555*if(now()=sysdate(),sleep(15),0)

That looks like a time-based blind SQL injection payload written for MySQL.

What it does (conceptually):

• 555 * IF(now() = sysdate(), sleep(15), 0)
• IF(condition, then, else) evaluates the condition.
• sleep(15) causes the database to pause for 15 seconds; otherwise the value is 0.
• NOW() and SYSDATE() both return the current date/time, but they can differ slightly in MySQL. In practice, now() = sysdate() is true only very rarely (it would require the two calls to resolve to exactly the same instant).
• If the condition is false, the expression yields 0 and nothing sleeps.
• If the condition were true, the server would sleep 15 seconds, which could be used in a timing attack to infer information about the database.

Notes:

• The multiplication by 555 doesn’t affect the sleep; the important part is the IF(..., SLEEP(15), 0) side effect.
• This is a classic example of a time-based blind SQL injection technique. It relies on observable delays to infer boolean results.

Important: use such techniques only in legal, controlled environments (your own lab, intentionally vulnerable apps like DVWA/WebGoat, or with explicit authorization). They are illegal and unethical on systems you don’t own or don’t have permission to test.

If you’re studying for an exam, I can help with:

• How time-based blind SQL injection works in general.
• How to defend against it (prepared statements, parameterized queries, input validation, least privilege, WAF, etc.).
• Safe practice examples in a lab setup. Would you like a quick example of safe lab exercises or a few practice questions?