Advanced SQL Injection Explained: SQLi Attack Flow from Input to Compromise

SQL Injection (SQLi) is one of the most dangerous and long-standing web application vulnerabilities. Despite being well known, it continues to cause massive data breaches due to poor input validation and insecure coding practices.

This post provides a deep, stage-by-stage explanation of how an advanced SQL Injection attack works, following the exact flow shown in the image: from vulnerable user input to full database compromise.

1. What Is SQL Injection?

Definition

SQL Injection (SQLi) is a web application vulnerability that occurs when an attacker is able to insert or manipulate SQL queries by injecting malicious input into an application’s database query.

Why SQLi Is Dangerous

Allows authentication bypass
Exposes sensitive data (users, passwords, credit cards)
Can lead to full database compromise
May result in complete system takeover

2. SQL Injection Attack Flow – High-Level Overview

An SQL Injection attack typically follows three major stages:

Vulnerable Input – Entry point where user input is not validated
Malicious Query Construction – Attacker manipulates SQL logic
Database Compromise – Unauthorized access and data extraction

3. Stage 1: Vulnerable Input (The Entry Point)

What Is Vulnerable Input?

A vulnerable input is any user-controlled field where data is directly passed to a database query without proper validation or sanitization.

Common Vulnerable Inputs

Login forms (username / password)
Search boxes
URL parameters
Cookies

What Goes Wrong

Instead of treating user input as plain data, the application blindly trusts the input and embeds it directly into an SQL query.

Example

The attacker enters malicious input such as:

' OR 1=1 --

This input is not expected data but SQL logic.

4. Stage 2: Malicious Query Construction (The Mechanism)

Vulnerable Backend Code

In insecure applications, backend code may look like this:

SELECT * FROM users
WHERE username = '$username'
AND password = '$password';

Here, user input is directly concatenated into the SQL query.

How the Attacker Manipulates the Query

When the attacker injects:

' OR 1=1 --

The resulting SQL query becomes:

SELECT * FROM users
WHERE username = '' OR 1=1 -- '
AND password = '';

Why This Works

OR 1=1 always evaluates to TRUE
The WHERE clause is bypassed
-- comments out the rest of the query

As a result, authentication is completely bypassed.

5. Stage 3: Database Compromise & Impact (The Consequence)

Database Executes the Malicious Query

The database does not know the intent of the query. It executes the altered SQL command as valid logic.

What the Attacker Gains

Login without credentials
Access to all user records
Extraction of password hashes
Administrative access

Attacker Console View

Attackers may retrieve:

Usernames
Passwords (hashed or plaintext)
Roles (admin, user)

Critical Impact

Unauthorized access
Data breach
Loss of confidentiality
Possible full system takeover

6. Types of SQL Injection (Important for Exams)

1. In-Band SQL Injection

Error-based SQLi
Union-based SQLi

2. Blind SQL Injection

Boolean-based
Time-based

3. Out-of-Band SQL Injection

Uses DNS or HTTP requests
Rare but powerful

7. Why SQL Injection Still Exists

Legacy applications
Improper input validation
Use of dynamic SQL queries
Lack of secure coding practices

8. SQL Injection Prevention (Critical Section)

1. Parameterized Queries (Prepared Statements)

User input is treated strictly as data, not executable code.

2. Input Validation & Sanitization

Whitelist validation
Reject unexpected characters

3. Least Privilege Principle

Database users should not have admin rights

4. Web Application Firewalls (WAF)

Detect and block SQL injection patterns.

5. Secure Coding Practices

Avoid dynamic SQL
Use ORM frameworks

9. SQL Injection in OWASP Top 10

SQL Injection is part of the OWASP Top 10 – Injection category and remains one of the highest-risk web vulnerabilities.

10. Career & Certification Relevance

Certifications

CEH
Security+
OSCP

Job Roles

Web Security Analyst
Application Security Engineer
Penetration Tester

Conclusion

SQL Injection demonstrates how a single vulnerable input field can compromise an entire database.

Understanding the full SQLi attack flow is essential for both attackers (red team) and defenders (blue team).

Secure coding, proper validation, and defense-in-depth are the only effective ways to eliminate SQL Injection risks.

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