Authentication middleware is application code that sits between a client and a protected service to check who is making the request. It often verifies sessions, cookies, tokens, or headers before the request reaches business logic. In web apps, this can prevent anonymous users from viewing dashboards, calling APIs, or opening privileged connections. It is a common defense because it centralizes identity checks and reduces duplicated code.
In security, middleware is only part of the control. Some endpoints, especially WebSocket upgrades, terminal routes, file handlers, or special admin paths, may bypass the normal middleware chain or need extra checks after the connection is established. Attackers look for those gaps to reach sensitive functions without authenticating. Defenders should treat middleware as a first gate, then add endpoint-level authorization, route-specific allowlists, and tests that confirm every critical path enforces identity before execution.