First-Party Sets and SameSite Cookies
draft-west-cookie-samesite-firstparty-latest

Abstract

This document proposes the addition of two new values to the SameSite cookie attribute defined in RFC6265bis [I-D.ietf-httpbis-rfc6265bis]: FirstPartyLax and FirstPartyStrict. These values are conceptually similar to the existing Lax and Strict values, but base the delivery checks on the First-Party Sets [first-party-set] of a request’s initiator and target, rather than on their respective registrable domains. This widens the scope of a given cookie’s applicability, enabling entities that have sharded themselves across multiple registrable domains to maintain HTTP state without exposing themselves to the risks of SameSite=None.

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This Internet-Draft will expire on November 8, 2019.

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Table of Contents

• 1. Introduction
• 2. Conventions and Definitions
• 2.1. Conformance
• 2.2. Syntax
• 2.3. Terms
• 3. The “FirstParty” value of the “SameSite” attribute
• 4. Security and Privacy Considerations
• 4.1. CSRF
• 4.2. Secure Transport
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Acknowledgments
• Author's Address

1. Introduction

The SameSite attribute enables developers to limit the scope of a given cookie’s delivery, mitigating the risks of some classes of cross-site request forgery (CSRF) attack by preventing certain cookies from being delivered along with requests that are initiated from a cross-site context.

For example, consider the exciting and dynamic https://internet-bookstore.example/, which uses SameSite=Lax cookies as one layer in its defense against CSRF attack. If https://example.com includes resources from https://internet-bookstore.example/, the request will be considered cross-site, and the authentication cookies will not be delivered. Without that state, CSRF attacks will be significantly less effective.

When the site expands into new locations, it may wish to register a domain under a localized TLD, perhaps https://internet-bookstore.测试/. Likewise, it may decide to shard itself into distinct brands, like https://internet-things-other-than-books-store.example/. Though the same entity controls each of these origins, they have distict registrable domains, and therefore the authentication cookie noted above will not be delivered from one site to resources on another. This frustrates a number of reasonable use cases, including single-sign on. Today, SameSite=None is necessary in order to support these use cases by enabling a given cookie to be delivered across registrable domains. SameSite=None, unfortunately, exposes the site to more risk than it would prefer, as it removes a layer of CSRF defense.

First-Party Sets [first-party-set] proposes a mechanism by which developers can bind each of their distinct registrable domains into a set which mutually agrees to be treated as a single entity. It would be helpful if this concept could be folded into the SameSite attribute, perhaps via new FirstPartyLax and FirstPartyStrict values. These could be conceptually similar to the existing Lax and Strict values, but base their delivery checks on the First-Party Sets of a given request’s initiator and target, rather than on their respective registrable domains.

This document spells out that proposal in a bit more detail.

2. Conventions and Definitions

2.1. Conformance

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2.2. Syntax

This document adjusts some syntax from [I-D.ietf-httpbis-rfc6265bis], and in doing so, relies upon the Augmented Backus-Naur Form (ABNF) notation of [RFC5234].

2.3. Terms

HTTP requests are considered “same-site” or “cross-site”, as defined in [I-D.ietf-httpbis-rfc6265bis].

First-Party Sets are defined in [first-party-set]. Two origins (“A” and “B”) are said to be in the same first-party set if the first-party set associated with “A” contains “B”.

A request is considered to be “first-party” if the target origin is in the same first-party set as the request’s initiator, and “third-party” otherwise. That is, for a given request (“r”), the following algorithm returns first-party or third-party:

1. If request is “same-site”, return “first-party”.
2. Let target be r’s current URL’s origin.
3. If target is in the same first-party set as r’s client’s origin, return first-party.
4. Return third-party.

A document is considered “first-party with its ancestors” if its origin is in the same First-Party Set with the origins of each of the document’s ancestors [HTML]. That is, for a given document (“d”), the following algorithm returns first-party or third-party:

1. If d’s browsing context is a top-level browsing context, return first-party.
2. Let set be d’s origin’s First-Party Set.
3. For each ancestor in d’s browsing context’s ancestor browsing contexts:
   1. If ancestor’s active document’s origin is not contained within set, return third-party.
4. Return first-party.

ISSUE: Move these definitions to the First-Party Sets spec, when one exists.

3. The “FirstParty” value of the “SameSite” attribute

[I-D.ietf-httpbis-rfc6265bis] defines three values for cookies’ SameSite attribute: None, which enables delivery for same-site and cross-site requests; Strict, which enables delivery only for same-site requests; and Lax, which enables delivery for same-site requests as well as for cross-site top-level navigations.

