Hypertext Transfer Protocol (HTTP/1.1): Authentication
draft-ietf-httpbis-p7-auth-25

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document defines the HTTP Authentication framework.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at http://lists.w3.org/Archives/Public/ietf-http-wg/.

The current issues list is at http://tools.ietf.org/wg/httpbis/trac/report/3 and related documents (including fancy diffs) can be found at http://tools.ietf.org/wg/httpbis/.

The changes in this draft are summarized in Appendix D.1.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on May 21, 2014.

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Table of Contents

• 1. Introduction
• 1.1. Conformance and Error Handling
• 1.2. Syntax Notation
• 2. Access Authentication Framework
• 2.1. Challenge and Response
• 2.2. Protection Space (Realm)
• 3. Status Code Definitions
• 3.1. 401 Unauthorized
• 3.2. 407 Proxy Authentication Required
• 4. Header Field Definitions
• 4.1. Authorization
• 4.2. Proxy-Authenticate
• 4.3. Proxy-Authorization
• 4.4. WWW-Authenticate
• 5. IANA Considerations
• 5.1. Authentication Scheme Registry
• 5.1.1. Procedure
• 5.1.2. Considerations for New Authentication Schemes
• 5.2. Status Code Registration
• 5.3. Header Field Registration
• 6. Security Considerations
• 6.1. Authentication Credentials and Idle Clients
• 6.2. Protection Spaces
• 7. Acknowledgments
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Changes from RFCs 2616 and 2617
• Appendix B. Imported ABNF
• Appendix C. Collected ABNF
• Appendix D. Change Log (to be removed by RFC Editor before publication)
• D.1. Since draft-ietf-httpbis-p7-auth-24
• Authors' Addresses

1. Introduction

This document defines HTTP/1.1 access control and authentication. It includes the relevant parts of RFC 2616 with only minor changes ([RFC2616]), plus the general framework for HTTP authentication, as previously defined in "HTTP Authentication: Basic and Digest Access Authentication" ([RFC2617]).

HTTP provides several OPTIONAL challenge-response authentication schemes that can be used by a server to challenge a client request and by a client to provide authentication information. The "basic" and "digest" authentication schemes continue to be specified in RFC 2617.

1.1. Conformance and Error Handling

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [Part1].

1.2. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 7 of [Part1]. Appendix B describes rules imported from other documents. Appendix C shows the collected ABNF with the list rule expanded.

2. Access Authentication Framework

2.1. Challenge and Response

HTTP provides a simple challenge-response authentication framework that can be used by a server to challenge a client request and by a client to provide authentication information. It uses a case-insensitive token as a means to identify the authentication scheme, followed by additional information necessary for achieving authentication via that scheme. The latter can either be a comma-separated list of parameters or a single sequence of characters capable of holding base64-encoded information.

Parameters are name-value pairs where the name is matched case-insensitively, and each parameter name MUST only occur once per challenge.

  auth-scheme    = token
  
  auth-param     = token BWS "=" BWS ( token / quoted-string )

  token68        = 1*( ALPHA / DIGIT /
                       "-" / "." / "_" / "~" / "+" / "/" ) *"=" 

The "token68" syntax allows the 66 unreserved URI characters ([RFC3986]), plus a few others, so that it can hold a base64, base64url (URL and filename safe alphabet), base32, or base16 (hex) encoding, with or without padding, but excluding whitespace ([RFC4648]).

The 401 (Unauthorized) [status.401] response message is used by an origin server to challenge the authorization of a user agent. This response MUST include a WWW-Authenticate [header.www-authenticate] header field containing at least one challenge applicable to the requested resource.

The 407 (Proxy Authentication Required) [status.407] response message is used by a proxy to challenge the authorization of a client and MUST include a Proxy-Authenticate [header.proxy-authenticate] header field containing at least one challenge applicable to the proxy for the requested resource.

  challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

• Note: Many clients fail to parse challenges containing unknown schemes. A workaround for this problem is to list well-supported schemes (such as "basic") first.

A user agent that wishes to authenticate itself with an origin server — usually, but not necessarily, after receiving a 401 (Unauthorized) [status.401] — can do so by including an Authorization [header.authorization] header field with the request.

A client that wishes to authenticate itself with a proxy — usually, but not necessarily, after receiving a 407 (Proxy Authentication Required) [status.407] — can do so by including a Proxy-Authorization [header.proxy-authorization] header field with the request.

Both the Authorization [header.authorization] field value and the Proxy-Authorization [header.proxy-authorization] field value contain the client's credentials for the realm of the resource being requested, based upon a challenge received in a response (possibly at some point in the past). When creating their values, the user agent ought to do so by selecting the challenge with what it considers to be the most secure auth-scheme that it understands, obtaining credentials from the user as appropriate.

  credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

Upon receipt of a request for a protected resource that omits credentials, contains invalid credentials (e.g., a bad password) or partial credentials (e.g., when the authentication scheme requires more than one round trip), an origin server SHOULD send a 401 (Unauthorized) [status.401] response that contains a WWW-Authenticate [header.www-authenticate] header field with at least one (possibly new) challenge applicable to the requested resource.

