What is the Authentication-related HTTP Error Code?

The primary authentication-related HTTP error codes are 401 Unauthorized and 403 Forbidden.

401 Unauthorized: This error code indicates that the client request lacks proper authentication credentials or the provided credentials are invalid. When a server responds with a 401 status, it typically includes the "WWW-Authenticate" header to prompt the client to supply valid credentials. This code signals that the user needs to authenticate to access the requested resource, but still needs to complete the authentication process successfully.

403 Forbidden: This code is returned when authentication has been provided, but the authenticated user does not have the necessary permissions to access the resource. Unlike a 401 error, a 403 error implies that the server has understood the request and the user's credentials, but access is explicitly denied due to insufficient permissions.

Both codes indicate authentication or authorization issues, but they differ in the cause and handling of the problem.