-1' OR 5*5=26 --

That string is a classic SQL injection payload. Here’s what each part does in general:

• -1' closes a string literal in a SQL query. If the application builds a query by concatenating user input, this can break out of the string context.
• OR introduces an additional condition to the WHERE clause (or other conditional part) of the query.
• 5*5=26 is a boolean expression that evaluates to false (25 ≠ 26). It’s being used as the injected condition.
• -- starts a SQL comment, so the rest of the query after it is ignored.

In a vulnerable query like: SELECT * FROM users WHERE id = '' this payload could transform it to: SELECT * FROM users WHERE id = '' OR 5*5=26 -- which, depending on the surrounding query, might bypass some checks or alter logic. A more common, successful injection for bypass is something like ' OR 1=1 --, which is always true.

Defensive notes:

• Do not build SQL queries by concatenating user input. Use parameterized queries (prepared statements).
• Validate and sanitize input; enforce type and range where applicable.
• Use least-privilege database accounts and proper error handling.
• Consider using ORM/DB libraries that automatically parameterize queries, or input validation layers and WAF rules.

Examples of safe parameterized usage:

• Python (psycopg2/sqlite3): cursor.execute("SELECT * FROM users WHERE id = %s", (user_input,))
• Java (PreparedStatement): PreparedStatement ps = conn.prepareStatement("SELECT * FROM users WHERE id = ?"); ps.setInt(1, Integer.parseInt(user_input));
• PHP (PDO): $stmt = $pdo->prepare("SELECT * FROM users WHERE id = :id"); $stmt->execute(['id' => $user_input]);

If you want, I can walk through a specific example (vulnerable vs. safe) or pose some exam-style questions about identifying and mitigating SQL injection.