Likewise, upon receipt of a request that requires authentication by proxies that omit credentials or contain invalid or partial credentials, a proxy SHOULD send a 407 (Proxy Authentication Required) [status.407] response that contains a Proxy-Authenticate [header.proxy-authenticate] header field with a (possibly new) challenge applicable to the proxy.

A server receiving credentials that are valid, but not adequate to gain access, ought to respond with the 403 (Forbidden) status code (Section 6.5.3 of [Part2]).

HTTP does not restrict applications to this simple challenge-response framework for access authentication. Additional mechanisms can be used, such as authentication at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, such additional mechanisms are not defined by this specification.

A proxy MUST forward the WWW-Authenticate [header.www-authenticate] and Authorization [header.authorization] header fields unmodified and follow the rules found in Section 4.1.

2.2. Protection Space (Realm)

The authentication parameter realm is reserved for use by authentication schemes that wish to indicate the scope of protection.

A protection space is defined by the canonical root URI (the scheme and authority components of the effective request URI; see Section 5.5 of [Part1]) of the server being accessed, in combination with the realm value if present. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which can have additional semantics specific to the authentication scheme. Note that a response can have multiple challenges with the same auth-scheme but different realms.

The protection space determines the domain over which credentials can be automatically applied. If a prior request has been authorized, the user agent MAY reuse the same credentials for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preferences (such as a configurable inactivity timeout). Unless specifically allowed by the authentication scheme, a single protection space cannot extend outside the scope of its server.

For historical reasons, a sender MUST only generate the quoted-string syntax. Recipients might have to support both token and quoted-string syntax for maximum interoperability with existing clients that have been accepting both notations for a long time.

3. Status Code Definitions

3.1. 401 Unauthorized

The 401 (Unauthorized) status code indicates that the request has not been applied because it lacks valid authentication credentials for the target resource. The origin server MUST send a WWW-Authenticate [header.www-authenticate] header field (Section 4.4) containing at least one challenge applicable to the target resource. If the request included authentication credentials, then the 401 response indicates that authorization has been refused for those credentials. The user agent MAY repeat the request with a new or replaced Authorization [header.authorization] header field (Section 4.1). If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user agent SHOULD present the enclosed representation to the user, since it usually contains relevant diagnostic information.

3.2. 407 Proxy Authentication Required

The 407 (Proxy Authentication Required) status code is similar to 401 (Unauthorized) [status.401], but indicates that the client needs to authenticate itself in order to use a proxy. The proxy MUST send a Proxy-Authenticate [header.proxy-authenticate] header field (Section 4.2) containing a challenge applicable to that proxy for the target resource. The client MAY repeat the request with a new or replaced Proxy-Authorization [header.proxy-authorization] header field (Section 4.3).

4. Header Field Definitions

This section defines the syntax and semantics of HTTP/1.1 header fields related to authentication.

4.1. Authorization

The "Authorization" header field allows a user agent to authenticate itself with an origin server — usually, but not necessarily, after receiving a 401 (Unauthorized) [status.401] response. Its value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested.

  Authorization = credentials

If a request is authenticated and a realm specified, the same credentials are presumed to be valid for all other requests within this realm (assuming that the authentication scheme itself does not require otherwise, such as credentials that vary according to a challenge value or using synchronized clocks).

See Section 3.2 of [Part6] for details of and requirements pertaining to handling of the Authorization field by HTTP caches.

4.2. Proxy-Authenticate

The "Proxy-Authenticate" header field consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the proxy for this effective request URI (Section 5.5 of [Part1]). It MUST be included as part of a 407 (Proxy Authentication Required) [status.407] response.

  Proxy-Authenticate = 1#challenge

Unlike WWW-Authenticate [header.www-authenticate], the Proxy-Authenticate header field applies only to the next outbound client on the response chain that chose to direct its request to the responding proxy. If that recipient is also a proxy, it will generally consume the Proxy-Authenticate header field (and generate an appropriate Proxy-Authorization [header.proxy-authorization] in a subsequent request) rather than forward the header field to its own outbound clients. However, if a recipient proxy needs to obtain its own credentials by requesting them from a further outbound client, it will generate its own 407 response, which might have the appearance of forwarding the Proxy-Authenticate header field if both proxies use the same challenge set.

Note that the parsing considerations for WWW-Authenticate [header.www-authenticate] apply to this header field as well; see Section 4.4 for details.

4.3. Proxy-Authorization

The "Proxy-Authorization" header field allows the client to identify itself (or its user) to a proxy that requires authentication. Its value consists of credentials containing the authentication information of the client for the proxy and/or realm of the resource being requested.

  Proxy-Authorization = credentials

Unlike Authorization [header.authorization], the Proxy-Authorization header field applies only to the next inbound proxy that demanded authentication using the Proxy-Authenticate [header.proxy-authenticate] field. When multiple proxies are used in a chain, the Proxy-Authorization header field is consumed by the first inbound proxy that was expecting to receive credentials. A proxy MAY relay the credentials from the client request to the next proxy if that is the mechanism by which the proxies cooperatively authenticate a given request.

