Solidity Game - DEX Attack

Inspired by OpenZeppelin's Ethernaut, Dex Level

Please see DexTwo for another Dex game.

⚠️Do not try on mainnet!

Task

The goal of this level is for you to hack the basic DEX contract below and steal the funds by price manipulation.

You will start with 10 tokens of token1 and 10 of token2. The DEX contract starts with 100 of each token.

You will be successful in this game if you manage to drain all of at least 1 of the 2 tokens from the contract, and allow the contract to report a "bad" price of the assets.

Quick note

Normally, when you make a swap with an ERC20 token, you have to approve the contract to spend your tokens for you. To keep with the syntax of the game, we've just added the approve method to the contract itself. So feel free to use contract.approve(contract.address, <uint amount>) instead of calling the tokens directly, and it will automatically approve spending the two tokens by the desired amount. Feel free to ignore the SwappableToken contract otherwise.

Hint:

1. How is the price of the token calculated?
2. How does the swap method work?
3. How do you approve a transaction of an ERC20?

What will you learn?

1. ERC20
2. SWAP

What is the most difficult challenge?

DEX 🤑

The idea of this game is that flash loan attacks are rampant in the DeFi world, and having some experience with some of the price issues is going to be a must for up and coming solidity developers.

Basically, the Dex is a really crappy uniswap. It reports a price based on liquidity pools, and a user can use this to flip back and forth and eventually drain the dex of the liquidity pools.

Source Code

⚠️This contract contains a bug or risk. Do not use on mainnet!

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.4 <0.9.0;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";

contract Dex {
  using SafeMath for uint256;
  address public token1;
  address public token2;

  constructor(address _token1, address _token2) public {
    token1 = _token1;
    token2 = _token2;
  }

  function swap(
    address from,
    address to,
    uint256 amount
  ) public {
    require((from == token1 && to == token2) || (from == token2 && to == token1), "Invalid tokens");
    require(IERC20(from).balanceOf(msg.sender) >= amount, "Not enough to swap");
    uint256 swap_amount = get_swap_price(from, to, amount);
    IERC20(from).transferFrom(msg.sender, address(this), amount);
    IERC20(to).approve(address(this), swap_amount);
    IERC20(to).transferFrom(address(this), msg.sender, swap_amount);
  }

  function add_liquidity(address token_address, uint256 amount) public {
    IERC20(token_address).transferFrom(msg.sender, address(this), amount);
  }

  function get_swap_price(
    address from,
    address to,
    uint256 amount
  ) public view returns (uint256) {
    return ((amount * IERC20(to).balanceOf(address(this))) / IERC20(from).balanceOf(address(this)));
  }

  function approve(address spender, uint256 amount) public {
    SwappableToken(token1).approve(spender, amount);
    SwappableToken(token2).approve(spender, amount);
  }

  function balanceOf(address token, address account) public view returns (uint256) {
    return IERC20(token).balanceOf(account);
  }
}

contract SwappableToken is ERC20 {
  constructor(
    string memory name,
    string memory symbol,
    uint256 initialSupply
  ) public ERC20(name, symbol) {
    _mint(msg.sender, initialSupply);
  }
}

Configuration

Install Truffle cli

Skip if you have already installed.

npm install -g truffle

Install Dependencies

yarn install

Test and Attack!💥

Run Tests

truffle develop
test

truffle(develop)> test
Using network 'develop'.


Compiling your contracts...
===========================
> Everything is up to date, there is nothing to compile.



  Contract: Hacker
    √ should check game state (482ms)
Start price manipulating...
Swap price: TKN2 -> TKN1 - 10
Balances: TKN1/DEX - 90, TKN2/DEX - 110, TKN1/HACKER - 20, TKN2/HACKER - 0
Swap price: TKN1 -> TKN2 - 24
Balances: TKN1/DEX - 110, TKN2/DEX - 86, TKN1/HACKER - 0, TKN2/HACKER - 24
Swap price: TKN2 -> TKN1 - 30
Balances: TKN1/DEX - 80, TKN2/DEX - 110, TKN1/HACKER - 30, TKN2/HACKER - 0
Swap price: TKN1 -> TKN2 - 41
Balances: TKN1/DEX - 110, TKN2/DEX - 69, TKN1/HACKER - 0, TKN2/HACKER - 41
Swap price: TKN2 -> TKN1 - 65
Balances: TKN1/DEX - 45, TKN2/DEX - 110, TKN1/HACKER - 65, TKN2/HACKER - 0
Swap price: TKN1 -> TKN2 - 110
Balances: TKN1/DEX - 90, TKN2/DEX - 0, TKN1/HACKER - 20, TKN2/HACKER - 110
    √ should drain the dex (8265ms)


  2 passing (9s)