12732

AttackingtheDeFiEcosystemwithFlashLoansforFunandProfit:?https://arxiv.org/abs/2003.03810

https://www.blocksecteam.com/

VEE今日漲幅達40.85%:根據OKEx交易平臺數據顯示，VEE最新成交價格為人民幣1.14元，24小時最高價達人民幣1.29元，最低價格為人民幣1.06元，24小時成交量33253877VEE。VEE是一個區塊鏈開發者的共享協議，幫助開發者創建區塊鏈上的智能數字對象以及提供一個塑造未來體驗經濟的基礎，并且也為處于高速發展的生態系統中的企業帶來全新的營收增長機遇。[2018/1/19]

NAS漲幅超80%:根據火幣交易平臺數據顯示，NAS24小時價格最高價格為人民幣85.74元，24小時最低價格為人民幣21.54元，目前的價格為人民幣39.41元，漲幅高達82.85%[2017/12/28]

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