First-Party Cookies
draft-west-first-party-cookies-00

HTTPbis                                                          M. West
Internet-Draft                                               Google, Inc
Updates: 6265 (if approved)                             October 27, 2014
Intended status: Standards Track
Expires: April 30, 2015

                          First-Party Cookies
                   draft-west-first-party-cookies-00

Abstract

   This document updates RFC6265, defining the "First-Party" attribute
   for cookies, which allows servers to mitigate the risk of cross-site
   request forgery and related information leakage attacks by asserting
   that a particular cookie should only be sent in a "first-party"
   context.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 30, 2015.

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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.  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" Attribute (Non-Normative)    4
   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
     6.1.  User Controls . . . . . . . . . . . . . . . . . . . . . .   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 cookies"
   which should be attached to requests if and only if they occur in a
   first-party context.

   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"
   attribute will simply store a non-first-party cookie, just as they do
   today.

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1.1.  Examples

   First-party cookies are set via the "First-Party" 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

   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 who they're talking to.

   Broadly speaking, then, a "first-party" request is 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 slightly more precise, given an HTTP request "request":

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

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   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" attribute.

3.1.  Grammar

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

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

   The "First-Party" 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 cookies if

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   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"
   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", the user agent MUST append an attribute to the "cookie-
   attribute-list" with an "attribute-name" of "First-Party" 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 "firstparty-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", set the cookie's "first-
           party-flag" to true.  Otherwise, set the cookie's "first-
           party-flag" to false.

       2.  If the cookie's "first-party-flag" is set to true, and the
           request which generated the cookie is not a first-party
           request (as defined in Section 2.1), 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 "first-party-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" 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 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 cookies.

6.  Privacy Considerations

6.1.  User Controls

   First-party 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.

   User agents, however, could offer users the ability to toggle a
   cookie's "first-party-flag" themselves, perhaps as part of a more
   general cookie management interface.  This could provide an
   interesting middle-ground between the options (e.g.  "Block all",

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   "Block third-party", and "Allow all") that many user agents offer to
   users.

7.  Security Considerations

7.1.  Limitations

   It is possible to bypass the protection that first-party 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 first-
   party 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 doing 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, 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 similar to (but
   stricter than) 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/>.

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

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   [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/

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