SQL Injection Attack

In this comprehensive guide, we'll explore the SQL injection attack in depth, including how it works, its different types, and prevention techniques with examples.

What is SQL Injection?

SQL injection is a type of cyber attack in which an attacker inserts malicious SQL queries into an application's input fields or parameters with the intent to manipulate the database behind the application. This can lead to serious consequences including data breach, data loss, unauthorized access, and even entire system compromise.

How SQL Injection Works?

SQLi attacks occur when a web application fails to properly validate and sanitize user input. When a user interacts with a web application, their input is often passed to a database server as part of an SQL query. If the application does not correctly validate and sanitize this input, an attacker can manipulate it to inject malicious SQL code.

Here's a simplified step-by-step explanation:

  1. User Input: The attacker identifies a vulnerable input field or parameter within a web application, such as a search bar, login form, or URL parameter.
  2. Malicious Input: The attacker provides input that includes malicious SQL code. This input is often referred to as the "payload."
  3. Injection Point: The malicious input is inserted into the SQL query constructed by the web application. This injection point is where the attacker's payload becomes part of the SQL query.
  4. Execution: The web application sends the modified SQL query to the database server for execution. Since the payload is part of the query, it gets executed along with the legitimate SQL code.
  5. Results: The database server processes the query, and the results are sent back to the web application. Depending on the payload, the attacker can extract sensitive data, modify the database, or gain unauthorized access.

Types and Examples

SQLi can be classified into three major categories:

1. In-band (Classic) SQL Injection

Classic SQL Injection, also known as in-band SQL Injection, is the most straightforward type. In this attack, the attacker directly communicates with the database server and retrieves data or performs actions.

There are two subtypes:

a. Error-Based SQL Injection

The attacker manipulates the input to trigger SQL errors. These errors can reveal information about the database schema or the data within it.

b. Union-Based SQL Injection

The attacker uses the UNION SQL operator to combine the results of their injected query with the original query's results. This allows them to extract data from the database.


Suppose we have a vulnerable login form on a website, and the SQL query used for authentication looks like this: SELECT * FROM users WHERE username = '$username' AND password = '$password'; The $username and $password variables are populated with user input. An attacker can manipulate the input to bypass authentication. Here's an example payload: Username: admin' OR '1'='1
Password: anything
When the application constructs the query, it becomes: SELECT * FROM users WHERE username = 'admin' OR '1'='1' AND password = 'anything'; The condition '1'='1' always evaluates to true, so the attacker gains access without knowing a valid password.

2. Inferential (Blind) SQL Injection

Blind SQL Injection, also known as inferential SQL Injection, is more challenging to exploit than classic SQL Injection because the attacker does not receive direct responses from the database server. Instead, they infer information through true/false responses from the application.

Blind SQL Injection comes in two main forms:

a. Boolean-Based Blind SQL Injection

The attacker crafts payloads that result in true or false conditions within the SQL query. By observing the application's behavior, they can deduce information about the database.

b. Time-Based Blind SQL Injection

In this variant, the attacker introduces time delays in the SQL query. They observe whether the application's response is delayed or not to infer whether their conditions are met.


In a blind SQL Injection scenario, the attacker cannot directly see query results but can infer information. Consider a URL parameter in a vulnerable website: http://example.com/search?query=apple' AND 1=IF(1=1, SLEEP(5), 0)-- In this payload, the attacker is testing whether the application experiences a 5-second delay when the condition 1=1 is true. If it does, they know the condition is true.

For time-based blind SQL Injection, the attacker may use a payload like this: http://example.com/product?id=1' AND IF(1=1, SLEEP(5), 0)-- Here, the attacker is testing whether the application delays for 5 seconds when the condition 1=1 is true. If there's a delay, it indicates a successful injection.

3. Out-of-band SQL Injection

This type of attack involves using a different channel, such as email or DNS, to retrieve the results of the attack. This technique relies on the database server's ability to make DNS or HTTP requests to deliver data to the attacker.

Prevention Techniques

Here are some of the best practices and techniques to prevent SQLi attacks:

1. Prepared Statements (Parameterized Queries)

Use prepared statements and parameterized queries provided by your programming language or framework. These mechanisms separate SQL code from user input, making it impossible for attackers to inject malicious SQL.

Example (Python using SQLite):

import sqlite3

conn = sqlite3.connect("mydatabase.db")
cursor = conn.cursor()

username = input("Enter username: ")
password = input("Enter password: ")

cursor.execute("SELECT * FROM users WHERE username = ? AND password = ?", (username, password))

# The query is automatically parameterized, preventing SQL Injection.

2. Input Validation and Sanitization

Validate and sanitize user input to ensure it adheres to expected formats and values. Reject input that does not meet validation criteria.

Example (PHP):

$username = $_POST['username'];
if (!preg_match('/^[a-zA-Z0-9]+$/', $username)) {
    die("Invalid username format");

// Continue with the query after validation.

3. Least Privilege Principle

Grant the minimum necessary database privileges to your application's database user. Restrict the application's access to only the required tables and operations.

4. Web Application Firewall (WAF)

Implement a Web Application Firewall that can detect and block SQL Injection attempts. WAFs can be configured to recognize and prevent common attack patterns.

5. Escaping User Input

If you can't use prepared statements or parameterized queries, make use of input escaping functions provided by your database library to sanitize user input before embedding it in SQL queries.

Example (Node.js with MySQL):

const mysql = require('mysql');
const connection = mysql.createConnection({
    host: 'localhost',
    user: 'user',
    password: 'password',
    database: 'mydb'

const username = connection.escape(req.body.username);
const password = connection.escape(req.body.password);

const query = `SELECT * FROM users WHERE username = ${username} AND password = ${password}`;

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