[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01

Network Working Group                                            M. West
Internet-Draft                                                    Google
Intended status: Standards Track                            May 10, 2019
Expires: November 11, 2019


                 First-Party Sets and SameSite Cookies
                draft-west-cookie-samesite-firstparty-01

Abstract

   This document proposes the addition of two new values to the
   "SameSite" cookie attribute defined in RFC6265bis
   [I-D.ietf-httpbis-rfc6265bis]: "FirstPartyLax" and
   "FirstPartyStrict".  These values are conceptually similar to the
   existing "Lax" and "Strict" values, but base the delivery checks on
   the First-Party Sets [first-party-set] of a request's initiator and
   target, rather than on their respective registrable domains.  This
   widens the scope of a given cookie's applicability, enabling entities
   that have sharded themselves across multiple registrable domains to
   maintain HTTP state without exposing themselves to the risks of
   "SameSite=None".

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 11, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



West                    Expires November 11, 2019               [Page 1]


Internet-Draft         cookie-samesite-firstparty               May 2019


   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
     2.1.  Conformance . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.3.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  The "FirstParty" value of the "SameSite" attribute  . . . . .   4
   4.  Security and Privacy Considerations . . . . . . . . . . . . .   7
     4.1.  CSRF  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Secure Transport  . . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Changes  . . . . . . . . . . . . . . . . . . . . . .   9
     A.1.  Since draft-west-cookie-samesite-firstparty-00  . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The "SameSite" attribute enables developers to limit the scope of a
   given cookie's delivery, mitigating the risks of some classes of
   cross-site request forgery (CSRF) attack by preventing certain
   cookies from being delivered along with requests that are initiated
   from a cross-site context.

   For example, consider the exciting and dynamic "https://internet-
   bookstore.example/", which uses "SameSite=Lax" cookies as one layer
   in its defense against CSRF attack.  If "https://example.com"
   includes resources from "https://internet-bookstore.example/", the
   request will be considered cross-site, and the authentication cookies
   will not be delivered.  Without that state, CSRF attacks will be
   significantly less effective.

   When the site expands into new locations, it may wish to register a
   domain under a localized TLD, perhaps "https://internet-
   bookstore.测试/".  Likewise, it may decide to shard itself into
   distinct brands, like "https://internet-things-other-than-books-
   store.example/".  Though the same entity controls each of



West                    Expires November 11, 2019               [Page 2]


Internet-Draft         cookie-samesite-firstparty               May 2019


   these origins, they have distict registrable domains, and therefore
   the authentication cookie noted above will not be delivered from one
   site to resources on another.  This frustrates a number of reasonable
   use cases, including single-sign on.  Today, "SameSite=None" is
   necessary in order to support these use cases by enabling a given
   cookie to be delivered across registrable domains.  "SameSite=None",
   unfortunately, exposes the site to more risk than it would prefer, as
   it removes a layer of CSRF defense.

   First-Party Sets [first-party-set] proposes a mechanism by which
   developers can bind each of their distinct registrable domains into a
   set which mutually agrees to be treated as a single entity.  It would
   be helpful if this concept could be folded into the "SameSite"
   attribute, perhaps via new "FirstPartyLax" and "FirstPartyStrict"
   values.  These could be conceptually similar to the existing "Lax"
   and "Strict" values, but base their delivery checks on the First-
   Party Sets of a given request's initiator and target, rather than on
   their respective registrable domains.

   This document spells out that proposal in a bit more detail.

2.  Conventions and Definitions

2.1.  Conformance

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Syntax

   This document adjusts some syntax from [I-D.ietf-httpbis-rfc6265bis],
   and in doing so, relies upon the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

2.3.  Terms

   HTTP requests are considered "same-site" or "cross-site", as defined
   in [I-D.ietf-httpbis-rfc6265bis].

   First-Party Sets are defined in [first-party-set].  Two origins ("A"
   and "B") are said to be in the same first-party set if the first-
   party set associated with "A" contains "B".

