Same-site Cookies
draft-west-first-party-cookies-06

HTTPbis                                                          M. West
Internet-Draft                                               Google, Inc
Updates: 6265 (if approved)                                   M. Goodwin
Intended status: Standards Track                                 Mozilla
Expires: July 28, 2016                                  January 25, 2016

                           Same-site Cookies
                   draft-west-first-party-cookies-06

Abstract

   This document updates RFC6265 by defining a "SameSite" attribute
   which allows servers to assert that a cookie ought not to be sent
   along with cross-site requests.  This assertion allows user agents to
   mitigate the risk of cross-origin information leakage, and provides
   some protection against cross-site request forgery attacks.

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 28, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and notation  . . . . . . . . . . . . . . . . . .   4
     2.1.  "Same-site" and "cross-site" Requests . . . . . . . . . .   4
       2.1.1.  Document-based requests . . . . . . . . . . . . . . .   5
       2.1.2.  Worker-based requests . . . . . . . . . . . . . . . .   6
   3.  Server Requirements . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Semantics of the "SameSite" Attribute (Non-Normative) . .   8
   4.  User Agent Requirements . . . . . . . . . . . . . . . . . . .   8
     4.1.  The "SameSite" attribute  . . . . . . . . . . . . . . . .   8
       4.1.1.  "Strict" and "Lax" enforcement  . . . . . . . . . . .   8
     4.2.  Monkey-patching the Storage Model . . . . . . . . . . . .   9
     4.3.  Monkey-patching the "Cookie" header . . . . . . . . . . .  10
   5.  Authoring Considerations  . . . . . . . . . . . . . . . . . .  10
     5.1.  Defense in depth  . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Top-level Navigations . . . . . . . . . . . . . . . . . .  11
     5.3.  Mashups and Widgets . . . . . . . . . . . . . . . . . . .  11
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Section 8.2 of [RFC6265] eloquently notes that cookies are a form of
   ambient authority, attached by default to requests the user agent
   sends on a user's behalf.  Even when an attacker doesn't know the
   contents of a user's cookies, she can still execute commands on the
   user's behalf (and with the user's authority) by asking the user
   agent to send HTTP requests to unwary servers.

   Here, we update [RFC6265] with a simple mitigation strategy that
   allows servers to declare certain cookies as "same-site", meaning
   they should not be attached to "cross-site" requests (as defined in
   section 2.1).

   Note that the mechanism outlined here is backwards compatible with
   the existing cookie syntax.  Servers may serve these cookies to all
   user agents; those that do not support the "SameSite" attribute will

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   simply store a cookie which is attached to all relevant requests,
   just as they do today.

1.1.  Goals

   These cookies are intended to provide a solid layer of defense-in-
   depth against attacks which require embedding an authenticated
   request into an attacker-controlled context:

   1.  Timing attacks which yield cross-origin information leakage (such
       as those detailed in [pixel-perfect]) can be substantially
       mitigated by setting the "SameSite" attribute on authentication
       cookies.  The attacker will only be able to embed unauthenticated
       resources, as embedding mechanisms such as "<iframe>" will yield
       cross-site requests.

   2.  Cross-site script inclusion (XSSI) attacks are likewise mitigated
       by setting the "SameSite" attribute on authentication cookies.
       The attacker will not be able to include authenticated resources
       via "<script>" or "<link>", as these embedding mechanisms will
       likewise yield cross-site requests.

   3.  Cross-site request forgery (CSRF) attacks which rely on top-level
       navigation (HTML "<form>" POSTs, for instance) can also be
       mitigated by treating these navigational requests as "cross-
       site".

   4.  Same-site cookies have some marginal value for policy or
       regulatory purposes, as cookies which are not delivered with
       cross-site requests cannot be directly used for tracking
       purposes.  It may be valuable for an origin to assert that its
       cookies should not be sent along with cross-site requests in
       order to limit its exposure to non-technical risk.

1.2.  Examples

   Same-site cookies are set via the "SameSite" attribute in the "Set-
   Cookie" header field.  That is, given a server's response to a user
   agent which contains the following header field:

   Set-Cookie: SID=31d4d96e407aad42; SameSite

   Subsequent requests from that user agent can be expected to contain
   the following header field if and only if both the requested resource
   and the resource in the top-level browsing context match the cookie.

