First-Party-Only Cookies
draft-west-first-party-cookies-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Author | Mike West | ||
| Last updated | 2015-02-20 | ||
| Replaced by | draft-ietf-httpbis-cookie-same-site | ||
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draft-west-first-party-cookies-01
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
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and may be updated, replaced, or obsoleted by other documents at any
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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
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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-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"
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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":
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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"
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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
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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.
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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/>.
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[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/
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