First-Party Sets

This document proposes a new web platform mechanism to declare a collection of related domains as being in a First-Party Set.

A Work Item of the Privacy Community Group.

Editors:

• Kaustubha Govind, Google
• David Benjamin, Google

Participate

• https://github.com/privacycg/first-party-sets/issues

Table of Contents

• Introduction
• Goals
• Non-goals
• Declaring a First Party Set
• Discovering First Party Sets
• Applications
• Design details
  • UA Policy
    • Defining acceptable sets
    • Static lists
    • Signed assertions
    • Open questions
    • Administrative controls
  • Cross-site tracking vectors
  • Service workers
  • UI Treatment
• Alternative designs
  • Origins instead of registrable domains
• Prior Art

Introduction

Browsers have proposed a variety of tracking policies and privacy models (Chromium, Edge, Mozilla, WebKit) which scope access to user identity to some notion of first-party. In defining this scope, we must balance two goals: the scope should be small enough to meet the user's privacy expectations, yet large enough to provide the user's desired functionality on the site they are interacting with.

One natural scope is the domain name in the top-level origin. However, the website the user is interacting with may be deployed across multiple domain names. For example, https://google.com, https://google.co.uk, and https://youtube.com are owned by the same entity, as are https://apple.com and https://icloud.com, or https://amazon.com and https://amazon.de.

We may wish to allow user identity to span related origins, where consistent with privacy requirements. For example, Firefox ships an entity list that defines lists of domains belonging to the same organization. This explainer discusses a mechanism to allow organizations to each declare their own list of domains, which is then accepted by a browser if the set conforms to its policy.

Goals

• Allow related domain names to declare themselves as the same first-party.
• Define a framework for browser policy on which declared names will be treated as the same site in privacy mechanisms.

Non-goals

• Third-party sign-in between unrelated sites.
• Information exchange between unrelated sites for ad targeting or conversion measurement.
• Other use cases which involve unrelated sites.

(Some of these use cases are covered by other explainers from the Privacy Sandbox.)

Declaring a First Party Set

A first-party set is identified by one owner registered domain and a list of secondary registered domains. (See alternative designs for a discussion of origins vs registered domains.)

An origin is in the first-party set if:

• Its scheme is https; and
• Its registered domain is either the owner or is one of the secondary domains.

The browser will consider domains to be members of a set if the domains opt in and the set meets UA policy, to incorporate both user and site needs. Domains opt in by hosting a JSON manifest at https://<domain>/.well-known/first-party-set. The secondary domains point to the owning domain while the owning domain lists the members of the set, a version number to trigger updates, and a set of signed assertions to inform UA policy (details below).

Suppose a.example, b.example, and c.example wish to form a first-party set, owned by a.example. The sites would then serve the following resources:

https://a.example/.well-known/first-party-set
{
  "owner": "a.example",
  "version": 1,
  "members": ["b.example", "c.example"],
  "assertions": { 
    "chrome-fps-v1" : "<base64 contents...>",
    "firefox-fps-v1" : "<base64 contents...>",
    "safari-fps-v1": "<base64 contents...>"
  }
}

https://b.example/.well-known/first-party-set
{ "owner": "a.example" }

https://c.example/.well-known/first-party-set
{ "owner": "a.example" }

The browser then imposes additional constraints on the owner's manifest:

• Entries in members that are not registrable domains are ignored.
• Only entries in members that meet UA policy will be accepted. The others will be ignored. If the owner is not covered by UA policy, the entire set is rejected.

Discovering First Party Sets

By default, every registrable domain is implicitly owned by itself. The browser discovers first-party sets as it makes network requests and stores the first-party set owner for each domain. On a top-level navigation, websites may send a Sec-First-Party-Set response header to inform the browser of its first-party set owner. For example https://b.example/some/page may send the following header:

  Sec-First-Party-Set: owner="a.example", minVersion=1

If this header does not match the browser's current information for b.example (either the owner does not match, or its saved first-party set manifest is too old), the browser pauses navigation to fetch the two manifest resources. Here, it would fetch https://a.example/.well-known/first-party-set and https://b.example/.well-known/first-party-set.

These requests must be uncredentialed and with suitably partitioned network caches to not leak cross-site information. In particular, the fetch must not share caches with browsing activity under a.example. See also discussion on cross-site tracking vectors.

If the manifests show the domain is in the set, the browser records a.example as the owner of b.example (but not c.example) in its first-party-set storage. It evicts all domains currently recorded as owned by a.example that no longer match the new manifest. Then it clears all state for domains whose owners changed, including reloading all active documents. This should behave like Clear-Site-Data: *. This is needed to unlink any site identities that should no longer be linked. Note this also means that execution contexts (documents, workers, etc.) are scoped to a particular first-party set throughout their lifetime. If the first-party owner changes, existing ones are destroyed.

