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

Versions: 00

Network Working Group                                            M. West
Internet-Draft                                                    Google
Intended status: Standards Track                             May 7, 2019
Expires: November 8, 2019


                      Incrementally Better Cookies
                  draft-west-cookie-incrementalism-00

Abstract

   This document proposes two changes to cookies inspired by the
   properties of the HTTP State Tokens mechanism proposed in
   [I-D.west-http-state-tokens].  First, cookies should be treated as
   "SameSite=Lax" by default.  Second, cookies that explicitly assert
   "SameSite=None" in order to enable cross-site delivery should also be
   marked as "Secure".

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 8, 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
   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




West                    Expires November 8, 2019                [Page 1]


Internet-Draft            cookie-incrementalism                 May 2019


   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
   3.  Monkey-Patches against RFC6265bis . . . . . . . . . . . . . .   3
     3.1.  "Lax" by Default  . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Requiring "Secure" for "SameSite=None"  . . . . . . . . .   4
   4.  Security and Privacy Considerations . . . . . . . . . . . . .   5
     4.1.  CSRF  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Secure Transport  . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Tracking  . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Implementation Considerations . . . . . . . . . . . . . . . .   6
     5.1.  Sequencing  . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Deployment  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The HTTP State Tokens proposal ([I-D.west-http-state-tokens]) aims to
   replace cookies with a state management mechanism that has better
   security and privacy properties.  That proposal is somewhat
   aspirational: it's going to take a long time to come to agreement on
   the exact contours of a cookie replacement, and an even longer time
   to actually do so.

   While we're debating the details of a new state management primitive,
   it seems quite reasonable to reevaluate some aspects of the existing
   primitive: cookies.  When we can find consensus on some aspect of
   HTTP State Tokens, we can apply those aspirations to cookies, driving
   incremental improvements to state management in the status quo.

   Based on conversations at [HTTP-Workshop-2019] and elsewhere, I'd
   suggest that we have something like agreement on at least two
   principles:

   1.  HTTP requests should not carry state along with cross-site
       requests by default (see Section 8.2 of [RFC6265bis]).




West                    Expires November 8, 2019                [Page 2]


Internet-Draft            cookie-incrementalism                 May 2019


   2.  HTTP requests should not carry state over non-secure channels
       (see Section 8.3 of [RFC6265bis], and [RFC7258]).

   With those principles in mind, this document proposes two changes
   that seem possible to deploy in the near-term.  User agents should:

   1.  Treat the lack of an explicit "SameSite" attribute as
       "SameSite=Lax".  That is, the "Set-Cookie" value "key=value" will
       produce a cookie equivalent to "key=value; SameSite=Lax".
       Cookies that require cross-site delivery can explicitly opt-into
       such behavior by asserting "SameSite=None" when creating a
       cookie.

       This is spelled out in more detail in Section 3.1.

   2.  Require the "Secure" attribute to be set for any cookie which
       asserts "SameSite=None" (similar conceptually to the behavior for
       the "__Secure-" prefix).  That is, the "Set-Cookie" value
       "key=value; SameSite=None; Secure" will be accepted, while
       "key=value; SameSite=None" will be rejected.

       This is spelled out in more detail in Section 3.2.

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 [RFC6265bis], and in doing so,
   relies upon the Augmented Backus-Naur Form (ABNF) notation of
   [RFC5234].

3.  Monkey-Patches against RFC6265bis

3.1.  "Lax" by Default

   The processing algorithm in Section 5.3.7 of [RFC6265bis] treats the
   absence of a "SameSite" attribute in a "Set-Cookie" header as
   equivalent to the presence of "SameSite=None".  Cookies are therefore
   available for cross-site delivery by default, and developers may opt-
   into more security by setting some other value explicitly.  Ideally,



West                    Expires November 8, 2019                [Page 3]


Internet-Draft            cookie-incrementalism                 May 2019


   we'd invert that such that developers who accepted the risks of
   cross-site delivery (see Section 8.2 of [RFC6265bis]) could opt into
   them, while developers who didn't make any explicit choice would be
   protected by default.

