HTTP Authentication with SASL
draft-vanrein-httpauth-sasl-03

Network Working Group                                        R. Van Rein
Internet-Draft                                                 ARPA2.net
Intended status: Standards Track                        January 20, 2020
Expires: July 23, 2020

                     HTTP Authentication with SASL
                     draft-vanrein-httpauth-sasl-03

Abstract

   Most application-level protocols standardise their authentication
   exchanges under the SASL framework.  HTTP has taken another course,
   and often ends up replicating the work to allow individual
   mechanisms.  This specification adopts full SASL authentication into
   HTTP.

Status of This Memo

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

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   This Internet-Draft will expire on July 23, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Embedding SASL in HTTP  . . . . . . . . . . . . . . . . . . .   3
     2.1.  HTTP Request and Response Messages  . . . . . . . . . . .   4
     2.2.  Authentication Field Definitions  . . . . . . . . . . . .   5
     2.3.  Caching Authentication Results  . . . . . . . . . . . . .   6
   3.  Viewing Users on HTTP Services  . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  HTTP Server Environment Variables  . . . . . . . . .  10
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   HTTP has historically followed its own path for client
   authentication, while many other end-user protocols standardised on
   SASL; examples of SASL protocols include SMTP, IMAP, POP, XMPP, LDAP
   and AMQP.  This specification introduces SASL to HTTP, so it may
   share in past and future work done for SASL in general.

   Among the work that could be shared is backend authentication
   integration, which is possible due to protocol-independent SASL
   exchanges for any given method, making it easy to take them out of
   one protocol and inserting them into another.  Although HTTP has
   adopted several SASL-compatible authentication methods, it uses
   various notations and so it still needs method-specific support at
   the HTTP level to translate them to a SASL backend.

   In front-ends, a similar situation has arisen.  The varying syntaxes
   for authentication methods have made it difficult to rely on support
   in most or all HTTP clients.  When such clients could externalise
   their SASL handling to generic software such as a SASL library, then
   any extension to a library automatically spills over into the HTTP
   sphere.  It is common for developers of web clients to also produce
   email clients, so a shared code base (and credential store) is not
   difficult to imagine.

   Sharing is beneficial in both directions.  HTTP benefits by being
   able to use anything from strong password mechanisms [RFC5802]
   without explicit support [RFC7804] in applications, up to GS2
   mechanisms [RFC5801] with channel binding [RFC5056] [RFC5554] to TLS
   [RFC5929] based on pinning either the certificate for the TLS server
   or even a unique part of the individual TLS connection; for instance

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   Kerberos5 [RFC4120] currently uses Negotiate authentication [RFC4559]
   which is not as secure as GS2-KRB5-PLUS over SASL.

   SASL also benefits; had it been the norm for HTTP, then the work to
   pass SAML over it [RFC6595] would probably have been done
   immediately.  In fact, HTTP can still benefit from receiving
   standardised SAML20 inquiries over SASL, becuase it resolves the need
   for configuration of initiation paths and practices.  Also, it
   removes authentication data from URIs, where they are not ideally
   placed.

   In terms of security for HTTP applications, it appears beneficial to
   have very good authentication capabilities in the layers below the
   application; this is specifically true for applications developed in
   HTML and JavaScript, which tend to load code from various places,
   including code that is not always in the end user's interest; since
   it already is a concern what identity information passes through
   these applications, it is certainly not advisable to use credentials
   in those places.  Browsers are in a better position to take control
   over these assets, at the protocol levels of HTTP and TLS, and
   conceal credentials and possibly also identity from applications
   running on top.  Inasfar as tokens are needed, they can be derived
   from session keys using generally accepted key derivation schemes,
   but the session keys can be isolated from dynamic layers above HTTP.

2.  Embedding SASL in HTTP

   This specification integrates the SASL framework [RFC4422] into
   mainstream HTTP [RFC2616].  The SASL Authentication scheme follows
   the general structure for HTTP Authentication [RFC7235].  It uses the
   WWW-Authenticate and Proxy-Authenticate headers in responses from web
   servers and web proxies, respectively, and correspondingly the
   Authorization and Proxy-Authorization request header to answer to
   requests.

   The SASL service name for the following embedding of SASL is HTTP;
   contrary to most other service names, it is spelled in uppercase, in
   line with what has become general practice in Kerberos and GSSAPI.

   Since SASL prescribes channel binding to occur relative to TLS
   instead of to the application protocol, we can add that when the
   HTTPS transport is used.

