: This treats user input as data, not as executable code.
: This part attempts to "break out" of the existing SQL command. The closing parenthesis ) is used to close a function or a nested query that the developer originally intended.
The string you provided is a specific used to test for vulnerabilities in a database. It is designed to trick a web application into running a second, unauthorized query and appending the results to the original one. Breakdown of the Payload {KEYWORD}) UNION ALL SELECT NULL,NULL,NULL,NULL,NULL,NULL#
: In MySQL, the hash symbol marks the rest of the line as a comment . This effectively deletes any remaining parts of the original developer's code (like a trailing WHERE clause or a closing quote) that would otherwise cause a syntax error. Why This Matters
: Only allow expected characters and formats. : This treats user input as data, not as executable code
If this payload successfully returns a blank page instead of an error, it confirms to a tester that the application is vulnerable. From there, they can replace the NULL s with commands to extract sensitive data, such as: Usernames and passwords. Database version and configuration details. The entire contents of specific tables. How to Prevent It
To protect your application from this type of attack, you should avoid building queries using simple string concatenation. Instead, use: The string you provided is a specific used
: This is the core of the attack. The UNION operator combines the results of two or more SELECT statements into a single result set. ALL ensures that duplicate rows are kept.