[Ethernaut] Wargame Writeup | 15. Naught Coin

Description

NaughtCoin is an ERC20 token and you're already holding all of them. The catch is that you'll only be able to transfer them after a 10 year lockout period. Can you figure out how to get them out to another address so that you can transfer them freely? Complete this level by getting your token balance to 0.

Things that might help

• The ERC20 Spec
• The OpenZeppelin codebase

Code

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "openzeppelin-contracts-08/token/ERC20/ERC20.sol";

contract NaughtCoin is ERC20 {
    // string public constant name = 'NaughtCoin';
    // string public constant symbol = '0x0';
    // uint public constant decimals = 18;
    uint256 public timeLock = block.timestamp + 10 * 365 days;
    uint256 public INITIAL_SUPPLY;
    address public player;

    constructor(address _player) ERC20("NaughtCoin", "0x0") {
        player = _player;
        INITIAL_SUPPLY = 1000000 * (10 ** uint256(decimals()));
        // _totalSupply = INITIAL_SUPPLY;
        // _balances[player] = INITIAL_SUPPLY;
        _mint(player, INITIAL_SUPPLY);
        emit Transfer(address(0), player, INITIAL_SUPPLY);
    }

    function transfer(address _to, uint256 _value) public override lockTokens returns (bool) {
        super.transfer(_to, _value);
    }

    // Prevent the initial owner from transferring tokens until the timelock has passed
    modifier lockTokens() {
        if (msg.sender == player) {
            require(block.timestamp > timeLock);
            _;
        } else {
            _;
        }
    }
}

Scenario

바로 contract의 transfer 함수를 호출하면 lockTokens modifier 때문에 timeLock 시간이 지나지 않는 이상 호출할 수 없다. ERC20 표준에 따르면 토큰을 전송하는 함수가 transfer 말고 transferFrom도 존재한다. approve() 함수를 통해 spender가 얼마만큼의 value를 전송할 수 있게 할 지 승인해주고 transferFrom 함수를 호출할 수 있다.

Exploit