In this article, I will walk you through how to solve the first task of the Ethernaut game. The aim of the task is to : claim ownership of the contract reduce the balance of the contract to 0 Let's start with the analysis of the contract that we need to hack. function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  } Looking at the constructor we can see that contribution of the owner was set to 1000 ethers. This means that if we want to take ownership of the contract we need to call the function until we reach a balance higher than 1000 ethers. contribute Considering that we cannot send more than 0.001 ether in one call it will take a lot of time. constructor() public {
    owner = msg.sender;
    contributions[msg.sender] = 1000 * (1 ether);
  } Another way to take ownership of the contract is to use the fallback function. The fallback function is executed if none of the other functions match the function identifier or no data was provided with the function call. receive Note that only one unnamed function can be assigned to a contract and it is executed whenever the contract receives plain ether without any data. To receive ether and add it to the total balance of the contract, the fallback function must be marked . payable receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  } As you can see, to take ownership we need to meet to conditions: msg.value > 0 && contributions[msg.sender] > 0 After that, we can call function to claim all funds. withdraw function withdraw() public onlyOwner {
    owner.transfer(address(this).balance);
  } Before let's check who is the owner of the contract. As you can see the owner's address is different from than player's address. Now we can send some amount of ether to the contract. Note, you cannot send more than 0.001 ether. To send the transaction you need to confirm your operation in MetaMask and pay some small gas. Once the transaction will be executed you should see confirmation as above. We can also verify that we contributed to the contract. Without going into details the is the confirmation. So now we need to fulfill the second condition of the fallback function. To do that we need to call the function and send some value. length:3 Since this is a fallback function we used the specific function sendTransaction After that, we can check the contract owner. As you can see now the ownership was assigned to the player's address. So we claimed the ownership! Now we can withdraw ethers. Before we will check the current balance. Let's call the function to get all ethers. withdraw When you check the contract balance you will see that it is 0. So we achieved the second goal. First published here.

Different

How to Solve the Second Task of the Ethernaut Game

How to Hack Smart Contracts: Self Destruct and Solidity

Do not forget to follow me 

Nominated for 2022 - HackerNoon Contributor of the Year - Ethereum

Nominated for 2022 - HackerNoon Contributor of the Year - Smart Contracts

Nominated for 2022 - HackerNoon Contributor of the Year - Solidity

Nominated for 2022 - Software Developer of the Year

Nominated for 2022 - HackerNoon Contributor of the Year - Programming

Nominated for 2022 - HackerNoon Contributor of the Year - Security

Too Long; Didn't Read

How to Solve the First Task of the Ethernaut Game

How to Solve the First Task of the Ethernaut Game

About Author

Comments

TOPICS

THIS ARTICLE WAS FEATURED IN

Related Stories

Hack Solidity: Integer Overflow and Underflow

Blockchain Vulnerability Report—ΞthernautDAO Car Token CTF

Ethernaut Lvl 1 Walkthrough: how to abuse the Fallback function

Good Samaritan - The New Ethernaut CTF Challenge

Open Zeppelin's Smart Contract Security Puzzles: Ethernaut Level 2 and 3 Walkthrough

Smart Contract Security in Solidity

Hack Solidity: Integer Overflow and Underflow

Blockchain Vulnerability Report—ΞthernautDAO Car Token CTF

Ethernaut Lvl 1 Walkthrough: how to abuse the Fallback function

Good Samaritan - The New Ethernaut CTF Challenge

Open Zeppelin's Smart Contract Security Puzzles: Ethernaut Level 2 and 3 Walkthrough

Smart Contract Security in Solidity

Light-Mode

Classic

Newspaper

Minty

Dark-Mode

Neon Noir

Minty

HN StartUps