(select(0)from(select(sleep(15)))v)/*'+(select(0)from(select(sleep(15)))v)+'"+(select(0)from(select(sleep(15)))v)+"*/

I can’t help with exploiting or modifying systems. But I can explain the concept at a high level and, since you’re studying for exams, how to defend against it and how such questions might appear on tests.

What the payload is trying to do (high-level)

• It resembles a time-based blind SQL injection payload. The core idea is to cause the database to sleep for a fixed time (e.g., sleep(15)) if certain conditions are met, so an attacker can infer information based on how long the server takes to respond.
• In time-based blind SQL injection, you don’t get data directly back in the query results. Instead, you infer truth values (true/false) by observing response delays.
• The extra characters (/* … */, quotes, etc.) are commonly used in attempts to bypass input parsing or comment out parts of the original query.

Defensive concepts you should know for exams

• What SQL injection is: placing or manipulating SQL statements in input data so an application's query changes behavior.
• Types of injection:
  • In-band: attacker gets data back in the same channel (e.g., union-based or error-based).
  • Blind (boolean-based): attacker infers data by sending conditions that return true/false and observing behavior.
  • Blind (time-based): attacker causes delays (e.g., sleep) to infer data from response time.
• Primary defenses:
  • Use parameterized queries / prepared statements everywhere (never interpolate user input into SQL strings).
  • Use stored procedures carefully (they can still be vulnerable if they use string concatenation with user input).
  • Implement proper input validation and whitelisting (reject disallowed characters/lengths, but don’t rely on it alone).
  • Least-privilege database accounts: the application should not have more permissions than needed (no DROP/ALTER if not necessary).
  • Disable multiple statements per query if your framework allows it.
  • Use an ORM correctly and keep its query-building safe.
  • Apply input/output encoding and proper escaping where appropriate (but encoding alone isn’t a substitute for parameterization).
  • Implement robust error handling and avoid exposing database errors to users.
  • Logging and anomaly detection for long-running queries to catch time-based techniques.
• How to test safely (in a lab or with authorization):
  • Use intentionally vulnerable training apps (e.g., DVWA, WebGoat, OWASP Web Security Academy) in a controlled environment.
  • Practice detecting and fixing vulnerabilities, not exploiting them on real systems.

If you want, I can:

• Provide exam-style questions about SQL injection and defenses (with model answers).
• Walk through safe, defensive coding examples in your language of choice (e.g., Java, Python, PHP) showing parameterized queries.
• Recommend lab setups and learning resources for ethical, legal security testing.

Would you like a set of practice questions on SQL injection defenses, or a quick walkthrough of safe coding patterns to prevent these vulnerabilities in your preferred language?