HTTPbis M. West
Internet-Draft Google, Inc
Updates: 6265 (if approved) April 15, 2015
Intended status: Standards Track
Expires: October 17, 2015
First-Party-Only Cookies
draft-west-first-party-cookies-02
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-origin information leakage, and
provides some minimal protection against cross-site request forgery
attacks.
Status of This Memo
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This Internet-Draft will expire on October 17, 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. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Limitations . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and notation . . . . . . . . . . . . . . . . . . 4
2.1. First-party and Third-party Requests . . . . . . . . . . 5
2.1.1. Document-based requests . . . . . . . . . . . . . . . 5
2.1.2. Worker-based requests . . . . . . . . . . . . . . . . 6
3. Server Requirements . . . . . . . . . . . . . . . . . . . . . 8
3.1. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Semantics of the "First-Party-Only" Attribute (Non-
Normative) . . . . . . . . . . . . . . . . . . . . . . . 8
4. User Agent Requirements . . . . . . . . . . . . . . . . . . . 8
4.1. The "First-Party" attribute . . . . . . . . . . . . . . . 8
4.2. Monkey-patching the Storage Model . . . . . . . . . . . . 9
4.3. Monkey-patching the "Cookie" header . . . . . . . . . . . 9
5. Authoring Considerations . . . . . . . . . . . . . . . . . . 10
5.1. Mashups and Widgets . . . . . . . . . . . . . . . . . . . 10
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
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 (as defined in section 2.1).
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. Goals
These first-party-only 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 "First-Party-Only" attribute on
authentication cookies. The attacker will only be able to embed
unauthenticated resources, as embedding mechanisms such as
"<iframe>" will not create first-party contexts.
2. Cross-site script inclusion (XSSI) attacks are likewise mitigated
by setting the "First-Party-Only" attribute on authentication
cookies. The attacker will not be able to include authenticated
resources via "<script>" or "<link>", as these embedding
mechanisms will not create first-party contexts.
First-party-only cookies also mitigate one specific kind of cross-
site request forgery (CSRF) attack by treating cross-origin "POST"
requests (including navigations) as as third-party requests.
Aside from these attack mitigations, first-party-only cookies can
also be useful for policy or regulatory purposes. That is, it may be
valuable for an origin to assert that its cookies should not be sent
along with third-party requests in order to limit its exposure to
non-technical risk.
1.2. Limitations
First-party-only cookies provide limited defense against one kind of
naive cross-site request forgery attack (CSRF). It does 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 first-party context (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 first-party contexts without the risk of user
detection.
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In addition to the usual server-side defenses (CSRF tokens, etc),
client-side techniques such as those described in [app-isolation] may
prove effective against CSRF, and are certainly worth exploring in
combination with first-party-only cookies. First-party-only cookies
on their own, however, are not a substantial barrier to CSRF attacks.
1.3. 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", "ancestor browsing context", "browsing
context", "document", "iframe srcdoc document", "parent browsing
context", and "top-level browsing context" are defined in the HTML
Living Standard [HTML].
"Web Workers", "dedicated workers", "owner document", "list of
relevant documents", and "shared workers" are defined in the Web
Workers specification [WORKERS].
"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].
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2.1. First-party and Third-party Requests
2.1.1. Document-based requests
When considering a request generated while parsing a document, or
executing script in its context, we need to consider the document's
origin as well as the origin of each of it's ancestors, in order to
determine whether the request should be considered "first-party".
For this kind of request, 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 origin of that
URI is, therefore, the "first-party origin".
In order to prevent the kinds of "multiple-nested scenarios"
described in Section 4 of [RFC7034], we must check the first-party
origin against the origins of each of a document's ancestor browsing
contexts' active documents. A document is considered a "first-party
context" if and only if the origin of its URI is the same as the
first-party origin, *and* if each of the active documents in its
ancestors' browsing contexts' is a first-party context.
This definition has a few implications:
o New windows create new first-party contexts (as the active
document is rendered into a top-level browsing context).
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.
