Alien Codex - The Ethernaut - Writeup

Sun, 31 May 2026 00:00:00 +0000

You’ve uncovered an Alien contract. Claim ownership to complete the level.

This is the code of the contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.5.0;

import "../helpers/Ownable-05.sol";

contract AlienCodex is Ownable {
    bool public contact;
    bytes32[] public codex;

    modifier contacted() {
        assert(contact);
        _;
    }

    function makeContact() public {
        contact = true;
    }

    function record(bytes32 _content) public contacted {
        codex.push(_content);
    }

    function retract() public contacted {
        codex.length--;
    }

    function revise(uint256 i, bytes32 _content) public contacted {
        codex[i] = _content;
    }
}

To be able to pass this challenge we need to understand how dynamic arrays are stored in the EVM storage. First, a whole slot (32 bytes) is assigned to that array using standard slot assigment rules. The number stored in this slot will correspond to the number of elements of the array. Let’s see that.

Array Underflow on manbolq's Blog

Alien Codex - The Ethernaut - Writeup