SQL Injection Attacks and Prevention Measures for Databases

Description
SQL injection is a common security vulnerability where attackers construct malicious inputs to alter the application's SQL query logic, thereby stealing, modifying, or destroying data in the database. This vulnerability typically occurs when the application fails to adequately validate or escape user input. Understanding its principles and prevention methods is crucial for database security.

Step-by-Step Explanation

1. Basic Principles of SQL Injection

   • Scenario Example: Suppose a login function verifies user identity via an SQL query. The original query is:

     SELECT * FROM users WHERE username = 'input_username' AND password = 'input_password';  
     

   • Attack Process: If an attacker enters admin' -- (-- denotes a comment in SQL) in the username field, the query becomes:

     SELECT * FROM users WHERE username = 'admin' --' AND password = 'ignored_part';  
     

     The query then verifies only the username admin, bypassing password checks and leading to unauthorized login.
   • Root Cause: User input is directly concatenated into the SQL statement, disrupting the original syntax structure.

2. Common Types of SQL Injection

   • Boolean-Based Blind Injection: Inferences about data are made based on true/false query results (e.g., differences between admin' AND 1=1 -- and admin' AND 1=2 --).
   • Union Query Injection: Uses the UNION keyword to merge malicious queries and steal data from other tables (e.g., ' UNION SELECT credit_card FROM payments --).
   • Time-Based Blind Injection: Determines conditions based on database response delays (e.g., admin' AND SLEEP(5) --).
   • Error-Based Injection: Intentionally triggers database errors to leak data through error messages (e.g., exploiting functions like extractvalue()).

3. Prevention Measures: Input Validation and Filtering

   • Allowlist Validation: Strictly restricts input formats (e.g., usernames may only contain alphanumeric characters).
   • Escaping Special Characters: Adds escape characters to quotes, semicolons, etc. (e.g., converting ' to \'), noting that escape rules may differ across databases.

4. Prevention Measures: Parameterized Queries (Prepared Statements)

   • Principle: Separates SQL statements from data. First, define a query template with placeholders (e.g., SELECT * FROM users WHERE username = ?), then bind user input as parameters. The database processes the input strictly according to data types, preventing it from being interpreted as SQL code.
   • Example (Java PreparedStatement):

     String sql = "SELECT * FROM users WHERE username = ?";  
     PreparedStatement stmt = connection.prepareStatement(sql);  
     stmt.setString(1, userInput); // Input is automatically escaped  
     

   • Advantages: Fundamentally prevents SQL injection and improves query performance.

5. Prevention Measures: Principle of Least Privilege

   • Assign the database account the minimum necessary permissions (e.g., read-only access) to prevent attackers from exploiting injection points for dangerous operations like table deletion.

6. Other Auxiliary Measures

   • Web Application Firewall (WAF): Detects and intercepts malicious request patterns.
   • Regular Security Scanning: Uses tools (e.g., SQLMap) to simulate attacks and test for vulnerabilities.
   • Error Message Hiding: Avoids exposing detailed database error information to users.

Summary
The core of SQL injection lies in the "confusion between data and code." Multi-layered defenses—including parameterized queries, input validation, and permission controls—can effectively reduce risks. In actual development, prepared statements should be prioritized, combined with secure coding practices for systematic protection.