   A request is considered to be "first-party" if the target origin is
   in the same first-party set as the request's initiator, and "third-



West                    Expires November 11, 2019               [Page 3]


Internet-Draft         cookie-samesite-firstparty               May 2019


   party" otherwise.  That is, for a given request ("r"), the following
   algorithm returns "first-party" or "third-party":

   1.  If "request" is "same-site", return "first-party".

   2.  Let "target" be "r"'s current URL's origin.

   3.  If "target" is in the same first-party set as "r"'s client's
       origin, return "first-party".

   4.  Return "third-party".

   A document is considered "first-party with its ancestors" if its
   origin is in the same First-Party Set with the origins of each of the
   document's ancestors [HTML].  That is, for a given document ("d"),
   the following algorithm returns "first-party" or "third-party":

   1.  If "d"'s browsing context is a top-level browsing context, return
       "first-party".

   2.  Let "set" be "d"'s origin's First-Party Set.

   3.  For each "ancestor" in "d"'s browsing context's ancestor browsing
       contexts:

       1.  If "ancestor"'s active document's origin is not contained
           within "set", return "third-party".

   4.  Return "first-party".

   ISSUE: Move these definitions to the First-Party Sets spec, when one
   exists.

3.  The "FirstParty" value of the "SameSite" attribute

   [I-D.ietf-httpbis-rfc6265bis] defines three values for cookies'
   "SameSite" attribute: "None", which enables delivery for same-site
   and cross-site requests; "Strict", which enables delivery only for
   same-site requests; and "Lax", which enables delivery for same-site
   requests as well as for cross-site top-level navigations.

   In the presence of first-party sets, it makes sense to extend this
   syntax a bit to include "FirstParty", which will allow delivery of
   cookies within a first-party set, and therefore will support the use
   cases that first-party sets addresses (a given first-party's single
   sign-on, for instance).  For example, given two distinct origins
   "https://sso.example/" and "https://application.example/" that are
   contained in the same first-party set:



West                    Expires November 11, 2019               [Page 4]


Internet-Draft         cookie-samesite-firstparty               May 2019


   o  Requests from "https://application.example/" to
      "https://sso.example/" may not contain any cookies set with
      "SameSite=Lax" or "SameSite=Strict", but only those set as
      "SameSite=None".

   o  Requests from "https://application.example/" to
      "https://sso.example/" may contain any cookies set with
      "SameSite=FirstParty" or "SameSite=None".

   To implement this change, adjust [I-D.ietf-httpbis-rfc6265bis] as
   follows:

   First, change the "samesite-value" definition from:

   samesite-value    = "Strict" / "Lax" / "None"

   to:

samesite-value    = "Lax" / "Strict" / "FirstPartyLax" / "FirstPartyStrict" / "None"

   Second, alter the "SameSite" attribute's processing algorithm
   (Section 5.3.7 of [I-D.ietf-httpbis-rfc6265bis]) to add a new step 4
   and 5:

4.  If cookie-av's attribute-value is a case-insensitive
    match for "FirstPartyLax", set `enforcement` to "FirstPartyLax".

5.  If cookie-av's attribute-value is a case-insensitive
    match for "FirstPartyStrict", set `enforcement` to "FirstPartyStrict".

   Third, alter the cookie storage model (Section 5.4 of
   [I-D.ietf-httpbis-rfc6265bis]) as follows:

   Change step 14.1 from:

   1.  If the cookie was received from a "non-HTTP" API, and
       the API was called from a context whose "site for
       cookies" is not an exact match for request-uri's host's
       registered domain, then abort these steps and ignore the
       newly created cookie entirely.

   to:









West                    Expires November 11, 2019               [Page 5]


Internet-Draft         cookie-samesite-firstparty               May 2019


   1.  If the cookie was received from a "non-HTTP" API:

       1.  If the cookie's `same-site-flag` is "Lax" or
           "Strict", and the API was called from a context
           whose "site for cookies" is not an exact match for
           request-uri's host's registered domain, then abort
           these steps and ignore the newly-created cookie
           entirely.

       2.  If the cookie's same-site-flag` is "FirstPartyLax"
           or "FirstPartyStrict", and the API was called from
           a context that is not first-party with its
           ancestors, then abort these steps and ignore the
           newly-created cookie entirely.

   Change step 14.2 from:

   2.  If the cookie was received from a "same-site" request,
       skip the remaining substeps and continue processing the
       cookie.

   to:

   2.  If the cookie's `same-site-flag` is "Lax" or "Strict",
       and the cookie was received from a "same-site" request,
       then skip the remaining substeps and continue processing
       the cookie.

   Add a new step 14.3 after the new step 14.2:

   3.  If the cookie's `same-site-flag` is "FirstPartyLax" or
       "FirstPartyStrict", and the cookie was received from a
       "first-party" request, then skip the remaining substeps
       and continue processing the cookie.

