Stranger login not allowed: authentication and authorization errors

What is a login in brief:

• User identification - checking that the user exists in the system
• Authentication - checking authenticity (password, token, etc.)
• Authorization - granting access

Let's consider the security issues of this process found during the bug bounty process.

These are logical issues:

• Changes in authentication data
• Bypassing authentication stages - 2fa
• Authorization issues

Let's try to figure out how these vulnerabilities could have been avoided at the design and development stage.

The report is intended for developers, QA, beginners in information security.