: This part attempts to break out of the existing SQL query structure. The closing parenthesis ) is used to "close" a likely function or subquery in the application's original code.
: The original table has exactly 5 columns. This confirms a vulnerability and allows the attacker to move to the next step: identifying which columns can display sensitive data. {KEYWORD}) UNION ALL SELECT NULL,NULL,NULL,NULL,NULL-- ZkhD
The string you provided is a designed to discover the number of columns in a database table. Breakdown of the Payload : This part attempts to break out of
: This is a comment indicator that tells the database to ignore the rest of the original query that follows. This confirms a vulnerability and allows the attacker
: The number of NULL values (5 in this case) does not match the number of columns in the original table.
An attacker (or security researcher) would send this payload to an application to see if it returns an error or a successful response.