The browser then retries the request (state has since been cleared) and completes navigation. As retrying POSTs is undesirable, we should ignore the Sec-First-Party-Set header directives on POST navigations. Sites that require a first-party set to be picked up on POST navigations should perform a redirect (as is already common), and have the Sec-First-Party-Set directive apply on the redirect.

Subresource requests and subframe navigations are simpler as they cannot introduce a new first-party context. If the request matches the first-party URL's owner's manifest but is not currently recorded as being in that first-party set, the browser validates membership as above before making the request. Any Sec-First-Party-Set headers are ignored and, in particular, the browser should never read or write state for a first-party set other than the current one. This simpler process also avoids questions of retrying requests. The minVersion parameter in the header ensures that the browser's view of the owner's manifest is up-to-date enough for this logic.

Applications

In support of the various browser privacy models, web platform features can use first-party sets to determine whether embedded content may or may not access its own state. For instance, sites may annotate individual cookies to be sent across same-party, cross-domain contexts by using the proposed SameParty cookie attribute. It may also be reasonable to use first-party sets to partition network caches, in cases where the tighter origin-based isolation is too expensive.

Web platform features should not use first-party sets to make one origin's state directly accessible to another origin in the set. First-party sets should only control when embedded content can access its own state. That is, if a.example and b.example are in the same first-party set, the same-origin policy should still prevent https://a.example from accessing https://b.example's IndexedDB databases. However, it may be reasonable to allow a https://b.example iframe within https://a.example to access the https://b.example databases.

Design details

UA Policy

Defining acceptable sets

We should have some notion of what sets are acceptable or unacceptable. For instance, a set containing the entire web, or a collection of completely unrelated sites, seems clearly unacceptable. Conversely, a set containing https://acme-corp-landing-page.example and https://acme-corp-online-store.example seems reasonable. There is a wide spectrum between these two scenarios. We should define where to draw the line.

Browsers implementing First-Party Sets will specify UA policy for which domains may be in the same set. While not required, it is desirable to have some consistency across UA policies. For a set of guiding principles in defining UA policy, we can look to how the various browser proposals describe first parties (emphasis added):

• A Potential Privacy Model for the Web (Chromium Privacy Sandbox): "The notion of "First Party" may expand beyond eTLD+1, e.g. as proposed in First Party Sets. It is reasonable for the browser to relax its identity-sharing controls within that expanded notion, provided that the resulting identity scope is not too large and can be understood by the user."
• Edge Tracking Protection Preview: "Not all organizations do business on the internet using just one domain name. In order to help keep sites working smoothly, we group domains owned and operated by the same organization together."
• Mozilla Anti-Tracking Policy: "A first party is a resource or a set of resources on the web operated by the same organization, which is both easily discoverable by the user and with which the user intends to interact."
• WebKit Tracking Prevention Policy: "A first party is a website that a user is intentionally and knowingly visiting, as displayed by the URL field of the browser, and the set of resources on the web operated by the same organization." and, under "Unintended Impact", "Single sign-on to multiple websites controlled by the same organization."

We expect the UA policies to evolve over time as use cases and abuse scenarios come up. For instance, otherwise unrelated sites forming a consortium in order to expand the scope of their site identities would be considered abuse.

Given the UA policy, policy decisions must be delivered to the user’s browser. This can use either static lists or signed assertions. Note first-party set membership requires being listed in the manifest in addition to meeting UA policy. This allows sites to quickly remove domains from their first-party set.

Static lists

The browser vendor could maintain a list of domains which meet its UA policy, and ship it in the browser. This is analogous to the list of domains owned by the same entity used by Edge and Firefox to control cross-site tracking mitigations.

A browser using such a list would then intersect first-party set manifests with the list. It would ignore the assertions field in the manifest. Note fetching the manifest is still necessary to ensure the site opts into being a set. This avoids problems if, say, a domain was transferred to another entity and the static list is out of date.

Static lists are easy to reason about and easy for others to inspect. At the same time, they can develop deployment and scalability issues. Changes to the list must be pushed to each user's browser via some update mechanism. This complicates sites' ability to deploy new related domains, particularly in markets where network connectivity limits update frequency. They also scale poorly if the list gets too large.

Signed assertions

Alternatively, the browser vendor, or some entities it designates, can sign assertions for domains which meet UA policy, using some private key. A signed assertion has the same meaning as membership in a static list: these domains meet the signer’s policy. The browser would trust the signers’ public key and, as above, only accept domains covered by suitable assertions.

Assertions are delivered in the assertions field, which contains a dictionary mapping from signer name to signed assertion. Browsers ignore unused assertions. This format allows sites to serve assertions from multiple signers, so they can handle policy variations more smoothly. In particular, we expect policies to evolve over time, so browser vendors may wish to run their own signers. Note these assertions solve a different problem from the Web PKI and are delivered differently. However, many of the lessons are analogous.

