How string concatenation creates vulnerabilities in SQL - Visual Walkthrough

Concept Flow - How string concatenation creates vulnerabilities

User Input Received

↓

Concatenate Input into SQL Query String

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Query String with User Input

↓

Send Query to Database

↓

Database Executes Query

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If Input Malicious -> Unexpected Behavior

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Security Vulnerability Occurs

User input is joined directly into a SQL query string, which the database executes. If input is malicious, it can change the query meaning, causing vulnerabilities.

Execution Sample

SQL

DECLARE @userInput NVARCHAR(100) = '1; DROP TABLE Users;';
DECLARE @sql NVARCHAR(MAX);
SET @sql = 'SELECT * FROM Products WHERE ProductID = ' + @userInput;
EXEC(@sql);

This code builds a SQL query by joining user input directly, then runs it. Malicious input can alter the query.

Execution Table

Step	Action	Value of @userInput	Value of @sql	Effect
1	Set user input	'1; DROP TABLE Users;'	NULL	User input contains malicious SQL
2	Concatenate input into query	'1; DROP TABLE Users;'	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'	Query string now includes malicious command
3	Execute query	'1; DROP TABLE Users;'	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'	Database runs both SELECT and DROP TABLE commands
4	Result	'1; DROP TABLE Users;'	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'	Users table is dropped, vulnerability exploited

💡 Execution stops after malicious query runs, causing data loss

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
@userInput	NULL	'1; DROP TABLE Users;'	'1; DROP TABLE Users;'	'1; DROP TABLE Users;'	'1; DROP TABLE Users;'
@sql	NULL	NULL	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'	'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'

Key Moments - 2 Insights

Why does concatenating user input directly into the SQL query cause a problem?

Because the input can contain SQL commands that change the query's meaning, as shown in step 2 and 3 of the execution table where the malicious DROP TABLE command is added and executed.

Why does the database execute both the SELECT and DROP TABLE commands?

Because the concatenated string includes both commands separated by a semicolon, so the database treats it as two commands to run, as seen in step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of @sql after concatenation in step 2?

A 'SELECT * FROM Products WHERE ProductID = 1;'

B 'SELECT * FROM Products WHERE ProductID = 1; DROP TABLE Users;'

C 'DROP TABLE Users;'

D 'SELECT * FROM Products WHERE ProductID = '

💡 Hint

Check the 'Value of @sql' column in step 2 of the execution table.

At which step does the database execute the malicious DROP TABLE command?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Effect' column in the execution table to see when the command runs.

If the user input was '5' instead of '1; DROP TABLE Users;', what would happen?

A The query would only select product with ID 5 safely.

B The database would drop the Users table anyway.

C The query would fail to run.

D The query would select all products.

💡 Hint

Think about how the concatenated query string changes with safe input, check variable_tracker for @userInput.

Concept Snapshot

String concatenation in SQL joins user input directly into query strings.
If input is malicious, it can add harmful commands.
This causes SQL injection vulnerabilities.
Always avoid direct concatenation; use parameterized queries instead.
This protects the database from unexpected commands.

Full Transcript

This visual execution shows how concatenating user input directly into a SQL query string creates vulnerabilities. The user input '@userInput' is set to a malicious string containing a DROP TABLE command. When concatenated into '@sql', the query string includes both a SELECT and DROP TABLE command. Executing '@sql' causes the database to run both commands, dropping the Users table. The variable tracker shows how '@userInput' and '@sql' change step-by-step. Key moments highlight why direct concatenation is dangerous and how the database executes multiple commands. The quiz tests understanding of the concatenated query and when the malicious command runs. The quick snapshot reminds to avoid string concatenation and use safe parameterized queries to prevent SQL injection.

Practice

1. What is the main security risk when using string concatenation to build SQL queries?

easy

A. It can allow attackers to inject malicious SQL code.

B. It makes queries run faster than parameterized queries.

C. It automatically encrypts user input.

D. It prevents any syntax errors in the query.

Solution

1. Step 1: Understand string concatenation in SQL queries

   Concatenating user input directly into SQL commands can mix code and data.

