HTTPbis                                                          M. West
Internet-Draft                                               Google, Inc
Updates: 6265 (if approved)                            February 20, 2015
Intended status: Standards Track
Expires: August 24, 2015


                        First-Party-Only Cookies
                   draft-west-first-party-cookies-01

Abstract

   This document updates RFC6265 by defining a "First-Party-Only"
   attribute which allows servers to assert that a cookie ought to be
   sent only in a "first-party" context.  This assertion allows user
   agents to mitigate the risk of cross-site request forgery attacks,
   and other related paths to cross-origin information leakage.

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/.

   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 August 24, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://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 August 24, 2015                [Page 1]

Internet-Draft             first-party-cookies             February 2015


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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and notation  . . . . . . . . . . . . . . . . . .   3
     2.1.  First-party and Third-party Requests  . . . . . . . . . .   3
   3.  Server Requirements . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Semantics of the "First-Party-Only" Attribute (Non-
           Normative)  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  User Agent Requirements . . . . . . . . . . . . . . . . . . .   5
     4.1.  The "First-Party" attribute . . . . . . . . . . . . . . .   5
     4.2.  Monkey-patching the Storage Model . . . . . . . . . . . .   5
     4.3.  Monkey-patching the "Cookie" header . . . . . . . . . . .   6
   5.  Authoring Considerations  . . . . . . . . . . . . . . . . . .   6
     5.1.  Mashups and Widgets . . . . . . . . . . . . . . . . . . .   6
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     7.1.  Limitations . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 "First-party-only",
   meaning they should be attached to requests if and only if those
   requests occur in a first-party context.  We define "first-party
   context" in terms of a user agent's top-level browsing context, which
   is the only security context a user can reasonably be expected to
   understand.

   Note that the mechanism outlined here is backwards compatible with
   the existing cookie syntax.  Servers may serve first-party cookies to
   all user agents; those that do not support the "First-Party-Only"



West                     Expires August 24, 2015                [Page 2]

Internet-Draft             first-party-cookies             February 2015


   attribute will simply store a cookie which is returned in all
   applicable contexts, just as they do today.

1.1.  Examples

   First-party-only cookies are set via the "First-Party-Only" 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; First-Party-Only

   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.

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", and "top-level browsing context" are
   defined in the HTML Living Standard.  [HTML]

   The term "origin" and the mechanism of deriving an origin from a URI
   are defined in [RFC6454].

2.1.  First-party and Third-party Requests

   The URL 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.

   With that in mind, we define a "first-party" request as an HTTP
   request for a resource whose URL's origin matches the origin of the
   URL the user sees in the address bar.  A "third-party" request is an
   HTTP request for a resource at any other origin.

   To be more precise, given an HTTP request "request":




West                     Expires August 24, 2015                [Page 3]

Internet-Draft             first-party-cookies             February 2015


   1.  Let "context" be the top-level browsing context in the window
       responsible for "request".

   2.  Let "top-origin" be the origin of the location of the active
       document in "context".

   3.  If the origin of "request"'s URL is the same as "top-origin",
       "request" is a *first-party request*. Otherwise, "request" is a
       *third-party request*.

   Note that we deal with the document's location in step 2 above, not
   with the document's origin.  For example, a top-level document from
   "https://example.com" which has been sandboxed into a unique origin
   still creates a non-unique first-party context for subsequent
   requests.

   This definition has a few implications:

   o  New windows create new first-party contexts.

   o  Full-page navigations create new first-party contexts.  Notably,
      this includes both HTTP and "<meta>"-driven redirects.

   o  "<iframe>"s do not create new first-party contexts; their requests
      MUST be considered in the context of the origin of the URL the
      user actually sees in the user agent's address bar.

3.  Server Requirements

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

3.1.  Grammar

   Add "First-Party-Only" 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 /
                         first-party-only-av / extension-av
   first-party-only-av = "First-Party-Only"








West                     Expires August 24, 2015                [Page 4]

Internet-Draft             first-party-cookies             February 2015


3.2.  Semantics of the "First-Party-Only" Attribute (Non-Normative)

   The "First-Party-Only" attribute limits the scope of the cookie such
   that it will only be attached to requests if those requests are
   "first-party", as described in Section 2.1.  For example, requests
   for "https://example.com/sekrit-image" will attach first-party-only
   cookies if and only if the top-level browsing context is currently
   displaying a document from "https://example.com".

