Why is UNION Query-Based SQLi the Fastest SQL Injection Type?

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The UNION-based SQL Injection technique is often considered one of the fastest ways to extract data from a vulnerable database. This is due to several reasons, including direct data retrieval, minimal execution complexity, and leveraging native SQL functions. Below is a detailed explanation.

1. Direct Data Retrieval in a Single Query

Unlike Blind SQL Injection (Boolean-based or Time-based), where the attacker has to infer data bit by bit or introduce artificial delays, UNION-based SQL Injection allows direct extraction of multiple rows in a single request.

Example:

Consider a vulnerable website where an attacker can manipulate the id parameter:

http://example.com/index.php?id=1

If the parameter is vulnerable to SQL Injection, the attacker can modify it as:

http://example.com/index.php?id=1 UNION SELECT username, password FROM users

Since the UNION operator merges results from two queries, the database directly returns usernames and passwords in the web application’s response. This eliminates the need for time-consuming brute-force inference used in Blind SQLi.

2. Leverages Native SQL Optimization

Databases optimize UNION queries efficiently because they are a standard SQL operation. The execution plan for a UNION query is optimized for speed as long as:

  • The columns match in count and data type.
  • Indexes exist on the tables being queried.

Most databases handle UNION queries quickly using indexing and query execution plans, making them faster than techniques requiring multiple separate queries or conditional execution (e.g., Blind SQLi).

3. Fewer Requests Compared to Other Techniques

Other SQLi techniques require multiple requests:

  • Boolean-based SQLi → Requires multiple requests to infer each character (A-Z, 0-9) of a string one at a time.
  • Time-based SQLi → Relies on delays (SLEEP() or WAITFOR DELAY), making extraction slow.
  • Error-based SQLi → Depends on error messages, which may be filtered or restricted.

In contrast, UNION-based SQLi extracts all necessary data in a single query, significantly reducing the number of requests and improving speed.

4. Works Well with Error-Based SQLi

Many applications return descriptive SQL errors, which help attackers determine the number of columns required for UNION-based injection.

For example, consider:

http://example.com/index.php?id=1 ORDER BY 5 --

If an error occurs (Unknown column '5' in 'order clause'), the attacker knows the number of columns is less than 5 and can adjust the UNION query accordingly.

Combining Error-based SQLi with UNION-based SQLi allows attackers to quickly craft a working exploit.

5. Minimal Need for Complex Logic

  • Boolean-based SQLi → Requires logical inference (IF condition THEN... ELSE...).
  • Time-based SQLi → Requires precise timing calculations.
  • Out-of-band SQLi → Requires secondary channels (e.g., DNS exfiltration).

UNION-based SQLi only requires knowing the table schema, making it simpler and faster to execute.

6. Exploits Readable Data Easily

If an application directly displays database results on a webpage, UNION-based SQLi instantly exposes sensitive data.

For example:

http://example.com/index.php?id=1 UNION SELECT username, password FROM users

Would directly return:

admin | admin123
user1 | password123

This instant access makes UNION-based SQLi significantly faster than Blind SQLi.

7. Bypasses Filtering in Many Cases

Many WAFs (Web Application Firewalls) and security filters focus on common SQLi payloads like:

  • 1=1
  • SLEEP(5)
  • AND 'a'='a'

However, UNION-based SQLi often bypasses naive filtering since:

  • UNION is a valid SQL keyword.
  • Filtering for UNION alone might break legitimate queries.
  • Encoding tricks (UNI%4F%4E) can further bypass protection.

This ease of exploitation contributes to its speed.

When is UNION-Based SQLi Not Feasible?

Despite being one of the fastest techniques, UNION-based SQL Injection may not always work in the following cases:

  1. Different Column Counts → The number of columns in the first query must match the number of columns in the second.
  2. Data Type Mismatch → The columns must have compatible data types.
  3. No Error Messages → If error-based information is hidden, attackers might struggle to identify column count.
  4. Limited User Privileges → If the SQL user lacks permission to query certain tables, UNION may fail.
  5. WAF Filtering → Some WAFs detect UNION-based SQLi and block such requests.

In such cases, attackers resort to Blind SQLi or Out-of-band (OOB) SQLi, which are slower.

Conclusion

UNION-based SQL Injection is the fastest method when:

  • The application returns query results on a webpage.
  • The attacker can determine the number and types of columns.
  • The database supports error-based information leaks.

It is faster than Blind and Time-based SQLi because: ✅ It retrieves full records in one query.
✅ It avoids multiple requests per character.
✅ It benefits from native SQL optimizations.

This efficiency makes UNION-based SQLi a preferred attack vector in real-world exploitation.

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