: The attacker uses a specific number of NULL values to match the number of columns in the original query's SELECT statement. NULL is used because it is compatible with almost any data type (strings, integers, dates), maximizing the chance that the injected query will succeed.
: 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
: This is a comment indicator that tells the database to ignore the rest of the original query that follows. : The attacker uses a specific number of
The string you provided is a designed to discover the number of columns in a database table. Breakdown of the Payload This confirms a vulnerability and allows the attacker
An attacker (or security researcher) would send this payload to an application to see if it returns an error or a successful response.
: This is likely a random string or a "signature" used by a vulnerability scanner (like Burp Suite or sqlmap) to track if the payload was successfully reflected in the application's response.
: The number of NULL values (5 in this case) does not match the number of columns in the original table.