   Add a new step after the existing step 14:

   15. If the cookie's `same-site-flag` is "FirstPartyLax" or
       "FirstPartyStrict", abort these steps and ignore the
       cookie entirely unles the cookie's `secure-only-flag`
       is true.

   Fourth, alter the last conditional in step 1 if the "Cookie" header
   algorithm (Section 5.5 of [I-D.ietf-httpbis-rfc6265bis]) from:







West                    Expires November 11, 2019               [Page 6]


Internet-Draft         cookie-samesite-firstparty               May 2019


   *   If the cookie's same-site-flag is not "None", and the
       HTTP request is cross-site then exclude the cookie
       unless all of the following statements hold:

       1.  The same-site-flag is "Lax".

       2.  The HTTP request's method is "safe".

       3.  The HTTP request's target browsing context is a
           top-level browsing context.

   to:

   *   If the HTTP request is cross-site, then exclude the
       cookie unless one of the following statements holds:

       1.  The cookie's `same-site-flag` is "None".

       2.  The cookie's `same-site-flag` is either "Lax" or
           "FirstPartyLax", the HTTP request's method is
           "safe", and the HTTP request's target browsing
           context is a top-level browsing context.

       3.  The cookie's `same-site-flag` is either
           "FirstPartyLax" or "FirstPartyStrict", and the HTTP
           request is a first-party request.

4.  Security and Privacy Considerations

4.1.  CSRF

   Both "FirstPartyLax" and "FirstPartyStrict" provide weaker defenses
   against CSRF than their "Lax" and "Strict" counterparts, as they
   enable authenticated requests from a larger set of initiating
   contexts.  That said, they also provide deployment benefits, as
   they're usable in some contexts where "Lax" and "Strict" would be too
   restrictive (e.g. the localized registrable domains in the
   introduction).

4.2.  Secure Transport

   First-Party Sets can only be created for secure origins, as
   unauthenticated transport doesn't give any guarantees that the
   assertions we use to build the set are in fact being delivered by the
   entity which controls the server.  This has the side-effect of
   ensuring that "FirstPartyLax" and "FirstPartyStrict" cookies can only
   be delivered to secure cross-site origins, which has the exciting




West                    Expires November 11, 2019               [Page 7]


Internet-Draft         cookie-samesite-firstparty               May 2019


   side effect of providing limited mitigation of monitoring by network
   attackers [RFC7258].

   Still, to ensure that the protections offered by secure transport
   accrue to users whose agents don't yet support these new "SameSite"
   values, we require that cookies asserting these values also include
   the "Secure" attribute.

5.  IANA Considerations

   This document has no IANA actions.

6.  References

6.1.  Normative References

   [first-party-set]
              West, M., "First-Party Sets", n.d.,
              <https://mikewest.github.io/first-party-sets/>.

   [I-D.ietf-httpbis-rfc6265bis]
              Barth, A. and M. West, "Cookies: HTTP State Management
              Mechanism", draft-ietf-httpbis-rfc6265bis-03 (work in
              progress), April 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [HTML]     Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
              P., and D. Denicola, "HTML", n.d.,
              <https://html.spec.whatwg.org/>.

   [I-D.west-http-state-tokens]
              West, M., "HTTP State Tokens", draft-west-http-state-
              tokens-00 (work in progress), March 2019.



West                    Expires November 11, 2019               [Page 8]


Internet-Draft         cookie-samesite-firstparty               May 2019


   [mixed-content]
              West, M., "Mixed Content", n.d.,
              <https://w3c.github.io/webappsec-mixed-content/>.

   [pref-cookie]
              Soltani, A., Peterson, A., and B. Gellman, "NSA uses
              Google cookies to pinpoint targets for hacking", December
              2013, <https://www.washingtonpost.com/news/the-
              switch/wp/2013/12/10/
              nsa-uses-google-cookies-to-pinpoint-targets-for-hacking/>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

Appendix A.  Changes

A.1.  Since draft-west-cookie-samesite-firstparty-00

   o  Added an explicit requirement to tag "FirstPartyLax" and
      "FirstPartyStrict" cookies as "Secure".

Acknowledgments

   Conversations with a number of folks at 2019's HTTP Workshop helped
   me clarify my thinking around the incremental improvements we can
   make to cookies.  In particular, Martin Thomson and Anne van Kesteren
   provided insightful feedback.

Author's Address

   Mike West
   Google

   Email: mkwst@google.com
   URI:   https://www.mikewest.org/











West                    Expires November 11, 2019               [Page 9]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/