4.4. WWW-Authenticate

The "WWW-Authenticate" header field consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the effective request URI (Section 5.5 of [Part1]).

It MUST be included in 401 (Unauthorized) [status.401] response messages and MAY be included in other response messages to indicate that supplying credentials (or different credentials) might affect the response.

  WWW-Authenticate = 1#challenge

User agents are advised to take special care in parsing the field value, as it might contain more than one challenge, and each challenge can contain a comma-separated list of authentication parameters. Furthermore, the header field itself can occur multiple times.

For instance:

  WWW-Authenticate: Newauth realm="apps", type=1,
                    title="Login to \"apps\"", Basic realm="simple"

This header field contains two challenges; one for the "Newauth" scheme with a realm value of "apps", and two additional parameters "type" and "title", and another one for the "Basic" scheme with a realm value of "simple".

• Note: The challenge grammar production uses the list syntax as well. Therefore, a sequence of comma, whitespace, and comma can be considered either as applying to the preceding challenge, or to be an empty entry in the list of challenges. In practice, this ambiguity does not affect the semantics of the header field value and thus is harmless.

5. IANA Considerations

5.1. Authentication Scheme Registry

The HTTP Authentication Scheme Registry defines the name space for the authentication schemes in challenges and credentials. It will be created and maintained at (the suggested URI) http://www.iana.org/assignments/http-authschemes.

5.1.1. Procedure

Registrations MUST include the following fields:

• Authentication Scheme Name
• Pointer to specification text
• Notes (optional)

Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).

5.1.2. Considerations for New Authentication Schemes

There are certain aspects of the HTTP Authentication Framework that put constraints on how new authentication schemes can work:

• HTTP authentication is presumed to be stateless: all of the information necessary to authenticate a request MUST be provided in the request, rather than be dependent on the server remembering prior requests. Authentication based on, or bound to, the underlying connection is outside the scope of this specification and inherently flawed unless steps are taken to ensure that the connection cannot be used by any party other than the authenticated user (see Section 2.3 of [Part1]).
• The authentication parameter "realm" is reserved for defining Protection Spaces as defined in Section 2.2. New schemes MUST NOT use it in a way incompatible with that definition.
• The "token68" notation was introduced for compatibility with existing authentication schemes and can only be used once per challenge or credential. New schemes thus ought to use the "auth-param" syntax instead, because otherwise future extensions will be impossible.
• The parsing of challenges and credentials is defined by this specification, and cannot be modified by new authentication schemes. When the auth-param syntax is used, all parameters ought to support both token and quoted-string syntax, and syntactical constraints ought to be defined on the field value after parsing (i.e., quoted-string processing). This is necessary so that recipients can use a generic parser that applies to all authentication schemes.
  
  Note: The fact that the value syntax for the "realm" parameter is restricted to quoted-string was a bad design choice not to be repeated for new parameters.
• Definitions of new schemes ought to define the treatment of unknown extension parameters. In general, a "must-ignore" rule is preferable over "must-understand", because otherwise it will be hard to introduce new parameters in the presence of legacy recipients. Furthermore, it's good to describe the policy for defining new parameters (such as "update the specification", or "use this registry").
• Authentication schemes need to document whether they are usable in origin-server authentication (i.e., using WWW-Authenticate [header.www-authenticate]), and/or proxy authentication (i.e., using Proxy-Authenticate [header.proxy-authenticate]).
• The credentials carried in an Authorization [header.authorization] header field are specific to the User Agent, and therefore have the same effect on HTTP caches as the "private" Cache-Control response directive (Section 5.2.2.6 of [Part6]), within the scope of the request they appear in.
  
  Therefore, new authentication schemes that choose not to carry credentials in the Authorization [header.authorization] header field (e.g., using a newly defined header field) will need to explicitly disallow caching, by mandating the use of either Cache-Control request directives (e.g., "no-store", Section 5.2.1.5 of [Part6]) or response directives (e.g., "private").

5.2. Status Code Registration

The HTTP Status Code Registry located at http://www.iana.org/assignments/http-status-codes shall be updated with the registrations below:

  BWS           = <BWS, defined in [Part1], Section 3.2.3>
  OWS           = <OWS, defined in [Part1], Section 3.2.3>
  quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
  token         = <token, defined in [Part1], Section 3.2.6>

Authorization = credentials

BWS = <BWS, defined in [Part1], Section 3.2.3>

OWS = <OWS, defined in [Part1], Section 3.2.3>

Proxy-Authenticate = *( "," OWS ) challenge *( OWS "," [ OWS
 challenge ] )
Proxy-Authorization = credentials

WWW-Authenticate = *( "," OWS ) challenge *( OWS "," [ OWS challenge
 ] )

auth-param = token BWS "=" BWS ( token / quoted-string )
auth-scheme = token

challenge = auth-scheme [ 1*SP ( token68 / [ ( "," / auth-param ) *(
 OWS "," [ OWS auth-param ] ) ] ) ]
credentials = auth-scheme [ 1*SP ( token68 / [ ( "," / auth-param )
 *( OWS "," [ OWS auth-param ] ) ] ) ]

quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>

token = <token, defined in [Part1], Section 3.2.6>
token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
 *"="