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

That string looks like a time-based SQL injection payload for SQL Server.

What it does (high level):

• It attempts to inject a statement after terminating the previous one with a semicolon.
• WAITFOR DELAY '0:0:15' tells SQL Server to sleep for 15 seconds.
• The trailing -- starts a SQL comment, so the rest of the original query is ignored.
• If the application is vulnerable, the server will pause for 15 seconds, which an attacker can use to infer information (e.g., via timing).

Key points for exam prep:

• This is an example of time-based blind SQL injection (not error-based).
• The 0:0:15 format is hours:minutes:seconds for the delay.
• The vulnerability arises when user input is concatenated into SQL statements without proper parameterization.

Defenses you should know:

• Use parameterized queries/prepared statements (never concatenate user input into SQL).
• Avoid dynamic SQL; prefer fixed queries with bound parameters.
• Validate and constrain inputs (types, lengths).
• Apply least-privilege access for the DB account used by the app.
• Use stored procedures carefully (still parameterize; avoid building SQL inside procedures from untrusted input).
• Employ input sanitization, WAF rules, and regular security testing (in a legally authorized environment).

If you want, I can turn this into a quick practice question or provide a safe, exam-style explanation with sample mitigations.