SQL Injection Attacks and Prevention

Description:
SQL injection is a common web security vulnerability where attackers insert malicious SQL code into an application's input fields, tricking the server into executing unintended database operations. This can lead to data leakage, tampering, or deletion, and even complete system compromise. For example, if a login form's username input field is not validated, an attacker might input ' OR '1'='1 to bypass password authentication.

Problem-Solving Process:

1. Understand the Principle of SQL Injection

   • Normal Scenario: The application executes an SQL query, such as SELECT * FROM users WHERE username='admin' AND password='123456', by dynamically concatenating SQL statements with user input.
   • Attack Scenario: If a user inputs the username ' OR '1'='1' --, the query becomes SELECT * FROM users WHERE username='' OR '1'='1' -- AND password='...'. Here, -- is a comment symbol that nullifies the password condition, and '1'='1' is always true, thus returning all user data.

2. Identify Injection Points

   • Testing Methods: Try entering special characters like single quotes (') or semicolons (;) into input fields. If the page returns database errors (e.g., MySQL syntax error messages), it may indicate an injection vulnerability.
   • Tool Assistance: Use automated tools like SQLMap to detect whether parameters are injectable.

3. Preventive Measures

   • Parameterized Queries (Prepared Statements):
     Separate SQL statements from data. For example, using Java's PreparedStatement:

     String sql = "SELECT * FROM users WHERE username = ? AND password = ?";  
     PreparedStatement stmt = connection.prepareStatement(sql);  
     stmt.setString(1, username); // Input values are automatically escaped  
     stmt.setString(2, password);  
     

     The database strictly distinguishes between code and data, preventing malicious input from being executed as SQL.
   • Input Validation:
     Apply whitelist filtering to user input (e.g., allowing only alphanumeric characters) or escape special characters (e.g., replacing ' with \'). However, escaping may fail due to database variations, making parameterized queries more reliable.
   • Principle of Least Privilege:
     Database accounts should be granted only necessary permissions (e.g., prohibiting ordinary business accounts from executing operations like DROP TABLE).

4. Practical Case Verification

   • Security Testing: Build a login interface with parameterized queries, attempt to input injection payloads, and confirm that the database does not return abnormal data.
   • Code Audit: Check whether SQL statements in the project use string concatenation and prioritize refactoring high-risk modules.

By following these steps, one can systematically understand, detect, and defend against SQL injection attacks.