SQL injection is a type of security vulnerability that occurs when an attacker is able to insert or “inject” arbitrary SQL code into a query. This typically happens due to insufficient validation or sanitization of user input within an application that interacts with a database. SQL injection can lead to unauthorized access to or manipulation of the database, allowing attackers to view, modify, or delete data.

Here’s a basic example to illustrate how an SQL injection might work:

1. Vulnerable SQL Query:

   In this query, user and pass are placeholders for user-provided input.

2. Injection Attack: Suppose the application constructs the SQL query by concatenating strings directly with user inputs:

   An attacker could input userInputUsername as admin' -- and leave the password field empty. This might result in the following query:

   This might allow the attacker to log in as the admin user without providing a password.

Common Types of SQL Injection:

1. Classic SQL Injection: The basic form where attackers manipulate query strings.
2. Blind SQL Injection: When the attacker cannot see the direct results of their injection but can infer information based on the application’s behavior.
3. Error-based SQL Injection: Exploiting database errors to gain information about the structure of the database.
4. Union-based SQL Injection: Using the UNION SQL operator to combine results from two or more SELECT statements.

Prevention Methods:

1. Parameterized Queries (Prepared Statements): Ensure user input is treated as data, not code.
2. Stored Procedures: Encapsulate SQL logic within the database to reduce the risk.
3. Input Validation and Sanitization: Validate and sanitize all user inputs to ensure they meet expected patterns.
4. Use of ORMs (Object-Relational Mappers): These tools can help manage database access more safely.
5. Least Privilege Principle: Ensure that database accounts have the minimum necessary permissions.

By following these practices, developers can significantly reduce the risk of SQL injection attacks.