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2.  Terminology and notation

   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].

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   Two sequences of octets are said to case-insensitively match each
   other if and only if they are equivalent under the "i;ascii-casemap"
   collation defined in [RFC4790].

   The terms "active document", "ancestor browsing context", "browsing
   context", "document", "WorkerGlobalScope", "sandboxed origin browsing
   context flag", "parent browsing context", "the worker's Documents",
   "nested browsing context", and "top-level browsing context" are
   defined in [HTML].

   "Service Workers" are defined in the Service Workers specification
   [SERVICE-WORKERS].

   The term "origin", the mechanism of deriving an origin from a URI,
   and the "the same" matching algorithm for origins are defined in
   [RFC6454].

   "Safe" HTTP methods include "GET", "HEAD", "OPTIONS", and "TRACE", as
   defined in Section 4.2.1 of [RFC7231].

   The term "public suffix" is defined in a note in Section 5.3 of
   [RFC6265] as "a domain that is controlled by a public registry".  For
   example, "example.com"'s public suffix is "com".  User agents SHOULD
   use an up-to-date public suffix list, such as the one maintained by
   Mozilla at [PSL].

   An origin's "registrable domain" is the origin's host's public suffix
   plus the label to its left.  That is, "https://www.example.com"'s
   registrable domain is "example.com".  This concept is defined more
   rigorously in [PSL].

   The term "request", as well as a request's "client", "current url",
   "method", and "target browsing context", are defined in [FETCH].

2.1.  "Same-site" and "cross-site" Requests

   A request is "same-site" if it's target's URI's origin's registrable
   domain is an exact match for the request's initiator's "site for
   cookies", and "cross-site" otherwise.  To be more precise, for a

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   given request ("request"), the following algorithm returns "same-
   site" or "cross-site":

   1.  If "request"'s client is "null", return "same-site".

   2.  Let "site" be "request"'s client's "site for cookies" (as defined
       in the following sections).

   3.  Let "target" be the registrable domain of "request"'s current
       url.

   4.  If "site" is an exact match for "target", return "same-site".

   5.  Return "cross-site".

2.1.1.  Document-based requests

   The URI displayed in a user agent's address bar is the only security
   context directly exposed to users, and therefore the only signal
   users can reasonably rely upon to determine whether or not they trust
   a particular website.  The registrable domain of that URI's origin
   represents the context in which a user most likely believes
   themselves to be interacting.  We'll label this domain the "top-level
   site".

   For a document displayed in a top-level browsing context, we can stop
   here: the document's "site for cookies" is the top-level site.

   For documents which are displayed in nested browsing contexts, we
   need to audit the origins of each of a document's ancestor browsing
   contexts' active documents in order to account for the "multiple-
   nested scenarios" described in Section 4 of [RFC7034].  These
   document's "site for cookies" is the top-level site if and only if
   the document and each of its ancestor documents' origins have the
   same registrable domain as the top-level site.  Otherwise its "site
   for cookies" is the empty string.

   Given a Document ("document"), the following algorithm returns its
   "site for cookies" (either a registrable domain, or the empty
   string):

   1.  Let "top-document" be the active document in "document"'s
       browsing context's top-level browsing context.

   2.  Let "top-origin" be the origin of "top-document"'s URI if "top-
       document"'s sandboxed origin browsing context flag is set, and
       "top-document"'s origin otherwise.

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   3.  Let "documents" be an empty list be a list containing "document"
       and each of "document"'s ancestor browsing contexts' active
       documents.

   4.  For each "item" in "documents":

       1.  Let "origin" be the origin of "item"'s URI if "item"'s
           sandboxed origin browsing context flag is set, and "item"'s
           origin otherwise.

       2.  If "origin"'s host's registrable domain is not an exact match
           for "top-origin"'s host's registrable domain, return the
           empty string.

