Understanding SQL Injection Attacks: Scenarios and Prevention

SQL Injection (SQLi) is one of the most notorious and dangerous web application vulnerabilities. It allows attackers to interfere with the queries an application makes to its database, potentially gaining unauthorized access to data, modifying it, or even executing administrative operations on the database.

This article dives into realistic scenarios where SQL injection attacks may occur, demonstrating how these vulnerabilities can be exploited and how developers can prevent them.

What is SQL Injection?

SQL Injection occurs when user input is improperly sanitized and then incorporated directly into an SQL query. Malicious inputs can manipulate the intended behavior of the query, often bypassing authentication or extracting sensitive data.

Example vulnerable query:

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

If a user inputs:
username = admin' --
password = anything

The resulting SQL becomes:

SELECT * FROM users WHERE username = 'admin' --' AND password = 'anything';

The -- comment ignores the rest of the query, effectively bypassing password verification.

Common SQL Injection Attack Scenarios

1. Authentication Bypass

Scenario: A login form accepts a username and password.
Malicious input:

Username: ' OR '1'='1  
Password: anything

Resulting SQL:

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

Since '1'='1' is always true, the attacker is logged in without knowing valid credentials.

2. Data Exfiltration

Scenario: A search field lets users find customers by name.
Malicious input:

' UNION SELECT card_number, cvv FROM credit_cards --

Resulting SQL:

SELECT * FROM customers WHERE name = '' UNION SELECT card_number, cvv FROM credit_cards --';

The attacker retrieves sensitive credit card information in the search results.

3. Blind SQL Injection

Scenario: An error-free login page provides no feedback on wrong inputs.
Malicious input (injected iteratively):

' AND SUBSTRING((SELECT password FROM users WHERE username='admin'), 1, 1) = 'a' --

Attackers use this technique to guess passwords character-by-character using timing or Boolean responses, even without visible output.

4. Database Destruction (Denial of Service)

Scenario: An update form accepts user IDs.
Malicious input:

1; DROP TABLE users; --

Resulting SQL:

SELECT * FROM users WHERE id = 1; DROP TABLE users; --;

The users table is deleted, causing a major outage or data loss.

5. Extracting Database Metadata

Scenario: An attacker explores the structure of the database.
Malicious input:

' UNION SELECT table_name, column_name FROM information_schema.columns WHERE table_schema = 'public' --

Resulting SQL:
Lists all table and column names in the database, aiding further exploitation.

Prevention Strategies

To defend against SQL injection:

Use Prepared Statements (Parameterized Queries)

Avoid dynamic SQL strings.

Example in Python:

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

ORMs (Object Relational Mappers)

Use frameworks like Django ORM, SQLAlchemy, or Hibernate which automatically parameterize queries.

Input Validation

Ensure inputs are within expected formats, types, and ranges.

Escape Inputs Properly

If dynamic SQL is unavoidable, escape inputs using the proper library functions.

Limit DB Privileges

Use a database user with only the necessary privileges.

Web Application Firewall (WAF)

A WAF can help detect and block SQL injection patterns.

Conclusion

SQL Injection remains a top web security threat, especially in legacy applications or when best practices are ignored. Understanding how attackers exploit these vulnerabilities is the first step toward building secure applications. By consistently using secure coding techniques, developers can effectively eliminate the risk of SQL injection attacks.