What is SQL Injection and How to Prevent It?

SQL Injection is one of the most common and dangerous vulnerabilities in web applications. This attack method allows malicious actors to gain unauthorized access to a database. It can be used to alter data, steal user credentials, or perform harmful operations on the server.

In this guide, we will cover how SQL Injection works and how to protect against it in detail.

1. How SQL Injection Works

SQL Injection attacks usually exploit vulnerabilities in user input forms, search boxes, or URL parameters.

For example, the following insecure SQL query is vulnerable:

$user = $_GET['user'];
$query = "SELECT * FROM users WHERE username = '$user'";
$result = mysqli_query($conn, $query);

If an attacker enters the following input:

' OR '1'='1

The resulting SQL query becomes:

SELECT * FROM users WHERE username = '' OR '1'='1'

This query always returns true, allowing the attacker to access all user data! 🔥

2. What Can Be Done with SQL Injection?

• ✅ Steal user credentials
• ✅ Gain unauthorized access to admin accounts
• ✅ Modify or delete database records
• ✅ Execute harmful commands on the server

3. How to Prevent SQL Injection

1️⃣ Use Prepared Statements

Prepared statements isolate user input from SQL logic, significantly improving security.

Secure Query Using PHP PDO:

$stmt = $pdo->prepare("SELECT * FROM users WHERE username = ?");
$stmt->execute([$user]);
$result = $stmt->fetch();

Secure Query Using MySQLi:

$stmt = $conn->prepare("SELECT * FROM users WHERE username = ?");
$stmt->bind_param("s", $user);
$stmt->execute();

✅ Advantages:

• Prevents SQL Injection
• Handles data safely
• Improves performance

2️⃣ Filter User Input

Validate and sanitize user inputs to block malicious values.

$username = filter_input(INPUT_GET, 'user', FILTER_SANITIZE_STRING);

❌ Incorrect Example:

$query = "SELECT * FROM users WHERE username = '" . $_GET['user'] . "'";

🚫 This is not secure!

3️⃣ Use a Web Application Firewall (WAF)

WAFs can automatically block malicious SQL requests. Recommended solutions:

• 🔹 Cloudflare WAF
• 🔹 ModSecurity
• 🔹 Sucuri Firewall

4️⃣ Restrict Database Privileges

Limit the privileges of database users to reduce the impact of potential attacks.

• ✅ Avoid using root user
• ✅ Grant only required permissions (e.g., SELECT only)
• ✅ Create separate users for different operations

GRANT SELECT ON database_name.* TO 'readonly_user'@'localhost' IDENTIFIED BY 'strongpassword';

Summary

SQL Injection attacks are easy to prevent with the right precautions:

• ✅ Always use prepared statements
• ✅ Sanitize and validate user inputs
• ✅ Implement firewalls and access control

By taking these steps, you can protect your database and users from one of the most dangerous web threats.