Difference between 403 Forbidden and 401 Unauthorized HTTP responses

In this post, I’ll share the differences between the 403 and 401 HTTP responses and what use cases are appropriate for each response.

What is the difference between 403 Forbidden and 401 Unauthorized HTTP responses?

Short answer:

401 Unauthorized: If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials.

403 Forbidden: The server understood the request, but is refusing to fulfill it.

Detailed answers:

There’s a problem with 401 Unauthorized, the HTTP status code for authentication errors. And that’s just it: it’s for authentication, not authorization. Receiving a 401 response is the server telling you, “you aren’t authenticated–either not authenticated at all or authenticated incorrectly–but please reauthenticate and try again.” To help you out, it will always include a WWW-Authenticate header that describes how to authenticate.

This is a response generally returned by your web server, not your web application.

It’s also something very temporary; the server is asking you to try again.

So, for authorization, I use the 403 Forbidden response. It’s permanent, it’s tied to my application logic, and it’s a more concrete response than a 401.

Receiving a 403 response is the server telling you, “I’m sorry. I know who you are–I believe who you say you are–but you just don’t have permission to access this resource. Maybe if you ask the system administrator nicely, you’ll get permission. But please don’t bother me again until your predicament changes.”

In summary, a 401 Unauthorized response should be used for missing or bad authentication, and a 403 Forbidden response should be used afterward when the user is authenticated but isn’t authorized to perform the requested operation on the given resource.

403 Forbidden vs 401 Unauthorized HTTP responses:

Something the other answers are missing is that it must be understood that Authentication and Authorization in the context of RFC 2616 refers ONLY to the HTTP Authentication protocol of RFC 2617. Authentication by schemes outside of RFC2617 is not supported in HTTP status codes and are not considered when deciding whether to use 401 or 403.

Brief and Terse

Unauthorized indicates that the client is not RFC2617 authenticated and the server is initiating the authentication process. Forbidden indicates either that the client is RFC2617 authenticated and does not have authorization or that the server does not support RFC2617 for the requested resource.

Meaning if you have your own roll-your-own login process and never use HTTP Authentication, 403 is always the proper response and 401 should never be used.

Detailed and In-Depth

From RFC2616

10.4.2 401 Unauthorized

The request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.47) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8).

and

10.4.4 403 Forbidden The server understood the request but is refusing to fulfil it. Authorization will not help and the request SHOULD NOT be repeated.

The first thing to keep in mind is that “Authentication” and “Authorization” in the context of this document refer specifically to the HTTP Authentication protocols from RFC 2617. They do not refer to any roll-your-own authentication protocols you may have created using login pages, etc. I will use “login” to refer to authentication and authorization by methods other than RFC2617

So the real difference is not what the problem is or even if there is a solution. The difference is what the server expects the client to do next.

401 indicates that the resource can not be provided, but the server is REQUESTING that the client log in through HTTP Authentication and has sent reply headers to initiate the process. Possibly there are authorizations that will permit access to the resource, possibly there are not, but let’s give it a try and see what happens.

403 indicates that the resource can not be provided and there is, for the current user, no way to solve this through RFC2617 and no point in trying. This may be because it is known that no level of authentication is sufficient (for instance because of an IP blacklist), but it may be because the user is already authenticated and does not have authority. The RFC2617 model is one-user, one-credentials so the case where the user may have a second set of credentials that could be authorized may be ignored. It neither suggests nor implies that some sort of login page or other non-RFC2617 authentication protocol may or may not help – that is outside the RFC2616 standards and definition.

Answer #3:

I think it is important to consider that, to a browser, 401 initiates an authentication dialog for the user to enter new credentials, while 403 does not. Browsers think that, if a 401 is returned, then the user should re-authenticate. So 401 stands for invalid authentication while 403 stands for a lack of permission.

Here are some cases under that logic where an error would be returned from authentication or authorization, with important phrases bolded.

  • A resource requires authentication but no credentials were specified.

401: The client should specify credentials.

  • The specified credentials are in an invalid format.

400: That’s neither 401 nor 403, as syntax errors should always return 400.

  • The specified credentials reference a user which does not exist.

401: The client should specify valid credentials.

  • The specified credentials are invalid but specify a valid user (or don’t specify a user if a specified user is not required).

401: Again, the client should specify valid credentials.

  • The specified credentials have expired.

401: This is practically the same as having invalid credentials in general, so the client should specify valid credentials.

  • The specified credentials are completely valid but do not suffice the particular resource, though it is possible that credentials with more permission could.

403: Specifying valid credentials would not grant access to the resource, as the current credentials are already valid but only do not have permission.

  • The particular resource is inaccessible regardless of credentials.

403: This is regardless of credentials, so specifying valid credentials cannot help.

  • The specified credentials are completely valid but the particular client is blocked from using them.

403: If the client is blocked, specifying new credentials will not do anything.

Summary:

These are the meanings:

401: User not (correctly) authenticated, the resource/page requires authentication

403: User’s role or permissions does not allow to access the requested resource, for instance the user is not an administrator and the requested page is for administrators.

Note: Technically, 403 is a superset of 401, since is legal to give 403 for the unauthenticated users too. Anyway is more meaningful to differentiate.

Hope you learned something from this post.

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Linux and Python enthusiast, in love with open source since 2014, Writer at programming-articles.com, India.

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