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

Short answer: it’s a time-based blind SQL injection payload designed to slow down the response if a certain time-based condition is met.

High-level breakdown (exam prep concepts)

• What the idea is:
  • The attacker injects SQL that, under a vulnerable query, can cause the database to sleep for 15 seconds.
  • By measuring response times, the attacker can infer information (e.g., whether a condition about the database is true or false) without seeing data directly.
• The components you see:
  • 5550' … 'Z and XOR: These are attempts to break out of an existing string literal in a query and concatenate new SQL fragments. The XOR parts are often used to bypass simple input filters or to craft the payload so the app-supplied string becomes valid SQL.
  • 555*if(now()=sysdate(),sleep(15),0): This is a conditional expression.
    • now() and sysdate() are time-related functions. In MySQL, NOW() is the time value evaluated at the start of the statement, while SYSDATE() can return a more “real-time” value.
    • if(now()=sysdate(), sleep(15), 0) means: if the two time values are equal (a rare timing coincidence), pause the database for 15 seconds; otherwise do nothing (return 0).
    • sleep(15) is the deliberate delay used to create a measurable pause.
• Why this works as an attack concept:
  • Instead of returning data, the attacker uses observable timing (slow response vs normal response) to deduce information. This is the essence of time-based blind SQL injection.
  • The specific use of NOW() vs SYSDATE() exploits how those functions behave differently in some DB engines or versions, turning a timing discrepancy into a controllable delay.

Important caveats for safety and ethics

• This kind of payload is used to test or exploit SQL injection vulnerabilities. Do not test against systems you do not own or do not have explicit authorization to test.
• In a class or lab, perform tests only in a controlled environment designed for security testing (e.g., your own VM with intentionally vulnerable apps) and follow all safety/legal guidelines.

Defensive takeaway (what to study for exams and real-world practice)

• How to prevent such injections:
  • Use prepared statements / parameterized queries (no string concatenation of user input into SQL).
  • Validate and sanitize inputs; enforce strict data types and length limits.
  • Apply least-privilege DB accounts (no write/delete rights for app accounts if not needed).
  • Avoid dynamic SQL inside stored procedures where user input can influence the SQL text.
  • Consider Web Application Firewalls (WAF) and proper error handling that doesn’t leak DB details.
  • Enable strict SQL modes and proper logging to detect anomalous queries or delayed responses.
• How to detect/time-based injections:
  • Look for unusual slow queries and increased sleep/delays in responses.
  • Use parameterized query testing in a controlled lab to see if replacing user input with sanitized values prevents delays.
• Related concepts to know:
  • Time-based blind vs error-based vs union-based SQL injection.
  • Differences between NOW() and SYSDATE() in various RDBMSs and how that can be leveraged or mitigated.
  • How input tampering can bypass naive filters (and why robust input handling is essential, not just filtering).

If you’d like, I can give a safe, non-exploitative example that demonstrates the same timing concept in a toy, labeled environment, or walk through how a secure code path would prevent such injections.