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防止 SQL 注入

了解 SQL 注入的工作原理，并使用参数化查询和预处理语句实施可靠的防御措施。

防止 SQL 注入 是 CoddyKit 上的免费 Secure Coding & OWASP Top 10 for Backend 课时。 这是第 1 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Secure Coding & OWASP Top 10 for Backend 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Secure Coding & OWASP Top 10 for Backend 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

What is SQL Injection?

Imagine you're talking to a database using a special language called SQL. Sometimes, bad actors can trick your application into sending unexpected SQL commands to the database.

This trick is called SQL Injection (SQLi). It happens when user input is treated as part of the SQL command itself, rather than just data.

SQLi can lead to:

  • Data theft or modification
  • Bypassing login screens
  • Gaining control over the database server

How SQLi Works: A Login Example

Let's say a login form takes your username and password. The application might build a SQL query like this to check if you exist:

SELECT * FROM users WHERE username = 'your_username' AND password = 'your_password';

What if 'your_username' isn't just a name, but also a piece of SQL code?

The Vulnerable Code

A common mistake is building SQL queries by directly combining (concatenating) user input with the query string. Here's a simplified example in Java:

public class Main {
  public static void main(String[] args) {
    String username = "admin"; // User input
    String password = "pass123"; // User input
    
    // DANGEROUS: String concatenation
    String query = "SELECT * FROM users WHERE username = '" + username + "' AND password = '" + password + "';";
    
    System.out.println("Executing query:\n" + query);
    // In a real app, this query would go to the database.
  }
}

Crafting the Malicious Input

Now, imagine a malicious user enters this as their username:

admin' OR '1'='1

And any text for the password. When combined with the vulnerable query, it becomes:

SELECT * FROM users WHERE username = 'admin' OR '1'='1' AND password = 'any_password';

The OR '1'='1' part makes the condition always true, effectively bypassing the password check and logging the attacker in!

Introducing Prepared Statements

The best defense against SQL Injection is using Prepared Statements (also known as Parameterized Queries).

They separate the SQL logic from the user-provided data. Instead of directly inserting input, you use placeholders (like ?) in your query.

The database then understands that anything provided for these placeholders is pure data, not part of the command.

Prepared Statements in Action

Here's how you'd use a prepared statement to safely query for a user by their ID. Notice the ? placeholder.

public class Main {
  public static void main(String[] args) {
    int userId = 123; // User input
    
    // Secure: Using a placeholder for the value
    String sql = "SELECT name, email FROM users WHERE id = ?;";
    
    // In a real application, you'd 'prepare' this SQL
    // and then 'set' the parameter value (userId) separately.
    System.out.println("Prepared SQL: " + sql);
    System.out.println("Parameter 1 (id): " + userId);
    // The database treats 'userId' as a value, not SQL code.
  }
}

How Parameters Prevent SQLi

When you use a prepared statement:

  • Separation of concerns: The SQL query structure is sent to the database first.
  • Data vs. Code: The database 'pre-compiles' the query. Then, when you provide values for the placeholders, it treats them strictly as data.
  • Automatic Escaping: The database automatically handles any special characters in your input, preventing them from being interpreted as SQL commands.

This ensures that malicious input like ' OR '1'='1 cannot alter the query's intent.

Securing the Login Example

Let's fix our vulnerable login example using prepared statements. Now, even if a user tries to inject SQL, it will be treated as part of their username/password, not as a command.

public class Main {
  public static void main(String[] args) {
    String username = "admin' OR '1'='1"; // Malicious input
    String password = "any_password"; // Malicious input
    
    // SECURE: Using prepared statement with placeholders
    String sql = "SELECT * FROM users WHERE username = ? AND password = ?;";
    
    // Simulating how parameters are set (conceptual)
    System.out.println("Prepared SQL: " + sql);
    System.out.println("Parameter 1 (username): " + username);
    System.out.println("Parameter 2 (password): " + password);
    
    System.out.println("\nDatabase will look for a user with the literal username 'admin' OR '1'='1' and the given password. This user likely won't exist. Attack thwarted!");
  }
}

Beyond Login: All SQL Operations

Prepared statements aren't just for SELECT queries. You should use them for ALL SQL operations that involve user input:

  • INSERT statements (e.g., adding a new user)
  • UPDATE statements (e.g., changing a user's profile)
  • DELETE statements (e.g., removing data)

Always assume user input is malicious until proven otherwise. Parameterized queries are your first line of defense.

Quick Check: Secure Query

Which of the following code snippets correctly uses parameterized queries to prevent SQL Injection when searching for a product by name?

Recap: SQL Injection Prevention

Great job! In this lesson, you learned about:

  • What SQL Injection is and its dangers.
  • How vulnerable applications can be exploited by concatenating user input directly into SQL queries.
  • The importance of using Prepared Statements (Parameterized Queries) as the primary defense mechanism.
  • How prepared statements separate data from code, preventing malicious input from altering query logic.

Always use prepared statements for any SQL operation involving user input to keep your backend secure!

常见问题解答

「防止 SQL 注入」课时是免费的吗？

是的 — 「防止 SQL 注入」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Secure Coding & OWASP Top 10 for Backend 课程的其余内容，请升级到 CoddyKit PRO。 Secure Coding & OWASP Top 10 for Backend 课程共包含 4 节课。

「防止 SQL 注入」这节课中我会学到什么？

了解 SQL 注入的工作原理，并使用参数化查询和预处理语句实施可靠的防御措施。 你通过在浏览器中直接运行的动手代码来练习 Secure Coding & OWASP Top 10 for Backend，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Secure Coding & OWASP Top 10 for Backend 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Secure Coding & OWASP Top 10 for Backend 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 1 节课，共 4 节。

「防止 SQL 注入」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Secure Coding & OWASP Top 10 for Backend 课中编写并运行代码吗？

能。每节 Secure Coding & OWASP Top 10 for Backend 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

  1. 防止 SQL 注入
  2. 命令与代码注入
  3. 后端中的跨站脚本攻击（XSS）
  4. 防止 XML 与 LDAP 注入
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