   5.  Return "top-site".

2.1.2.  Worker-based requests

   Worker-driven requests aren't as clear-cut as document-driven
   requests, as there isn't a clear link between a top-level browsing
   context and a worker.  This is especially true for Service Workers
   [SERVICE-WORKERS], which may execute code in the background, without
   any document visible at all.

   Note: The descriptions below assume that workers must be same-origin
   with the documents that instantiate them.  If this invariant changes,
   we'll need to take the worker's script's URI into account when
   determining their status.

2.1.2.1.  Dedicated and Shared Workers

   Dedicated workers are simple, as each dedicated worker is bound to
   one and only one document.  Requests generated from a dedicated
   worker (via "importScripts", "XMLHttpRequest", "fetch()", etc) define
   their "site for cookies" as that document's "site for cookies".

   Shared workers may be bound to multiple documents at once.  As it is
   quite possible for those documents to have distinct "site for cookie"
   values, the worker's "site for cookies" will be the empty string in
   cases where the values diverge, and the shared value in cases where
   the values agree.

   Given a WorkerGlobalScope ("worker"), the following algorithm returns
   its "site for cookies" (either a registrable domain, or the empty
   string):

   1.  Let "site" be "worker"'s origin's host's registrable domain.

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   2.  For each "document" in "worker"'s Documents:

       1.  Let "document-site" be "document"'s "site for cookies" (as
           defined in Section 2.1.1).

       2.  If "document-site" is not an exact match for "site", return
           the empty string.

   3.  Return "site".

2.1.2.2.  Service Workers

   Service Workers are more complicated, as they act as a completely
   separate execution context with only tangential relationship to the
   Document which registered them.

   Requests which simply pass through a service worker will be handled
   as described above: the request's client will be the Document or
   Worker which initiated the request, and its "site for cookies" will
   be those defined in Section 2.1.1 and Section 2.1.2.1

   Requests which are initiated by the Service Worker itself (via a
   direct call to "fetch()", for instance), on the other hand, will have
   a client which is a ServiceWorkerGlobalScope.  Its "site for cookies"
   will be the registrable domain of the Service Worker's URI.

   Given a ServiceWorkerGlobalScope ("worker"), the following algorithm
   returns its "site for cookies" (either a registrable domain, or the
   empty string):

   1.  Return "worker"'s origin's host's registrable domain.

3.  Server Requirements

   This section describes extensions to [RFC6265] necessary to implement
   the server-side requirements of the "SameSite" attribute.

3.1.  Grammar

   Add "SameSite" to the list of accepted attributes in the "Set-Cookie"
   header field's value by replacing the "cookie-av" token definition in
   Section 4.1.1 of [RFC6265] with the following ABNF grammar:

   cookie-av      = expires-av / max-age-av / domain-av /
                    path-av / secure-av / httponly-av /
                    samesite-av / extension-av
   samesite-av    = "SameSite" / "SameSite=" samesite-value
   samesite-value = "Strict" / "Lax"

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3.2.  Semantics of the "SameSite" Attribute (Non-Normative)

   The "SameSite" attribute limits the scope of the cookie such that it
   will only be attached to requests if those requests are "same-site",
   as defined by the algorithm in Section 2.1.  For example, requests
   for "https://example.com/sekrit-image" will attach same-site cookies
   if and only if initiated from a context whose "site for cookies" is
   "example.com".

   If the "SameSite" attribute has no value, if the value is "Strict",
   or if the value is invalid, the cookie will only be sent along with
   "same-site" requests.  If the value is "Lax", the cookie will be sent
   with "same-site" requests, and with "cross-site" top-level
   navigations, as described in Section 4.1.1.

   The changes to the "Cookie" header field suggested in Section 4.3
   provide additional detail.

4.  User Agent Requirements

   This section describes extensions to [RFC6265] necessary in order to
   implement the client-side requirements of the "SameSite" attribute.

4.1.  The "SameSite" attribute

   The following attribute definition should be considered part of the
   the "Set-Cookie" algorithm as described in Section 5.2 of [RFC6265]:

   If the "attribute-name" case-insensitively matches the string
   "SameSite", the user agent MUST process the "cookie-av" as follows:

   1.  Let "enforcement" be "Strict".

   2.  If "cookie-av"'s "attribute-value" is a case-insensitive match
       for either "Strict" or "Lax", set "enforcement" to "cookie-av"'s
       "attribute-value".