To be more precise, given an HTTP request "request", the following
algorithm returns "First-Party" if "request" is a first-party
request, and "Third-Party" otherwise:
1. Let "document" be the document responsible for "request".
2. If "document" is a first-party context, and "request"'s URI's
origin is the same as the origin of the URI of the active
document in the top-level browsing context of "document", then
return "First-Party".
3. Return "Third-Party".
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Given a Document "document", the following algorithm returns "First-
Party" if "document" is a first-party context, and "Third-Party"
otherwise:
1. Let "top-origin" be the origin of the URI of the active document
in the top-level browsing context of the document responsible for
"request".
2. If "document"'s URI's origin is not "top-origin", return "Third-
Party".
3. While "document" has a parent browsing context:
1. Let "document" be "document"'s parent browsing context's
active document.
2. If "document"'s URI's origin is not "top-origin" and
"document" is not an iframe srcdoc document, return "Third-
Party".
4. Return "First-Party".
Note that we deal with the document's location in steps 2, 3, and 4.2
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.
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 Workers
Dedicated workers are fairly straightforward to categorize, as each
dedicated worker is bound to one and only one Document. Requests
generated from a dedicated worker (via "importScripts",
"XMLHttpRequest", "fetch()", and so on) are first-party requests if
and only if the worker's owner document is a first-party context.
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To be more precise, given an HTTP request "request", the following
algorithm returns "First-Party" if "request" is a first-party
request, and "Third-Party" otherwise:
1. Let "worker" be the dedicated worker responsible for "request".
2. Let "document" be "worker"'s owner document.
3. If "document" is a first-party context, and "request"'s URI's
origin is the same as the origin of the URI of the active
document in the top-level browsing context of "document", then
return "First-Party".
4. Return "Third-Party".
2.1.2.2. Shared Workers
Shared Workers introduce the complexity of bindings to multiple
Documents. As it is quite possible for a shared worker to be bound
at the same time to one Document that is a first-party context, and
another that isn't, we'll need to walk through all the documents
associated with the worker to determine its status. If and only if
all associated documents are first-party contexts, then the worker is
a first-party context.
To be more precise, given an HTTP request "request", the following
algorithm returns "First-Party" if "request" is a first-party
request, and "Third-Party" otherwise:
1. Let "worker" be the dedicated worker responsible for "request".
2. For each "document" in "worker"'s list of relevant Documents:
1. Return "Third-Party" if "document" is not a first-party
context (as defined in section 2.1.1).
2. Return "Third-Party" if "request"'s URI's origin is not the
same as the origin of the URI of the active document in the
top-level browsing context of "document".
3. Return "First-Party".
2.1.2.3. Service Workers
Service Workers are more complex still, as they act as a completely
separate execution context, with very little relationship to the
Document which registered them.
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Until we have more implementation experience, we will consider
Service Workers as third-party contexts in all cases.
Note: Requests which simply pass through a service worker will be
handled as described above; the only requests which will be effected
by this categorization are those which the service worker itself
initiates (via "fetch()", for instance).
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"
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]:
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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
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 requirements 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).
* If the cookie's "first-party-only-flag" is true, then exclude
the cookie if the HTTP request's method is not "GET" and the
origin of the document which originated the request is not the
same as the origin of the HTTP request's URI.
Note that the modifications suggested here concern themselves only
with the origins of ancestor browsing contexts and the origin of the
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resource being requested. The cookie's "domain", "path", and
"secure" attributes do not come into play for these comparisons.
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.
7. References
7.1. 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.
[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.
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[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
[RFC7034] Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
Options", RFC 7034, October 2013.
[SERVICE-WORKERS]
Russell, A., Song, J., and J. Archibald, "Service
Workers", n.d., <http://www.w3.org/TR/service-workers/>.
[WORKERS] Hickson, I., "Web Workers", n.d.,
<http://www.w3.org/TR/workers/>.
7.2. Informative References
[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>.
The first-party 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.
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Author's Address
Mike West
Google, Inc
Email: mkwst@google.com
URI: https://mikewest.org/
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