   We could accomplish this goal by first altering the processing
   algorithm, replacing the current step 1:

   1.  Let "enforcement" be "None".

   with the following two steps:

   1.  Let "enforcement" be "Lax".

   2.  If cookie-av's attribute-value is a case-insensitive
       match for "None", set "enforcement" to "None".

   And then by, altering step 13 of the cookie storage model
   (Section 5.4 of [RFC6265bis]) from:

   13. If the cookie-attribute-list contains an attribute
       with an attribute-name of "SameSite", set the cookie's
       same-site-flag to attribute-value (i.e. either "Strict",
       "Lax", or "None"). Otherwise, set the cookie's
       same-site-flag to "None".

   to:

   13. If the cookie-attribute-list contains an attribute
       with an attribute-name of "SameSite", set the
       cookie's same-site-flag to attribute-value. Otherwise,
       set the cookie's same-site-flag to "Lax".

   This would have the effect of mapping the default behavior in the
   absence of an explicit "SameSite" attribute, as well as the presence
   of any unknown "SameSite" value, to the "Lax" behavior, protecting
   developers by making cross-site delivery an explicit choice, as
   opposed to an implicit default.

3.2.  Requiring "Secure" for "SameSite=None"

   Cookies sent over plaintext HTTP are visible to anyone on the
   network.  As section 8.3 of [RFC6265bis] points out, this visibility
   exposes substantial amounts of data to network attackers.  We know,
   for example, that long-lived and stable cookies have enabled
   pervasive monitoring [RFC7258] in the past (see Google's PREF cookie
   [pref-cookie]), and we know that a secure transport layer provides
   significant confidentiality protections against this kind of attack.



West                    Expires November 8, 2019                [Page 4]


Internet-Draft            cookie-incrementalism                 May 2019


   We can, to a reasonable extent, mitigate this threat by ensuring that
   cookies intended for cross-site delivery (and therefore likely to be
   more prevalent on the wire than cookies scoped down to same-site
   requests) require secure transport.

   That is, we can require that any cookie which asserts "SameSite=None"
   must also assert the "Secure" attribute (Section 4.1.2.5 of
   [RFC6265bis]) by altering the storage model defined in Section 5.4 of
   [RFC6265bis], inserting the following step after the existing step
   14:

   15. If the cookie's "same-site-flag" is "None", abort
       these steps and ignore the cookie entirely unless
       the cookie's secure-only-flag is true.

   This is conceptually similar to the requirements put into place for
   the "__Secure-" prefix (Section 4.1.3.1 of [RFC6265bis]).

4.  Security and Privacy Considerations

4.1.  CSRF

   "SameSite" is a reasonably robust defense against some classes of
   cross-site request forgery attacks, as described in Section 8.8.1 of
   [RFC6265bis], but developers need to opt-into its protections in
   order for them to have any effect.  That is, developers are
   vulnerable to CSRF attacks by default, and must do some work to shift
   themselves into a more defensible position.

   The change proposed in Section 3.1 would invert that requirement,
   placing the burden on the small number of developers who are building
   services that require state in cross-site requests.  Those developers
   would be empowered to opt-into the status quo's less-secure model,
   while developers who don't intend for their projects to be embedded
   in cross-site contexts are protected by default.

4.2.  Secure Transport

   As discussed in Section 8.3 of [RFC6265bis], cookies delivered over
   plaintext channels are exposed to intermediaries, and thereby enable
   pervasive monitoring [RFC7258].  The change proposed in Section 3.2
   above would set secure transport as a baseline requirement for all
   stateful cross-site requests, thereby reducing the risk that these
   cookies can be cataloged or modified by network attackers.

