-1' OR 5*5=25 --

This looks like an SQL injection payload.

What it does conceptually:

• The -1' part tries to terminate an existing string in a query.
• OR 55=25 adds a condition that is always true (since 55 equals 25).
• -- starts a comment, so the rest of the query is ignored.

Net effect (in a vulnerable query like a login check):

• The WHERE clause becomes something like: username = '-1' OR 5*5=25
• Since 5*5=25 is true, the condition is true for all rows, which can bypass authentication or reveal data.

Why this is risky:

• It exploits unsafely constructed SQL queries that embed user input directly into the SQL string.

How to defend (best practices):

• Use parameterized queries / prepared statements. Do not concatenate user input into SQL strings.
  • Example (conceptual):
    • Python with psycopg2: cur.execute("SELECT * FROM users WHERE username = %s", (user_input,))
    • Java with PreparedStatement: ps = conn.prepareStatement("SELECT * FROM users WHERE username = ?"); ps.setString(1, userInput);
• Validate and sanitize inputs, and apply least privilege on the database user.
• Disable or restrict multi-statement queries if possible.
• Use stored procedures with proper parameter binding.
• Consider additional protections like input whitelisting and a Web Application Firewall.

If you’d like, I can show concrete examples in a particular language/framework or tailor it to a typical exam question.