-1' OR 5*5=25 --

What you posted is a classic SQL injection payload: -1' OR 5*5=25 --.

What it does:

• If user input is directly concatenated into an SQL query, the input can close a string literal (-1' ends the username string) and inject an OR condition.
• 5*5=25 is a tautology (true), so the WHERE clause can become true for all rows.
• The -- starts a SQL comment, effectively ignoring the rest of the query. This can bypass authentication or alter logic.

Common defense principles:

• Use parameterized queries (prepared statements) so user input is never concatenated into SQL.
• Validate and/or restrict input types (e.g., usernames as fixed character sets, numeric fields as numbers).
• Store passwords securely (hash with a strong algorithm and use salted hashes; compare hashes, not plain text).
• Apply least-privilege database accounts (the app uses a DB user with only needed permissions).
• Consider additional protections like input validation, ORM abstractions, and application-layer guards.

Quick examples of safe usage (conceptual):

• Python (sqlite3/psycopg2): cursor.execute("SELECT * FROM users WHERE username = ? AND password_hash = ?", (username, hash(password)))
• Java JDBC: PreparedStatement ps = conn.prepareStatement("SELECT * FROM users WHERE username = ? AND password_hash = ?"); ps.setString(1, username); ps.setString(2, hash(password));
• PHP PDO: $stmt = $pdo->prepare("SELECT * FROM users WHERE username = :username AND password_hash = :password"); $stmt->execute(['username' => $username, 'password' => $hash]);

If you’re studying for an exam, I can tailor explanations or practice questions around SQL injection and defenses—just tell me your course language or framework. And remember: only test injection techniques in authorized, isolated environments.