-1' OR 5*5=26 --

That string is a classic-looking SQL injection payload. Here’s a concise breakdown and safe, exam-focused guidance.

What the parts do (at a high level)

• -1' closes a string literal in many SQL dialects.
• OR introduces an additional boolean condition into the WHERE clause.
• 5*5=26 is a boolean expression; it evaluates to false (25 ≠ 26). If it were 25=25, it would be true.
• -- starts a comment in SQL, so the rest of the line is ignored.
• The overall effect depends on the surrounding query. A vulnerable query might become something like: ... WHERE username = '-1' OR 5*5=26 -- AND password = ... which reduces to: username = '-1' OR false, and the rest is ignored due to the comment.

Important caution

• This demonstrates the general idea of boolean-based SQL injection: injecting a condition that can alter the logic of a query. In many real-world cases, if the injected condition becomes always true (e.g., OR 1=1 or OR 25=25), you can bypass authentication. In your exact string, 5*5=26 is false, so it wouldn’t bypass by itself, but the technique illustrates why such injections are dangerous.

Defensive takeaway (exam-relevant)

• Use parameterized queries/prepared statements everywhere. Do not concatenate user input into SQL strings.
• Validate and sanitize inputs, enforce least privilege for the database user, and apply input length limits.
• Consider using ORM features or stored procedures with proper parameter binding.
• Implement a Web Application Firewall or input-encoding strategy as a defense-in-depth measure.
• Be able to recognize insecure versus secure code:
  • Insecure: SELECT * FROM users WHERE username = '$username' AND password = '$password';
  • Secure: SELECT * FROM users WHERE username = :username AND password = :password (via prepared statements).

If you want, tell me what language you’re studying (e.g., PHP with PDO, Java with PreparedStatement, Python with psycopg2) and I can show a concrete secure vs insecure example and a quick explanation of why it’s safe.