{keyword}) Union All Select Null,null,null,null,null-- Zkhd -

An attacker (or security researcher) would send this payload to an application to see if it returns an error or a successful response.

: 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. {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. An attacker (or security researcher) would send this

: The number of NULL values (5 in this case) does not match the number of columns in the original table. : This is a comment indicator that tells

: 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 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.

: This command combines the result set of the original query with a new set of data. UNION ALL is used instead of UNION because it is often faster and does not remove duplicates, which can be useful for certain types of data extraction.