Ethernaut Challenge #4 Solution — Telephone

This is Part 4 of the "Let's play OpenZeppelin Ethernaut CTF" series, where I will explain how to solve each challenge.

The Ethernaut is a Web3/Solidity based wargame created by OpenZeppelin. Each level is a smart contract that needs to be 'hacked'. The game acts both as a tool for those interested in learning ethereum, and as a way to catalogue historical hacks in levels. Levels can be infinite and the game does not require to be played in any particular order.

Challenge #4: Telephone

Claim ownership of the contract below to complete this level.

Level author: Kyle Riley

For this challenge, our end goal is to be able to claim the ownership of the contract.

Study the contracts

The Telephone contract is pretty small, so it will be fast to read and understand how to solve the challenge.

The owner state variable is initialized in the constructor. The only function that will update the owner is changeOwner :

function changeOwner(address _owner) public

It is a public function that take only one parameter address _owner. If the tx.origin value is different from msg.sender it will update the owner with the function input parameter _owner.

To solve this challenge, we need to understand what are msg.sender and tx.origin.

If we look at the Block and Transaction Properties documentation page from the Solidity official doc, we find this definition:

• tx.origin (address): sender of the transaction (full call chain)
• msg.sender (address): sender of the message (current call)

Both tx.origin and msg.sender are "special variables" which always exist in the global namespace and are mainly used to provide information about the blockchain or are general-use utility functions.

But we need to pay attention to this:

• The values of all members of msg, including msg.sender and msg.value can change for every external function call. This includes calls to library functions.
• tx.origin will return the address that have sent (in origin) the transaction, while msg.sender will return the value that have originated the external call.

What does this mean?

Let's make an example and see the different values for both of them

Scenario A: Alice (EOA) call directly Telephone.changeOwner(Bob)

• tx.origin: Alice address
• msg.sender: Alice address

Scenario B: Alice (EOA) call a smart contract Forwarder.forwardChangeOwnerRequest(Bob) that will call Telephone.changeOwner(Bob)

Inside Forwarder.forwardChangeOwnerRequest

• tx.origin: Alice address
• msg.sender: Alice address

Inside Telephone.changeOwner(Bob)

• tx.origin: Alice address
• msg.sender: Forwarder (contract) address

This happens because while tx.origin will always return the address that have created the transaction, msg.sender will return the address who made the last external call.

Solution code

We just need to create a contract that will be in the middle of the call to the Telephone contract.

contract Exploiter {
    function exploit(Telephone level) public {
        level.changeOwner(msg.sender);
    }
}

And in our solution code, just deploy it and call it

function exploitLevel() internal override {
    vm.startPrank(player, player);

    Exploiter exploiter = new Exploiter();

    vm.stopPrank();
}

If you have paid attention to our previous blog post, you already saw the startPrank cheat code. startPrank has another overloaded version

// Sets all subsequent calls' msg.sender to be the input address until `stopPrank` is called
function startPrank(address) external;

// Sets all subsequent calls' msg.sender to be the input address until `stopPrank` is called, and the tx.origin to be the second input
function startPrank(address, address) external;

In this case, we are using the second one because we need to also override the initial tx.orgin that otherwise would be address(this): the address of the test contract itself!

You can read the full solution of the challenge opening Telephone.t.sol

Further reading

• Block and Transaction Properties
• SWC-115: Authorization through tx.origin
• Security Consideration about tx.origin from Solidity docs
• Consensys Ethereum Smart Contract Best Practices: tx.origin
• Vitalik Buterin: Do NOT assume that tx.origin will continue to be usable or meaningful

Disclaimer

All Solidity code, practices and patterns in this repository are DAMN VULNERABLE and for educational purposes only.

I do not give any warranties and will not be liable for any loss incurred through any use of this codebase.

DO NOT USE IN PRODUCTION.