   Whether channel binding is used SHOULD remain a configuration choice
   in HTTP software, as it might interfere with intentional HTTPS
   proxying.  Unintended proxying on the other hand, might lead to
   tapping of credentials under certain SASL mechanisms, and it may be

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   considered helpful to prevent such situations by relying on channel
   binding for at least those mechanisms.

2.1.  HTTP Request and Response Messages

   This section defines a few names for HTTP request and response
   messages, to be used in the remainder of this specification.

   Initial Responses are those HTTP responses that set a status code 401
   or 407, and that are sent when the HTTP server decides to initiate an
   authentication exchange.

   Initial Requests are those HTTP requests that a client sends to
   initiate a fresh SASL authentication.  User-Aware Requests are a
   variation defined further below, intended for attempts to address
   public resources under a given user name.

   Intermediate Responses are HTTP responses to SASL authentication,
   with a status code set to 401 or 407.  Intermediate Requests are
   those HTTP requests that a client sends to continue a SASL
   authentication after an Intermediate Response.

   Final Responses either set a 200 or 403 status code, the first
   depicting success and the second depicting failure.  Information in a
   Final 200 Response is provided in an Authentication-Info or Proxy-
   Authentication-Info header [RFC7615] instead of the headers used in
   Initial Responses and Intermediate Responses [RFC7235].  Note that
   proper interpretation of the Final 200 Response requires client state
   indicating that SASL authentication was used, or else the optional
   fields are not completely reliable information sources; cryptographic
   markers in the c2c field MAY be used to overcome this in a manner
   that defies abuse by rogue servers.  The Final 403 Response never
   contains authentication-related headers.

   The following fields, defined in upcoming sections, MUST and MAY be
   present in HTTP authentication exchanges for SASL:

   Request or Response   | MUST have fields    | MAY have fields
   ----------------------+---------------------+----------------------
   User Viewing Request  | userview            | mech
   Initial Response      | mech,s2s,realm      | text
   Initial Request       | mech,s2s,c2c,realm  | c2s,userview
   Intermediate Response | c2c,s2c,s2s         | text
   Intermediate Request  | c2c,c2s,s2s         | userview
   Final 200 Response    | c2c                 | s2s
   Final 403 Response    | c2c                 |

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2.2.  Authentication Field Definitions

   Data for SASL is transported in the following fields:

   c2s   holds SASL mechanism-specific data from client to server.

   s2c   holds SASL mechanism-specific data from server to client.

   s2s   holds opaque server data which the client MUST reflect in
         Intermediate Requests, to implement stateless SASL handling in
         the server.  This is a requirement for the HTTP Authentication
         framework [Section 5.1.2 of [RFC7235]].  It MAY also be used to
         pass cached Section 2.3 authentication results.

   c2c   holds opaque client data which the server MUST reflect in
         Intermediate Responses and Final 200/403 Responses.  This can
         help to also make the client stateless.

   It is not generally safe for SASL mechanisms to exchange c2s and s2c
   messages over unprotected transports.  The c2c and s2s fields MUST be
   protected against tampering by rogue peers, and such protection also
   protects against tampering by rogue intermediates when using an
   unprotected transport.  In addition, c2c and s2s fields may also need
   to be concealed from peers and intermediates.

   Whether s2c is supplied in a Final 200 Response depends on the SASL
   mechanism, which may or may not have additional data to provide in
   this phase.  Note that SASL requires empty s2c messages to be
   distinguished from absence thereof.  When the server provides c2s
   and/or s2s data in a Final 200 Response, then it indicates that the
   supplied fields MAY provide one-step re-authentication with an empty
   s2c string, but the server MAY revoke this privilege at any time and
   for any reason; it would respond with an Initial Response in case of
   such revocation, but with a quick Final 200 Response if the one-step
   re-authentication is still acceptable.

   The following fields support SASL within the HTTP Authentication
   Framework:

   userview  selects a user view on the resources accessible over HTTP.
         This data selection purpose is unrelated to authentication.  As
         a general principle, the userview MAY lead to changes in
         authentication triggers for URI paths, but MUST NOT change
         authentication triggers for the underlying resources.

   realm optionally names a scope of authorisation under the combination
         of scheme, server host name and userview.  This does not
         signify a realm in the sense of realm crossover.

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   mech  In an Initial Response, the field is filled with a space-
         separated list of SASL mechanism names; In an Initial Request,
         the client chooses one SASL mechanism name; In a User Viewing
         Request, the field is fixated to ANONYMOUS [RFC4505] if it is
         provided.

   text  is a user-oriented text explaining what information is needed.

