SQL Injection (SQLi) is a particularly dangerous and common type of code injection attack that targets data-driven applications, especially web applications. It exploits vulnerabilities in an application's input validation and database query construction. An attacker inserts malicious SQL code into input fields (such as login forms, search bars, or URL parameters) that are then improperly handled by the application, leading to the database executing unintended commands.

The core vulnerability lies in applications that build SQL queries by directly concatenating (stitching together) user-supplied input strings without proper sanitization or parameterization.

Example of a Vulnerable Query (in application code):

String userInputUsername = request.getParameter("username"); // User provides 'JohnDoe'
String userInputPassword = request.getParameter("password"); // User provides 'mypass'
String sqlQuery = "SELECT * FROM Users WHERE Username = '" + userInputUsername + "' AND Password = '" + userInputPassword + "';";

If userInputUsername = 'JohnDoe' and userInputPassword = 'mypass', the query becomes:

SELECT * FROM Users WHERE Username = 'JohnDoe' AND Password = 'mypass';

• Data Theft: Unauthorized access to sensitive information (credit card numbers, PII, intellectual property).
• Data Alteration/Destruction: Modifying or deleting database records.
• Privilege Escalation: Gaining administrative privileges on the database server.
• Denial of Service: Making the database unavailable.
• Remote Code Execution: In some cases, executing arbitrary commands on the database server.

Preventing SQL Injection is paramount and relies on fundamental secure coding practices:
1. Parameterized Queries (Prepared Statements): The Golden Rule
• This is the single most effective defense against SQL Injection. Instead of concatenating user input directly into the SQL string, the query structure is defined first, with placeholders for parameters. The values for these parameters are then passed separately to the DBMS.
• The DBMS treats the parameters as data values, not as executable SQL code, thus neutralizing any malicious code injected by the user.
• Example (using prepared statement):

      String sql = "SELECT * FROM Users WHERE Username = ? AND Password = ?;";
      PreparedStatement statement = connection.prepareStatement(sql);
      statement.setString(1, userInputUsername); // Pass username as a parameter
      statement.setString(2, userInputPassword); // Pass password as a parameter
      ResultSet rs = statement.executeQuery();

1. Input Validation and Sanitization:
   • Validate User Input: Always check if user input conforms to expected data types, formats, lengths, and ranges. For example, if a field expects a number, reject any input that contains letters or special characters.
   • Sanitize Input: Remove or escape potentially dangerous characters from user input if parameterization isn't possible (though parameterization is always preferred). This is a last resort, as it's prone to error.
2. Principle of Least Privilege:
   • Database users (especially those used by applications) should only have the absolute minimum privileges required to perform their functions. For example, a web application's database user should typically only have SELECT, INSERT, UPDATE, DELETE on specific tables, never DROP TABLE or GRANT privileges.
3. Robust Error Handling:
   • Avoid displaying verbose database error messages directly to users, as these can reveal sensitive information about your database schema, version, and underlying architecture, which attackers can exploit. Log errors securely instead.
4. Web Application Firewalls (WAFs):
   • A WAF can provide an additional layer of defense by inspecting incoming web traffic and blocking known SQL injection attack patterns before they reach the application. This is a good complementary measure but not a substitute for secure coding.