-1" OR 5*5=25 --

That string is a classic SQL injection payload.

What it does:

• The quote closes a string literal in a vulnerable query.
• The OR 55=25 adds a condition that is always true (55 equals 25).
• The trailing -- starts a SQL comment, so the rest of the query is ignored.

Example vulnerable query: SELECT * FROM users WHERE username = "-1" OR 5*5=25 --;

Effect:

• The WHERE clause becomes true for all rows, so the query returns all users (if the injection works).

How to defend (exam-style takeaway):

• Use parameterized queries/prepared statements instead of string concatenation.
  • Example (pseudo):
    • In Python with psycopg2: cur.execute("SELECT * FROM users WHERE username = %s", (username,))
    • In Java with JDBC: PreparedStatement ps = conn.prepareStatement("SELECT * FROM users WHERE username = ?");
• Validate and sanitize inputs; apply allow-listing where possible.
• Use least-privilege database accounts and avoid dynamic SQL.
• Consider ORM layers that automatically parameterize queries.
• Enable proper error handling to avoid leaking database details.

If you want, I can give you a few practice questions or show how to fix a vulnerable snippet in a specific language. Do you want that?