A penetration tester enters an invalid user ID on the login page of a web...

The application is giving distinct error messages for valid vs. invalid usernames. This is a classic case of user enumeration, where an attacker can determine valid accounts before proceeding to brute-force or password attacks.

From the CompTIA PenTest+ PT0-003 Official Study Guide (Chapter 6 – Vulnerability Identification):

“Authentication systems that return different error messages based on the validity of the username can allow attackers to enumerate valid accounts.”