   3.  Append an attribute to the "cookie-attribute-list" with an
       "attribute-name" of "SameSite" and an "attribute-value" of
       "enforcement".

4.1.1.  "Strict" and "Lax" enforcement

   By default, same-site cookies will not be sent along with top-level
   navigations.  As discussed in Section 5.2, this might or might not be
   compatible with existing session management systems.  In the
   interests of providing a drop-in mechanism that mitigates the risk of
   CSRF attacks, developers may set the "SameSite" attribute in a "Lax"

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   enforcement mode that carves out an exception which sends same-site
   cookies along with cross-site requests if and only if they are top-
   level navigations which use a "safe" (in the [RFC7231] sense) HTTP
   method.

   Lax enforcement provides reasonable defense in depth against CSRF
   attacks that rely on unsafe HTTP methods (like "POST"), but do not
   offer a robust defense against CSRF as a general category of attack:

   1.  Attackers can still pop up new windows or trigger top-level
       navigations in order to create a "same-site" request (as
       described in section 2.1), which is only a speedbump along the
       road to exploitation.

   2.  Features like "<link rel='prerender'>" [prerendering] can be
       exploited to create "same-site" requests without the risk of user
       detection.

   When possible, developers should use a session management mechanism
   such as that described in Section 5.2 to mitigate the risk of CSRF
   more completely.

4.2.  Monkey-patching the Storage Model

   Note: There's got to be a better way to specify this.  Until I figure
   out what that is, monkey-patching!

   Alter Section 5.3 of [RFC6265] as follows:

   1.  Add "samesite-flag" to the list of fields stored for each cookie.
       This field's value is one of "None", "Strict", or "Lax".

   2.  Before step 11 of the current algorithm, add the following:

       1.  If the "cookie-attribute-list" contains an attribute with an
           "attribute-name" of "SameSite", set the cookie's "samesite-
           flag" to "attribute-value" ("Strict" or "Lax").  Otherwise,
           set the cookie's "samesite-flag" to "None".

       2.  If the cookie's "samesite-flag" is not "None", and the
           request which generated the cookie's client's "site for
           cookies" is not an exact match for "request-uri"'s host's
           registrable domain, then abort these steps and ignore the
           newly created cookie entirely.

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4.3.  Monkey-patching the "Cookie" header

   Note: There's got to be a better way to specify this.  Until I figure
   out what that is, monkey-patching!

   Alter Section 5.4 of [RFC6265] as follows:

   1.  Add the following requirement to the list in step 1:

       *  If the cookie's "samesite-flag" is not "None", and the HTTP
          request is cross-site (as defined in Section 2.1 then exclude
          the cookie unless all of the following statements hold:

          1.  "samesite-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.

   Note that the modifications suggested here concern themselves only
   with the "site for cookies" of the request's client, and the
   registrable domain of the resource being requested.  The cookie's
   "domain", "path", and "secure" attributes do not come into play for
   these comparisons.

5.  Authoring Considerations

5.1.  Defense in depth

   "SameSite" cookies offer a robust defense against CSRF attack when
   deployed in strict mode, and when supported by the client.  It is,
   however, prudent to ensure that this designation is not the extent of
   a site's defense against CSRF, as same-site navigations and
   submissions can certainly be executed in conjunction with other
   attack vectors such as cross-site scripting.

   Developers are strongly encouraged to deploy the usual server-side
   defenses (CSRF tokens, ensuring that "safe" HTTP methods are
   idempotent, etc) to mitigate the risk more fully.

   Additionally, client-side techniques such as those described in
   [app-isolation] may also prove effective against CSRF, and are
   certainly worth exploring in combination with "SameSite" cookies.

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5.2.  Top-level Navigations

   Setting the "SameSite" attribute in "strict" mode provides robust
   defense in depth against CSRF attacks, but has the potential to
   confuse users unless sites' developers carefully ensure that their
   session management systems deal reasonably well with top-level
   navigations.

