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

Short answer: This is a time-based blind SQL injection-style payload in MySQL. It’s designed to cause a delay (sleep) if a certain condition is true, so an attacker can infer information by measuring response time. The 555* is just obfuscation; the important part is the IF with SLEEP.

What the expression does (conceptually)

• 555 * if(now() = sysdate(), sleep(15), 0)
  • The IF checks a condition: now() = sysdate().
  • If true, it executes sleep(15) (causing the server to sleep for 15 seconds).
  • If false, it returns 0.
  • The result is multiplied by 555, but since SLEEP returns 0 after sleeping, the final numeric value is usually 0 in either branch; the key effect is the delay.

Notes on the condition

• NOW() and SYSDATE() are both current timestamps, but in MySQL they can reflect slightly different moments:
  • NOW() is the time at the start of the statement.
  • SYSDATE() is evaluated at the time it’s encountered.
• The equality now() = sysdate() is a timing trick that may be true only if evaluated within the same second, so it’s a fragile condition used to induce a deliberate delay for probing.

Why this is used

• Attackers use the observable delay to infer information about the query or database (e.g., whether a guess about data is true). It’s a classic time-based blind SQL injection technique.

Defensive takeaway (for exams and real-world safety)

• Use parameterized queries and prepared statements; never concatenate user input into SQL.
• Implement input validation and proper escaping.
• Apply least privilege to the database user (no SLEEP or heavy functions exposed if unnecessary).
• Consider WAFs and query logging/auditing to detect long-running or unusual queries.
• Disable or restrict dangerous functions where possible (e.g., SLEEP) or restrict their use via privileges.
• Test security in a controlled, authorized environment.

If you’re studying for an exam, you’ll likely be asked to recognize that this pattern reveals an injection risk and to discuss how timing-based techniques work and how to defend against them—not to implement or misuse them. If you want, I can give you a safe, exam-focused example or a checklist for defending against time-based injections.