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Intermediate Topic 24 of 51 Security & Cryptography

SQL Injection

Attack inserting malicious SQL through user input.

How it works

SQL injection occurs when an application builds a database query by inserting untrusted input into SQL text, letting an attacker change the query's structure, for example by supplying input such as a quote followed by an OR condition that is always true. Consequences include reading or modifying any data, bypassing login, and sometimes executing commands on the server. Variants include in-band, blind and out-of-band injection.

Step-by-step walkthrough

  1. The app builds SQL with string concatenation.
  2. The attacker types ' OR '1'='1.
  3. The query becomes always true.
  4. The database returns all rows.
  5. The fix is parameterized queries, which keep input as data.

Where it is used

Understanding it matters for every developer writing database code. The defense is parameterized queries or prepared statements, which keep data separate from code, with ORMs that use them by default.

Common pitfall

String concatenation and escaping by hand are fragile. Dynamic parts such as table names or ORDER BY columns cannot be parameterized and must be validated against an allow-list. Stored procedures built from concatenated strings remain vulnerable. Apply least privilege to database accounts, hide detailed error messages and use web application firewalls only as an extra layer. Test with security scanners and code review.

Where it fits in the course

SQL Injection is topic 24 of 51 in Security & Cryptography, rated Intermediate. It follows Message Authentication Codes. The next topic is Cross-Site Scripting.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Security & Cryptography quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is SQL Injection?
Attack inserting malicious SQL through user input. It belongs to Security & Cryptography and is an intermediate-level topic.
Where do people use SQL Injection?
Understanding it matters for every developer writing database code.
What is the most common mistake with SQL Injection?
String concatenation and escaping by hand are fragile.
What should I learn before this topic?
Start with Message Authentication Codes, the topic just before it in Security & Cryptography.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Security & Cryptography quiz.
Is SQL Injection hard to learn?
It is an Intermediate topic. Be comfortable with the Beginner topics in this area first, then read the common pitfall carefully, since it is where most mistakes happen.
How long does it take to learn SQL Injection?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to SQL Injection?
Message Authentication Codes, Cross-Site Scripting, Block Cipher Modes are closely connected. The full list is in the related topics section of this page.
What should I study after SQL Injection?
Continue with Cross-Site Scripting, the next topic in Security & Cryptography.

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