   Consider the scenario in which a user reads their email at MegaCorp
   Inc's webmail provider "https://example.com/".  They might expect
   that clicking on an emailed link to "https://projects.com/secret/
   project" would show them the secret project that they're authorized
   to see, but if "projects.com" has marked their session cookies as
   "SameSite", then this cross-site navigation won't send them along
   with the request. "projects.com" will render a 404 error to avoid
   leaking secret information, and the user will be quite confused.

   Developers can avoid this confusion by adopting a session management
   system that relies on not one, but two cookies: one conceptualy
   granting "read" access, another granting "write" access.  The latter
   could be marked as "SameSite", and its absence would provide a
   reauthentication step before executing any non-idempotent action.
   The former could drop the "SameSite" attribute entirely, or choose
   the "Lax" version of enforcement, in order to allow users access to
   data via top-level navigation.

5.3.  Mashups and Widgets

   The "SameSite" attribute is inappropriate for some important use-
   cases.  In particular, note that content intended for embedding in a
   cross-site contexts (social networking widgets or commenting
   services, for instance) will not have access to such cookies.  Cross-
   site cookies may be required in order to provide seamless
   functionality that relies on a user's state.

   Likewise, some forms of Single-Sign-On might require authentication
   in a cross-site context; these mechanisms will not function as
   intended with same-site cookies.

6.  Privacy Considerations

   Same-site cookies in and of themselves don't do anything to address
   the general privacy concerns outlined in Section 7.1 of [RFC6265].
   The attribute is set by the server, and serves to mitigate the risk
   of certain kinds of attacks that the server is worried about.  The
   user is not involved in this decision.  Moreover, a number of side-
   channels exist which could allow a server to link distinct requests
   even in the absence of cookies.  Connection and/or socket pooling,

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   Token Binding, and Channel ID all offer explicit methods of
   identification that servers could take advantage of.

7.  References

7.1.  Normative References

   [FETCH]    van Kesteren, A., "Fetch", n.d.,
              <https://fetch.spec.whatwg.org/>.

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

   [PSL]      "Public Suffix List", n.d., <https://publicsuffix.org/
              list/>.

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

   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790, DOI 10
              .17487/RFC4790, March 2007,
              <http://www.rfc-editor.org/info/rfc4790>.

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

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

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454, DOI 10
              .17487/RFC6454, December 2011,
              <http://www.rfc-editor.org/info/rfc6454>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
              10.17487/RFC7231, June 2014,
              <http://www.rfc-editor.org/info/rfc7231>.

   [SERVICE-WORKERS]
              Russell, A., Song, J., and J. Archibald, "Service
              Workers", n.d., <http://www.w3.org/TR/service-workers/>.

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7.2.  Informative References

   [RFC7034]  Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
              Options", RFC 7034, DOI 10.17487/RFC7034, October 2013,
              <http://www.rfc-editor.org/info/rfc7034>.

   [app-isolation]
              Chen, E., Bau, J., Reis, C., Barth, A., and C. Jackson,
              "App Isolation - Get the Security of Multiple Browsers
              with Just One", n.d., <http://www.collinjackson.com/
              research/papers/appisolation.pdf>.

   [pixel-perfect]
              Stone, P., "Pixel Perfect Timing Attacks with HTML5",
              n.d., <http://www.contextis.com/documents/2/
              Browser_Timing_Attacks.pdf>.

   [prerendering]
              Bentzel, C., "Chrome Prerendering", n.d.,
              <https://www.chromium.org/developers/design-documents/
              prerender>.

   [samedomain-cookies]
              Goodwin, M. and J. Walker, "SameDomain Cookie Flag", 2011,
              <http://people.mozilla.org/~mgoodwin/SameDomain/
              samedomain-latest.txt>.

Appendix A.  Acknowledgements

   The same-site cookie concept documented here is indebited to Mark
   Goodwin's and Joe Walker's [samedomain-cookies].  Michal Zalewski,
   Artur Janc, and Ryan Sleevi provided particularly valuable feedback
   on this document.

Authors' Addresses

   Mike West
   Google, Inc

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

   Mark Goodwin
   Mozilla

   Email: mgoodwin@mozilla.com
   URI:   https://www.computerist.org/

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