Introduction to SQL Injection

SQL injection is a code injection technique used to exploit security vulnerabilities in an application's database layer. Specifically, it is where malicious SQL statements are inserted into an entry field for execution. It is a common and serious threat to web application security.

Let's look at a simple example. Suppose you have a login form that checks a username and password against a database using the following SQL:

query = "SELECT * FROM users WHERE username = '" + username + "' AND password = '" + password + "'";

An attacker can use SQL injection to bypass the login mechanism by entering the following:

Username: admin' -- Password: any password

This results in the following query:

query = "SELECT * FROM users WHERE username = 'admin' --' AND password = 'any password'";

Because -- is the SQL comment syntax, everything after -- is ignored, effectively nullifying the password check. The SQL now simply checks if there is a user named "admin", and if so, allows access.

Prevention Techniques

Prepared Statements (Parameterized Queries): Rather than constructing SQL statements with string concatenation, use prepared statements which work by defining all the SQL code first and then passing in each parameter to the query later. This ensures that an attacker cannot change the intent of a query.
Stored Procedures: Like prepared statements, stored procedures separate the data from the query, and are defined and stored in the database itself.
Input Validation: Inputs should be validated using a whitelist approach, where only specified input is accepted. Avoid a blacklist approach where specified input is rejected, as this can be hard to maintain and can be bypassed by an attacker.
Escaping: All user supplied input should be escaped. In this context, escaping means to tell the database to interpret input as data, rather than SQL code.
Least Privilege: Your database user should only have the minimum level of privilege necessary to perform their actions. This limits potential damage if an injection attack occurs.
Web Application Firewalls (WAFs): A WAF can help filter out malicious data by inspecting HTTP traffic before it reaches your application.

Please note that not all of these measures are needed in all languages/environments. For example, if you're using an ORM (Object Relational Mapping) library or modern data access tools, SQL injection might be mostly taken care of. It's always important to understand the specific security considerations of your specific technologies and libraries.

Detecting and Preventing SQL Injection Attacks:
- Description: SQL Injection is a type of cyber attack where malicious SQL statements are inserted into an input field to manipulate a database. Detection involves monitoring for unusual database behavior, while prevention includes input validation and the use of parameterized queries.
- Code Example (Preventing SQL Injection with Parameterized Query):
```
# Using parameterized query in Python with SQLite
import sqlite3

def execute_query(username):
    connection = sqlite3.connect("example.db")
    cursor = connection.cursor()

    # Parameterized query to prevent SQL Injection
    cursor.execute("SELECT * FROM Users WHERE username=?", (username,))

    results = cursor.fetchall()

    connection.close()

    return results
```

SQL Injection Examples:

Description: Examples of SQL Injection involve manipulating input fields to inject malicious SQL code, such as UNION-based attacks, error-based attacks, and time-based blind attacks.

Example:

-- UNION-based SQL Injection
SELECT username, password FROM Users WHERE username = 'admin' UNION SELECT 1, 'hacked';

-- Error-based SQL Injection
SELECT username, password FROM Users WHERE username = 'admin' AND 1=CONVERT(int, (SELECT @@version));

-- Time-based Blind SQL Injection
SELECT username, password FROM Users WHERE username = 'admin' AND IF(1=1, SLEEP(5), 0);

How to Secure Against SQL Injection:

Description: Techniques to secure against SQL Injection include input validation, using parameterized queries, and employing proper access controls.

Code Example (Parameterized Query):

# Parameterized query in Python with MySQL
import mysql.connector

def execute_query(username):
    connection = mysql.connector.connect(
        host="localhost",
        user="user",
        password="password",
        database="example"
    )
    cursor = connection.cursor()

    # Parameterized query to prevent SQL Injection
    cursor.execute("SELECT * FROM Users WHERE username=%s", (username,))

    results = cursor.fetchall()

    connection.close()

    return results

Parameterized Queries to Prevent SQL Injection:

Description: Parameterized queries use placeholders for input values, preventing direct insertion of user inputs into SQL statements.

Code Example (Parameterized Query):

// Parameterized query in Java with JDBC
PreparedStatement statement = connection.prepareStatement("SELECT * FROM Users WHERE username=?");
statement.setString(1, username);
ResultSet results = statement.executeQuery();

Sanitizing User Input to Avoid SQL Injection:
- Description: Sanitizing involves cleaning and validating user inputs to ensure they do not contain malicious code.
- Code Example (Python):
```
import re

def sanitize_input(input_str):
    # Remove any non-alphanumeric characters
    sanitized_str = re.sub(r'[^a-zA-Z0-9]', '', input_str)
    return sanitized_str
```
SQL Injection Testing Tools and Methods:
- Description: Tools like SQLMap and methods like manual testing are used to identify and address SQL Injection vulnerabilities.
- Example (SQLMap):
```
sqlmap -u "http://example.com/login" --data="username=admin&password=test" --level=5 --risk=3
```
Impact of SQL Injection on Database Security:
- Description: SQL Injection can lead to unauthorized access, data manipulation, and exposure of sensitive information. It poses a severe threat to database security.
- Example (Unauthorized Access):
```
-- SQL Injection to bypass authentication
SELECT * FROM Users WHERE username='admin' OR 1=1 --' AND password='password';
```