Provide an example of testing using the gray box method.

Testing the user interface of a web application for validation errors by using knowledge of the architecture and interactions between the frontend and backend.

Example:

• Knowledge: The "Email" input field on the frontend expects a string of a certain format (validated with a regular expression) and sends it to the backend, which also performs validation and stores the data in the database.
• Test: Instead of standard black-box test cases (valid/invalid email format in UI), we can additionally check:
  • Sending deliberately invalid data directly to the backend, bypassing frontend validation (for example, via a POST request using Postman). This helps identify backend validation errors or SQL injection vulnerabilities.
  • Sending excessively long strings that exceed the database field size (using knowledge of the database structure).
  • Testing the interaction between the frontend and backend when validation errors are received from the backend (for example, verifying the correct display of error messages on the UI).