How SQL injection exploits queries - Why You Should Know This

The Big Idea

What if a tiny trick in your login box lets strangers steal all your secrets?

The Scenario

Imagine you have a website login form where users type their username and password. You check these by manually building a query string that looks like: SELECT * FROM users WHERE username = 'user' AND password = 'pass'. You do this by just adding the typed words into the query text.

The Problem

This manual way is risky because if someone types special characters or code instead of a normal username, they can trick your query to do things you never wanted. This can let them see secret data or even change your database without permission.

The Solution

Understanding how SQL injection works helps you see why you must never just add user input directly into queries. Instead, you use safe methods like prepared statements or parameterized queries that keep user input separate from the command, stopping attackers from changing your query's meaning.

Before vs After

✗ Before

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

✓ After

query = "SELECT * FROM users WHERE username = ? AND password = ?"; execute(query, [user_input, pass_input]);

What It Enables

By preventing SQL injection, you protect your data and users, making your applications safe and trustworthy.

Real Life Example

A hacker types ' OR '1'='1 as a password, turning your query into something that always returns true, letting them log in without a real password. Knowing this helps you stop such attacks.

Key Takeaways

Manually building queries with user input is dangerous.

SQL injection lets attackers change your query's meaning.

Using safe query methods protects your data and users.

Practice

1. What is the main risk of SQL injection attacks on a database?

easy

A. Slower query performance

B. Unauthorized access to or manipulation of data

C. Syntax errors in SQL queries

D. Increased storage space usage

Solution

1. Step 1: Understand what SQL injection does

   SQL injection allows attackers to insert malicious SQL code into queries to manipulate the database.

2. Step 2: Identify the main consequence

   This manipulation can lead to unauthorized access or changes to data, which is a serious security risk.

3. Final Answer:

   Unauthorized access to or manipulation of data -> Option B

4. Quick Check:

   SQL injection = Unauthorized data access [OK]

Hint: SQL injection lets attackers access or change data illegally [OK]

Common Mistakes:

• Confusing SQL injection with performance issues
• Thinking it only causes syntax errors
• Assuming it increases storage use

2. Which of the following SQL query snippets is vulnerable to SQL injection if user input is not sanitized?
SELECT * FROM users WHERE username = '" + userInput + "';

easy

A. SELECT * FROM users WHERE username = '" + userInput + "';

B. SELECT * FROM users WHERE username = ?;

C. SELECT * FROM users WHERE username = 'admin';

D. SELECT * FROM users WHERE username = 'admin' AND password = 'pass';

Solution

1. Step 1: Identify how user input is used

   The query concatenates userInput directly into the SQL string without sanitization.

2. Step 2: Recognize vulnerability

   This direct insertion allows attackers to inject malicious SQL code, making it vulnerable.

3. Final Answer:

   SELECT * FROM users WHERE username = '" + userInput + "'; -> Option A

4. Quick Check:

   Direct input concatenation = Vulnerable [OK]

Hint: Directly adding user input to query string is unsafe [OK]

Common Mistakes:

• Choosing parameterized queries as vulnerable
• Confusing safe hardcoded queries with vulnerable ones
• Ignoring the role of input sanitization

3. Given the vulnerable query:
SELECT * FROM users WHERE username = '" + userInput + "';
If userInput is admin' OR '1'='1, what will the query become and what is the likely result?

medium

A. Syntax error due to unmatched quotes

B. SELECT * FROM users WHERE username = 'admin OR 1=1'; -- Returns no users

C. SELECT * FROM users WHERE username = 'admin'; -- Returns only admin user

D. SELECT * FROM users WHERE username = 'admin' OR '1'='1'; -- Returns all users

Solution

1. Step 1: Substitute userInput into the query

   The query becomes: SELECT * FROM users WHERE username = 'admin' OR '1'='1';

2. Step 2: Understand the effect of OR '1'='1'

   The condition '1'='1' is always true, so the WHERE clause matches all rows.

3. Final Answer:

   SELECT * FROM users WHERE username = 'admin' OR '1'='1'; -- Returns all users -> Option D

4. Quick Check:

   Injected OR '1'='1' returns all rows [OK]

Hint: OR '1'='1' always true returns all rows [OK]

Common Mistakes:

• Thinking it causes syntax error
• Assuming it returns only admin user
• Misreading the injected condition

4. You have this vulnerable SQL query in your code:
query = "SELECT * FROM users WHERE username = '" + userInput + "';"
Which fix correctly prevents SQL injection?

medium

A. Remove the WHERE clause entirely

B. Escape single quotes in userInput manually

C. Use parameterized queries or prepared statements

D. Convert userInput to uppercase before using

Solution

1. Step 1: Identify secure coding practice

   Parameterized queries separate code from data, preventing injection.

2. Step 2: Evaluate other options

   Manual escaping is error-prone, removing WHERE breaks logic, uppercase conversion doesn't prevent injection.

3. Final Answer:

   Use parameterized queries or prepared statements -> Option C

4. Quick Check:

   Parameterized queries = Safe from injection [OK]

Hint: Always use parameterized queries to stop injection [OK]

Common Mistakes:

• Relying on manual escaping only
• Removing query conditions instead of fixing
• Thinking input case changes prevent injection

5. Consider a login system using this vulnerable query:
SELECT * FROM users WHERE username = '" + userInput + "' AND password = '" + passInput + "';
If an attacker inputs admin' -- as username and anything as password, what happens and why?

hard

A. Login succeeds without password check because -- comments out rest

B. Login fails due to syntax error from unmatched quotes

C. Login succeeds only if password matches exactly

D. Login fails because input is rejected by the system

Solution

1. Step 1: Substitute attacker input into query

   The query becomes: SELECT * FROM users WHERE username = 'admin' --' AND password = 'anything';

2. Step 2: Understand effect of -- comment

   The -- makes the rest of the query a comment, so password check is ignored.

3. Step 3: Result of query

   The query checks only username = 'admin', allowing login without password.

4. Final Answer:

   Login succeeds without password check because -- comments out rest -> Option A

5. Quick Check:

   -- comments out password check = Login bypass [OK]

Hint: SQL comment -- skips password check for login bypass [OK]

Common Mistakes:

• Assuming syntax error occurs
• Thinking password is still checked
• Believing input is blocked automatically