-1' OR 5*5=25 --

That looks like a classic SQL injection payload used for testing or exploiting a vulnerable login/query. Here’s the safe, conceptual breakdown:

What it does

• -1' closes an open string in the SQL query.
• OR 55=25 adds a condition that is always true (55=25 evaluates to true).
• -- ends the rest of the SQL statement as a comment, so any trailing part of the query is ignored.

Why this can bypass a login

• If a query is built by concatenating user input, e.g.: SELECT * FROM users WHERE username = 'USER' AND password = 'PASS'; a malicious USER value like -1' OR 5*5=25 -- can transform it into something that always evaluates to true, effectively bypassing the intended authentication check (depending on the exact query and DB).

Notes

• The exact behavior depends on the database and how the query is constructed, but the pattern is the same: close the string, inject a tautology, and comment out the rest.
• This is a defensive topic for exams or security awareness, not a guide for wrongdoing.

Defensive measures (how to prevent)

• Use parameterized/prepared statements so user input is never concatenated into SQL.
  • Example idea: SELECT * FROM users WHERE username = ? AND password = ? (bind username and password as parameters).
• Avoid building SQL by string concatenation; use ORM or proper escaping.
• Store passwords securely (hash with a strong algorithm like bcrypt, scrypt, or Argon2) and compare hashes, not plaintext.
• Implement least privilege for the database user (e.g., no executive rights; only what's needed).
• Validate and constrain inputs (type checks, length limits, etc.).
• Use server-side error handling and avoid exposing detailed SQL errors to users.
• Consider additional defenses like input sanitization where appropriate, WAF rules, and disabling multi-statement queries if possible.

If you’d like, I can show language-specific examples of safe, prepared-statements code (e.g., Python, Java, PHP, Node.js) or create exam-style practice questions about SQL injection and mitigation.