In the presence of first-party sets, it makes sense to extend this syntax a bit to include FirstParty, which will allow delivery of cookies within a first-party set, and therefore will support the use cases that first-party sets addresses (a given first-party’s single sign-on, for instance). For example, given two distinct origins https://sso.example/ and https://application.example/ that are contained in the same first-party set:

• Requests from https://application.example/ to https://sso.example/ may not contain any cookies set with SameSite=Lax or SameSite=Strict, but only those set as SameSite=None.
• Requests from https://application.example/ to https://sso.example/ may contain any cookies set with SameSite=FirstParty or SameSite=None.

To implement this change, adjust [I-D.ietf-httpbis-rfc6265bis] as follows:

First, change the samesite-value definition from:

samesite-value    = "Strict" / "Lax" / "None"

to:

samesite-value    = "Lax" / "Strict" / "FirstPartyLax" / "FirstPartyStrict" / "None"

Second, alter the SameSite attribute’s processing algorithm (Section 5.3.7 of [I-D.ietf-httpbis-rfc6265bis]) to add a new step 4 and 5:

4.  If cookie-av's attribute-value is a case-insensitive
    match for "FirstPartyLax", set `enforcement` to "FirstPartyLax".

5.  If cookie-av's attribute-value is a case-insensitive
    match for "FirstPartyStrict", set `enforcement` to "FirstPartyStrict".

Third, alter the cookie storage model (Section 5.4 of [I-D.ietf-httpbis-rfc6265bis]) as follows:

Change step 14.1 from:

1.  If the cookie was received from a "non-HTTP" API, and
    the API was called from a context whose "site for
    cookies" is not an exact match for request-uri's host's
    registered domain, then abort these steps and ignore the
    newly created cookie entirely.

to:

1.  If the cookie was received from a "non-HTTP" API:

    1.  If the cookie's `same-site-flag` is "Lax" or
        "Strict", and the API was called from a context
        whose "site for cookies" is not an exact match for
        request-uri's host's registered domain, then abort
        these steps and ignore the newly-created cookie
        entirely.

    2.  If the cookie's same-site-flag` is "FirstPartyLax"
        or "FirstPartyStrict", and the API was called from
        a context that is not first-party with its
        ancestors, then abort these steps and ignore the
        newly-created cookie entirely.

Change step 14.2 from:

2.  If the cookie was received from a "same-site" request,
    skip the remaining substeps and continue processing the
    cookie.

to:

2.  If the cookie's `same-site-flag` is "Lax" or "Strict",
    and the cookie was received from a "same-site" request,
    then skip the remaining substeps and continue processing
    the cookie.

Add a new step 14.3 after the new step 14.2:

3.  If the cookie's `same-site-flag` is "FirstPartyLax" or
    "FirstPartyStrict", and the cookie was received from a
    "first-party" request, then skip the remaining substeps
    and continue processing the cookie.

Fourth, alter the last conditional in step 1 if the Cookie header algorithm (Section 5.5 of [I-D.ietf-httpbis-rfc6265bis]) from:

*   If the cookie's same-site-flag is not "None", and the
    HTTP request is cross-site then exclude the cookie
    unless all of the following statements hold:
    
    1.  The same-site-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.

to:

*   If the HTTP request is cross-site, then exclude the
    cookie unless one of the following statements holds:

    1.  The cookie's `same-site-flag` is "None".

    2.  The cookie's `same-site-flag` is either "Lax" or
        "FirstPartyLax", the HTTP request's method is
        "safe", and the HTTP request's target browsing
        context is a top-level browsing context.

    3.  The cookie's `same-site-flag` is either
        "FirstPartyLax" or "FirstPartyStrict", and the HTTP
        request is a first-party request.

4. Security and Privacy Considerations

4.1. CSRF

Both FirstPartyLax and FirstPartyStrict provide weaker defenses against CSRF than their Lax and Strict counterparts, as they enable authenticated requests from a larger set of initiating contexts. That said, they also provide deployment benefits, as they’re usable in some contexts where Lax and Strict would be too restrictive (e.g. the localized registrable domains in the introduction).

4.2. Secure Transport

First-Party Sets can only be created for secure origins, as unauthenticated transport doesn’t give any guarantees that the assertions we use to build the set are in fact being delivered by the entity which controls the server. This has the side-effect of ensuring that FirstPartyLax and FirstPartyStrict cookies can only be delivered to secure cross-site origins, which has the exciting side effect of providing limited mitigation of monitoring by network attackers [RFC7258].

5. IANA Considerations

This document has no IANA actions.

6. References

6.1. Normative References

6.2. Informative References

Acknowledgments

Conversations with a number of folks at 2019’s HTTP Workshop helped me clarify my thinking around the incremental improvements we can make to cookies. In particular, Martin Thomson and Anne van Kesteren provided insightful feedback.

Author's Address