5550'XOR(555*if(now()=sysdate(),sleep(15),0))XOR'Z

That string is a time-based blind SQL injection payload, likely aimed at MySQL. It’s designed to be injected into an SQL query so that the database will sleep for 15 seconds if a certain condition is true, allowing an attacker to infer information from the response time.

What the pieces do (high level):

• 5550' ... 'Z: Aiming to break out of a quoted string in an existing query and inject extra SQL.
• XOR(... ) XOR: Uses the XOR operator (a boolean operator in SQL) as part of the injection syntax.
• 555*if(now()=sysdate(),sleep(15),0): The inner part asks MySQL to evaluate now() = sysdate(); if true, it runs sleep(15) (delay of 15 seconds); otherwise it evaluates to 0. The sleep(15) introduces a measurable delay.
• The overall effect in a vulnerable query is a pause in the server’s response, which an attacker can use to detect whether the condition was true and, over multiple trials, infer information about the database.

Important context:

• Time-based blind SQLi relies on observed delays rather than returned data. If a vulnerable application’s responses slow down by specific amounts, an attacker can deduce whether certain conditions hold (e.g., whether a value equals something).
• The inclusion of NOW() and SYSDATE() is a common trick to create a time-based condition, since those functions return the current time and can be compared.

Defensive measures (recommended for exam preparation and real-world defense):

• Use parameterized queries / prepared statements everywhere. Do not concatenate user input into SQL strings.
• Validate and sanitize inputs, and apply a strict least-privilege policy for the database user (e.g., no rights to sleep or to execute dangerous functions if not needed).
• Prefer stored procedures with proper binding rather than dynamic SQL.
• Use input whitelisting where feasible (e.g., allow only numeric IDs with proper typing).
• Implement a web application firewall (WAF) with SQLi detection rules and monitor for unusual sleep() calls or other time-based patterns.
• Disable or restrict dangerous functions at the application level or via database configuration if possible (e.g., limit or monitor the use of sleep-based calls).
• Error handling: don’t leak detailed database error messages to users; log them server-side for forensics.
• Auditing and monitoring: look for odd delays, repeated attempts, or patterns that suggest time-based injections.
• In a lab or training scenario: practice with safe, legal environments (DVWA, OWASP Juice Shop, bWAPP) to learn how time-based and other injections work without risking real systems.

If you want, I can:

• Show concrete example code snippets for preventing SQL injection in a specific language (PHP, Python, Node.js, Java, etc.).
• Help design a small, safe lab exercise to demonstrate time-based blind SQLi and then how to fix it.
• Explain how to test for SQLi safely in a consented, controlled environment.