InsecureEtherVault.sol

// SPDX-License-Identifier: BSL-1.0 (Boost Software License 1.0)

//-------------------------------------------------------------------------------------//
// Copyright (c) 2022 - 2023 serial-coder: Phuwanai Thummavet (mr.thummavet@gmail.com) //
//-------------------------------------------------------------------------------------//

// For more info, please refer to my article:
//  - On Medium: https://medium.com/valixconsulting/solidity-smart-contract-security-by-example-01-integer-underflow-c1147c2e507b
//  - On serial-coder.com: https://www.serial-coder.com/post/solidity-smart-contract-security-by-example-01-integer-underflow/

pragma solidity 0.6.12;

import "./Dependencies.sol";

contract InsecureEtherVault is ReentrancyGuard {
    mapping (address => uint256) private userBalances;

    function deposit() external payable {
        userBalances[msg.sender] += msg.value;
    }

    function withdraw(uint256 _amount) external noReentrant {
        uint256 balance = getUserBalance(msg.sender);
        require(balance - _amount >= 0, "Insufficient balance");

        userBalances[msg.sender] -= _amount;
        
        (bool success, ) = msg.sender.call{value: _amount}("");
        require(success, "Failed to send Ether");
    }

    function getEtherBalance() external view returns (uint256) {
        return address(this).balance;
    }

    function getUserBalance(address _user) public view returns (uint256) {
        return userBalances[_user];
    }
}