2. Step 2: Recognize the risk of SQL injection

   This mixing allows attackers to add harmful SQL commands, called SQL injection.

3. Final Answer:

   It can allow attackers to inject malicious SQL code. -> Option A

4. Quick Check:

   SQL injection risk = A [OK]

Hint: Concatenating user input risks injection attacks [OK]

Common Mistakes:

• Thinking concatenation speeds up queries
• Believing concatenation encrypts input
• Assuming concatenation prevents errors

2. Which of the following SQL query constructions using string concatenation is syntactically correct?

easy

A. SELECT * FROM users WHERE name = '\'' + userName + '\'';

B. SELECT * FROM users WHERE name = '\'' + userName + '\'

C. SELECT * FROM users WHERE name = ' + userName + "';

D. SELECT * FROM users WHERE name = '\'' + userName + "';

Solution

1. Step 1: Check string delimiters and concatenation syntax

   In SQL, string literals are enclosed in single quotes. Concatenation in code requires proper quotes and plus signs.

2. Step 2: Identify correct syntax for concatenation in code

   SELECT * FROM users WHERE name = '\'' + userName + '\''; correctly escapes single quotes for SQL string literals and concatenates userName with plus signs.

3. Final Answer:

   "SELECT * FROM users WHERE name = '\'' + userName + '\'';" -> Option A

4. Quick Check:

   Proper quotes and concatenation = A [OK]

Hint: Look for matching quotes and plus signs [OK]

Common Mistakes:

• Missing semicolon at end
• Mismatched or missing quotes
• Using wrong concatenation operators

3. Given the query built by concatenation: "SELECT * FROM users WHERE username = '" + input + "';" and input = admin' OR '1'='1, what will be the effect when executed?

medium

A. Returns only the user with username 'admin'.

B. Returns all users because the WHERE clause is always true.

C. Causes a syntax error and query fails.

D. Returns no rows because input is invalid.

Solution

1. Step 1: Substitute input into the query string

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

2. Step 2: Analyze the WHERE clause logic

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

3. Final Answer:

   Returns all users because the WHERE clause is always true. -> Option B

4. Quick Check:

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

Hint: Injected OR '1'='1' makes WHERE always true [OK]

Common Mistakes:

• Thinking it returns only 'admin'
• Assuming syntax error occurs
• Believing no rows are returned

4. Identify the error in this SQL query construction using string concatenation: query = "SELECT * FROM users WHERE id = " + userId; where userId is a string input.

medium

A. Using plus sign for concatenation is invalid in SQL.

B. No error; query is correct.

C. Query is safe from SQL injection.

D. Missing quotes around userId value causing syntax error.

Solution

1. Step 1: Check how userId is inserted in query

   The query concatenates userId without quotes, so if userId is a string, SQL syntax breaks.

2. Step 2: Understand SQL string literal requirements

   String values must be enclosed in single quotes in SQL to be valid.

3. Final Answer:

   Missing quotes around userId value causing syntax error. -> Option D

4. Quick Check:

   String values need quotes = B [OK]

Hint: String inputs need quotes in SQL queries [OK]

Common Mistakes:

• Thinking plus sign is invalid in code
• Assuming query is safe from injection
• Ignoring missing quotes causes errors

5. You want to prevent SQL injection when building queries with user input. Which approach best avoids vulnerabilities caused by string concatenation?

hard

A. Remove all spaces from user input before concatenation.

B. Escape all single quotes in user input manually before concatenation.

C. Use parameterized queries or prepared statements instead of concatenation.

D. Convert user input to uppercase before concatenation.

Solution

1. Step 1: Understand the problem with string concatenation

   Concatenation mixes code and data, allowing injection if input is malicious.

2. Step 2: Identify secure alternatives

   Parameterized queries separate code from data, preventing injection regardless of input.

3. Final Answer:

   Use parameterized queries or prepared statements instead of concatenation. -> Option C

4. Quick Check:

   Parameterized queries prevent injection = C [OK]

Hint: Always prefer parameterized queries over concatenation [OK]

Common Mistakes:

• Relying on manual escaping which is error-prone
• Thinking removing spaces prevents injection
• Assuming uppercase conversion secures input