2.3.  Caching Authentication Results

   When an HTTP server sends a Final 200 Response, it MAY include an
   "s2s" field.  If it does this, then it SHOULD prepare to accept the
   field value for authentication in an Initial Request.  However,
   credentials can expire or fall in disgrace for other reasons, so the
   server MAY still choose to reject the provided field.

   When an HTTP client receives a Final 200 Response with an "s2s"
   field, it MAY memorise the fields for future reuse in an Initial
   Request, either with or without preceding Initial Response from the
   server.  The HTTP client MUST use the realm as part of the decision
   which cached result to use, but it MAY extrapolate the results from
   one resource retrieval in an attempt to authenticate another.

   When cached field result in a Final 407 Response then the HTTP client
   SHOULD remove the failing cache entry, and it SHOULD try again by
   going through a full SASL authentication cycle.  The stateless nature
   of HTTP authentication is helpful in the sense that a new Initial
   Request can be sent to an older Initial Response.

3.  Viewing Users on HTTP Services

   The initial authentication mechanisms for HTTP were Basic and Digest
   [RFC2617] but these are no longer considered secure.  These forms
   used the username in the URI together with a password, a combination
   that is now officially deprecated [Section 3.2.1 of [RFC3986]].

   The use of a user name in a URI has not been deprecated, but has been
   withdrawn from HTTP because it was not included in the core HTTP
   specification [RFC2616].  With client identities that support realm
   crossover, it is possible to add this to SASL Authorization headers,
   through a new userview field, to be used like this:

   GET / HTTP/1.1
   Host: www.example.net
   Authorization: SASL userview=snowboarders

   There is a clear need for users in HTTP URIs, as can be seen from a
   wild variation of mappings into paths and sub-domains.  The actual

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   use of the username part of the URI is however preferrable, and the
   URI format defines it in the authority part [Section 3.2 of
   [RFC3986]] which seems to match with the intended uses.  A URI-based
   user name will probaly still be mapped to a path on a server, but
   need not be shown in the path in the URI.  Better standardisation of
   user names is supportive of better integration with tools on both the
   client and server side.

   We emphasise that the userview and client identity are orthogonal
   concepts, except for the special case where the client is viewing his
   own resources.  This implies that it can be used with or without
   authentication.  The userview field indicates the name space of
   resources beig addressed, while the client identity is involved in
   access rights.

   The reason to integrate the userview with SASL is one of identity
   control; when the userview changes, one is visiting another part of a
   HTTP resource space, and it may then be proper to switch to another
   identity.  This is in line with the scoping of protection spaces
   [Section 2.2 of [RFC7235]] to combinations of URI scheme, URI
   authority section and a server-defined realm string, where the URI
   authority section includes the user name in the URI.

   Browsers currently show varying behaviours when supplied with a user
   name.  This is mostly due to the deprecation [Section 3.2.1 of
   [RFC3986]] of userparts with a password embedded.  This has created
   uncertainty and variation in handling of HTTP URIs with user names
   but no password.  The userview field is intended to present a
   constructive alternative, where user names may once more be used to
   scope the resource name space to that of a user, without saying
   anything about authentication.

4.  Security Considerations

   The SASL exchange may be at risk of tampering when the sequence of
   HTTP messages is not secured to form one stream.  The termination of
   such a secure layer MUST also terminate an ongoing SASL handshake.

   SASL EXTERNAL can be a very efficient mechanism to combine with a
   secure transport layer if that includes authentication.  This may be
   the case for TLS, especially when client-side authentication is
   deployed.  Mechanisms other than EXTERNAL should take into account
   that a relation may exist between identities negotiated in the
   protective layer and the SASL exchange over HTTP.  For example, a
   login account may be exchanged for an alias or group identity.

   Channel binding is available in some SASL mechanisms.  When used with
   HTTP SASL over TLS, it binds to the TLS channel, by default using the

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   type tls-unique [Section 3 of [RFC5929]].  When doing so, it is vital
   that either there be no renegotiation of the TLS handshake, or both
   secure renegotiation [RFC5746] and the extended master secret
   [RFC7627] are used.

5.  IANA Considerations

   This specification extends the "Hypertext Transfer Protocol (HTTP)
   Authentication Scheme Registry" with an "Authentication Scheme Name"
   SASL, referencing this specification.

