555-1); waitfor delay '0:0:15' --

That string is a classic time-based SQL injection payload aimed at SQL Server.

What it does conceptually:

• The input ends the previous statement with 555-1); so the application would have been building a SQL statement by string concatenation.
• It then executes WAITFOR DELAY '0:0:15', which causes the database to pause for 15 seconds.
• The trailing -- comments out the rest of the line, so any following part of the original query is ignored.

Why this matters:

• If user input is concatenated into SQL without proper parameterization, an attacker can cause delays (and in other cases exfiltrate data or modify data). This is a time-based blind SQL injection technique.

Defenses (exam/real-world tips):

• Use parameterized queries / prepared statements everywhere. Do not concatenate user input into SQL strings.
  • Examples:
    • In Python (psycopg2): cursor.execute("SELECT * FROM users WHERE phone = %s", (user_input,))
    • In Java (PreparedStatement): PreparedStatement ps = conn.prepareStatement("SELECT ... WHERE phone = ?");
• Avoid dynamic SQL whenever possible; prefer static queries with parameters.
• Validate and sanitize input (e.g., enforce types/formats like digits for phone numbers).
• Apply least privilege to the DB account used by the app (limit permissions, disable dangerous operations).
• Use proper error handling to avoid leaking DB error details.
• Consider a WAF or code scanners that detect injection patterns.
• In tests, use a secure lab with permission and avoid testing on production systems.

If you want, I can show a quick side-by-side comparison (unsafe concatenation vs. safe parameterization) in a language of your choice.