HTTP 401 status code

401 Unauthorized

The current request requires user authentication. The response MUST include a WWW-Authenticate header appropriate to the requested resource that asks for user information. The client can repeatedly submit a request containing the appropriate Authorization header. If the current request already included Authorization certificates, the 401 response indicates that the server verification has rejected those certificates. If the 401 response contains the same authentication query as the previous response, and the browser has attempted authentication at least once, the browser should display the entity information contained in the response to the user, because this entity information may contain relevant diagnostic information .