   Requiring secure transport for cookies intended for cross-site usage
   has the exciting secondary effect of increasing pressure on entities
   that produce embeddable content to migrate their products to HTTPS.



West                    Expires November 8, 2019                [Page 5]


Internet-Draft            cookie-incrementalism                 May 2019


   That has security benefits for those third-party products themselves,
   but also has the effect of removing the potential of mixed content
   ([mixed-content]) as a blocker to first-party migration to HTTPS.

   Note that in the long term, it seems quite reasonable to take the
   additional step of requiring the "Secure" attribute for all cookies,
   regardless of their "SameSite" value.  That would have more
   substantial impact on pervasive monitoring and network attackers
   generally.  This document's proposal limits itself to "SameSite=None"
   because that seems like a low-hanging, high-value change that's
   deployable in the near term.  User agents are encouraged to find
   additional subsets for which "Secure" can be required.

4.3.  Tracking

   The proposals in this document do not in themselves mitigate the
   privacy risks described in Section 7.1 of [RFC6265bis].  Entities who
   wish to use cookies to track user activity from cross-site contexts
   can continue to do so by setting cookies that declare themselves as
   "SameSite=None".

   Requiring that explicit declaration, however, gives user agents the
   ability to easily distinguish cookies used for stateful cross-site
   requests from those with narrower scope.  After the change proposed
   in Section 3.1, only those cookies that make an explicit
   "SameSite=None" declaration can be directly used for cross-site
   tracking.  It may make sense for user agents to use that information
   to give users different controls for these cookies, or to apply
   different policies for expiration and delivery.

5.  Implementation Considerations

5.1.  Sequencing

   The steps described in this document don't need to be taken at the
   same time.  It's quite possible that it will be less disruptive to
   deploy "SameSite=Lax" as a default first, and then to require the
   "Secure" attribute for any explicitly "SameSite=None" cookie as a
   subsequent step.

   User agents are encouraged to adopt these recommendations in whatever
   order they believe will lead to the widest, most expedient
   deployment.








West                    Expires November 8, 2019                [Page 6]


Internet-Draft            cookie-incrementalism                 May 2019


5.2.  Deployment

   It's possible that a middle-ground between "SameSite=Lax" and
   "SameSite=None" could be a better balance between doing what
   developers want by default, and mitigating CSRF by default.
   [I-D.west-cookie-samesite-firstparty] explores the possibility of
   integrating First-Party Sets [first-party-set] with the "SameSite"
   attribute in order to allow entities that shard themselves across
   multiple registrable domains to maintain stateful communication
   between them (to support single-sign on, for example).

   It's possible that user agents who support First-Party Sets could
   reduce the deployment overhead for developers, and increase the
   robustness of a site's CSRF defense for cross-site-but-not-cross-
   party cookies by defaulting to something like that document's
   "FirstPartyLax" instead of "Lax".

6.  IANA Considerations

   This document has no IANA actions.

7.  References

7.1.  Normative References

   [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>.

   [RFC6265bis]
              Barth, A. and M. West, "Cookies: HTTP State Management
              Mechanism", draft-ietf-httpbis-rfc6265bis-03 (work in
              progress), April 2019.

   [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>.








West                    Expires November 8, 2019                [Page 7]


Internet-Draft            cookie-incrementalism                 May 2019


7.2.  Informative References

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

   [HTTP-Workshop-2019]
              Nottingham, M., "HTTP Workshop 2019: Report", April 2019,
              <https://github.com/HTTPWorkshop/workshop2019/wiki/
              Report>.

   [I-D.west-cookie-samesite-firstparty]
              West, M., "First-Party Sets and SameSite Cookies", May
              2019, <https://tools.ietf.org/html/
              draft-west-cookie-samesite-firstparty-00>.

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

   [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>.

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








West                    Expires November 8, 2019                [Page 8]


Internet-Draft            cookie-incrementalism                 May 2019


   Mike West
   Google

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














































West                    Expires November 8, 2019                [Page 9]


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