   This specification defines an additional entry in the registry
   "Generic Security Service Application Program Interface
   (GSSAPI)/Kerberos/Simple Authentication and Security Layer (SASL)
   Service Names" namely:

   Service Name: HTTP
   Usage:        Web authentication using the SASL framework
   Reference:    TBD:this specification

6.  References

6.1.  Normative References

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <https://www.rfc-editor.org/info/rfc4422>.

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   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
              <https://www.rfc-editor.org/info/rfc4559>.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
              <https://www.rfc-editor.org/info/rfc5056>.

   [RFC5554]  Williams, N., "Clarifications and Extensions to the
              Generic Security Service Application Program Interface
              (GSS-API) for the Use of Channel Bindings", RFC 5554,
              DOI 10.17487/RFC5554, May 2009,
              <https://www.rfc-editor.org/info/rfc5554>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <https://www.rfc-editor.org/info/rfc5746>.

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
              July 2010, <https://www.rfc-editor.org/info/rfc5801>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <https://www.rfc-editor.org/info/rfc5929>.

   [RFC6595]  Wierenga, K., Lear, E., and S. Josefsson, "A Simple
              Authentication and Security Layer (SASL) and GSS-API
              Mechanism for the Security Assertion Markup Language
              (SAML)", RFC 6595, DOI 10.17487/RFC6595, April 2012,
              <https://www.rfc-editor.org/info/rfc6595>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7615]  Reschke, J., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <https://www.rfc-editor.org/info/rfc7615>.

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   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <https://www.rfc-editor.org/info/rfc7627>.

6.2.  Informative References

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <https://www.rfc-editor.org/info/rfc2617>.

   [RFC4505]  Zeilenga, K., "Anonymous Simple Authentication and
              Security Layer (SASL) Mechanism", RFC 4505,
              DOI 10.17487/RFC4505, June 2006,
              <https://www.rfc-editor.org/info/rfc4505>.

   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,
              <https://www.rfc-editor.org/info/rfc5802>.

   [RFC7804]  Melnikov, A., "Salted Challenge Response HTTP
              Authentication Mechanism", RFC 7804, DOI 10.17487/RFC7804,
              March 2016, <https://www.rfc-editor.org/info/rfc7804>.

Appendix A.  HTTP Server Environment Variables

   We define a number of variables that SHOULD be passed from an HTTP
   SASL stack to applications run on top of it.  The intention of
   defining these is to obtain maximum interoperability between these
   layers of software.

   A common practice is to set environment variables with a given name
   to values that may be meaningful to applications.  Those applications
   should be mindful about the possible meaning of absent variables.

   The following variable MAY be available in both the SASL
   authenticated and unauthenticated state:

   SASL_USERVIEW  refers to the user name in the URI

   SASL_USERVIEW  that MUST NOT be accompanied by a password.  It
         refines the view on resources held by the web server, usually
         from a general site to one that is user-specific.  The URI user

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         is considered local to the web server (and, as a result of
         that, often its domain or security realm).  This variable is
         only set when it is provided through the siteview field in the
         SASL exchange.

   The following variables MUST NOT be available until SASL
   authentication is successful; it would be available when the server
   could send a 200 OK response:

   SASL_SECURE  is only "yes" (without the quotes) when a client is
         authenticated to the current resource.  It never has another
         value; it is simply undefined when not secured by SASL.

   SASL_REALM  is the realm for which the secure exchange succeeded.  A
         realm is not always used, because sites only need it when there
         are more than one in the same name space.  When undefined in
         the SASL flow, this variable will not be set.

   SASL_CLIENTID  is the identity as confirmed through SASL
         authentication.  Its content is formatted like an email
         address, and includes a domain name.  That domain need not be
         related to the web server; it is possible for a web server to
         welcome foreign clients.

   SASL_MECH  indicates the mechanism used, and is one of the
         standardised SASL mechanism names.  It may be used to detect
         the level of security.

   SASL_S2S  holds the accepted s2s field, and could be used as a random
         session identifier.  It would normally be encrypted
         information.

   SASL_S2S_  is a prefix for extra information that the server may
         extract from the s2s field in the HTTP SASL protocol flow.
         This depends on the authentication stack used in the web
         server.

Appendix B.  Acknowledgements

   Thanks to Henri Manson for making the first implementation of this
   specification and for feedback on the header formats.

Author's Address

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   Rick van Rein
   ARPA2.net
   Haarlebrink 5
   Enschede, Overijssel  7544 WP
   The Netherlands

   Email: rick@openfortress.nl

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