   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 "First-Party-Only"
   attribute.

4.1.  The "First-Party" 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 "First-
   Party-Only", the user agent MUST append an attribute to the "cookie-
   attribute-list" with an "attribute-name" of "First-Party-Only" and an
   empty "attribute-value".

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 "first-party-only-flag" to the list of fields stored for each
       cookie.

   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 "First-Party-Only", set the cookie's
           "first-party-only-flag" to true.  Otherwise, set the cookie's
           "first-party-only-flag" to false.

       2.  If the cookie's "first-party-only-flag" is set to true, and
           the request which generated the cookie is not a first-party




West                     Expires August 24, 2015                [Page 5]

Internet-Draft             first-party-cookies             February 2015


           request (as defined in Section 2.1), then abort these steps
           and ignore the newly created cookie entirely.

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 "first-party-only-flag" is true, then exclude
          the cookie if the HTTP request is a third-party request (see
          Section 2.1).

   Note that the modifications suggested here concern themselves only
   with the origin of the top-level browsing context and the origin of
   the resource being requested.  The cookie's "domain", "path", and
   "secure" attributes do not come into play for this comparison.

5.  Authoring Considerations

5.1.  Mashups and Widgets

   The "First-Party-Only" attribute is inappropriate for some important
   use-cases.  In particular, note that content intended for embedding
   in a third-party context (social networking widgets or commenting
   services, for instance) will not have access to first-party-only
   cookies.  Non-first-party 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 third-party context; these mechanisms will not function as
   intended with first-party-only cookies.

6.  Privacy Considerations

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




West                     Expires August 24, 2015                [Page 6]

Internet-Draft             first-party-cookies             February 2015


   We recommend, therefore, that servers interested in reducing the
   ambient authority of requests generated in a third-party context use
   such identification mechanisms only in addition to first-party-only
   cookies, and not as a replacement for them.

7.  Security Considerations

7.1.  Limitations

   It is possible to bypass the protection that first-party-only cookies
   offer against cross-site request forgery attacks by creating first-
   party contexts in which to execute the attack.  Consider, for
   instance, the URL "https://example.com/logout" which logs the current
   user out of "example.com".  If the user's session cookie is a first-
   party-only cookie, then embedding the logout URL in an "<iframe>"
   element or an "<img>" element won't log her out, as the cookie won't
   be sent.  Popping up a new window, or triggering a top-level
   navigation, on the other hand, will create a first-party context,
   attach cookies, and perform the logout.

   Note, though, that popping up a window, or doing a top-level
   navigation are both significantly more visible to the user than
   loading a subresource.  Users will at least have the opportunity to
   notice that something strange is going on, which hopefully reduces an
   attacker's ability to perform untargeted attacks.

   Further, note that certain kinds of attacks are no longer possible if
   a first-party context is required.  Information leakage attacks which
   rely on visible side-effects of loading a session-protected image,
   for example, can no longer access those side-effects if the image is
   loaded in a new window.  Timing attacks like those Paul Stone
   outlines in [pixel-perfect] are no longer possible if the session
   cookie is first-party-only, as they rely on "<iframes>" to contain
   the protected content in a way the attacker can manipulate.

8.  Acknowledgements

   The first-party cookie concept documented here is indebited to Mark
   Goodwin's and Joe Walker's [samedomain-cookies].

9.  References

9.1.  Normative References

   [HTML]     Hickson, I., "HTML Living Standard", n.d.,
              <https://html.spec.whatwg.org/>.





West                     Expires August 24, 2015                [Page 7]

Internet-Draft             first-party-cookies             February 2015


   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4790]  Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
              Application Protocol Collation Registry", RFC 4790, March
              2007.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454, December
              2011.

9.2.  Informative References

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

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

Author's Address

   Mike West
   Google, Inc

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
















West                     Expires August 24, 2015                [Page 8]