An authentication filter is a servlet-layer control that inspects an incoming HTTP request before it reaches application code. Its job is to decide whether the request is already authenticated, must be redirected to login, or should be rejected. In Java web apps, this often sits early in the request pipeline, alongside routing and session handling.
This control matters because it defines the login boundary. If the filter is misconfigured, bypassed by an alternate URL, or applied inconsistently to forwarded requests, an attacker may reach sensitive endpoints without a valid session. That can turn a simple access-control mistake into pre-authentication exposure, including data access or server-side code execution in severe cases. Defenders should verify that the filter covers every route, including custom controllers, internal forwards, and API paths, and that authorization checks are repeated at the business-logic layer.