As with a static list, signers maintain a full list of currently checked domains. They should publish this list at a well-known location, such as https://fps-signer.example/first-party-sets.json. Although browsers will not consume the list directly, this allows others to audit the list. The signer may wish to incorporate a Certificate-Transparency-like mechanism for stronger guarantees.

The signer then regularly produces fresh signed assertions for the current list state. For extensibility, the exact format and contents of this assertion are signer-specific (browsers completely ignore unknown signers, so there is no need for a common format). However, there should be a recommended format to avoid common mistakes. Each signed assertion must contain:

• The domains that have been checked against the signer’s policy
• An expiration time for the signature
• A signature over the above, made by the signer’s private key

Assertion lifetimes should be kept short, say two weeks. This reduces the lifetime of any mistakes. The browser vendor may also maintain a blocklist of revoked assertions to react more quickly, but the reduced lifetime reduces the size of such a list.

To avoid operational challenges for sites, the signer makes the latest assertions available at a well-known location, such as https://fps-signer.example/assertions/<owner-domain>. We will provide automated tooling to refresh the manifest from these assertions, and sites with more specialized needs can build their own. To support such automation, the URL patterns must be standard across signers.

Note any duplicate domains in the assertions and members attribute should compress well with gzip.

Open questions

• Should the recommended format include intermediate certificates? X.509 certificates are typically issued from a shorter-lived intermediate certificate signed by the root. This allows keeping the more sensitive root key offline.
• Should the recommended format include extensions? Too many extension points, particularly around cryptographic algorithms, can introduce complexity and security risks.
• Should the assertions treat the owner as distinct from the member domains, or is a flat list sufficient? That is, is the signer’s policy likely to treat the owner distinct from other members?

Extensibility by signer names means formats can always be extended by updating browsers to expect new signers. But we must ensure that this does not increase operational burden on sites by designing the tooling correctly. For instance, the chrome-fps-v1 and chrome-fps-v2 signers could share an assertion URL, which provides a set of assertions. The tooling would then automatically include each in the manifest.

Administrative controls

For enterprise usages, browsers typically offer administrators options to control web platform behavior. UA policy is unlikely to cover private domains, so browsers might expose administrative options for locally-defined first-party sets.

Cross-site tracking vectors

This design requires the browser remember state about first-party sets, and use that state to influence site behavior. We must ensure this state does not introduce a cross-site tracking vector for two sites not in the same first-party set. For instance, a site may be able to somehow encode a user identifier into the first-party set and have that identifier be readable in another site. Additionally, first-party sets are discovered and validated on-demand, so this could leak information about which sites have been visited.

Our primary mitigation for these attacks is to treat first-party sets as first-party-only state. We heavily restrict how first-party set state interacts with subresources. Thus we never query or write to first-party set information for any set other than the current one. Even if first-party set membership were personalized, that membership should only influence the set itself.

We can further mitigate personalized first-party sets, as well as information leaks during validation, by fetching manifests without credentials and from appropriate network partitions (double-keyed HTTP cache, etc.).

Finally, first-party set state must be cleared whenever other state for some first-party is cleared, such as if the user cleared cookies from the browser UI.

Some additional scenarios to keep in mind:

• The decision to validate a first-party set must not be based on not-yet-readable data, otherwise side channel attacks are feasible. For instance, we cannot optimize the subresource logic to only validate sets if a SameParty cookie exists.
• When validating a first-party set from a top-level navigation, it is important to fetch both manifests unconditionally, rather than use the cached version of the owner manifest. Otherwise one site can learn if the user has visited the other by claiming to be in a first-party set and measuring how long the browser took to reject it.
• If two first-party sets jointly own a set of "throwaway" domains (so state clearing does not matter), they can communicate a user identifier in which throwaway domains one set grabs from the other. This can be prevented if UA policy includes each domain in at most one entity. However note that, immediately after a domain changes ownership, policies using signed assertions may briefly accept either of two entities while the old assertions expire. The browser can push a revocation list to clear old assertions faster. Mitigating personalized sets also partially addresses this attack (if not personalized, the sites must coordinate via a global signal like time).

Service workers

Service workers complicate first-party sets. We must consider network requests made from a service worker, subresource fetches made from a document with a service worker attached, as well as how a site which uses a service worker may adopt first-party sets.

Changing a domain's first-party owner clears all state, including service worker registrations. This means service workers, like documents, are scoped to a given first-party set. Network requests from a service worker then behave like subresources.

If a document has a service worker attached, its subresource fetches go through the service worker. This does not trigger first-party set logic as this fetch is, at this point, a funny IPC. If the service worker makes a request in response, the first-party set logic will fire as above.

Finally, if a site already has a service worker, it should still be able to deploy first-party sets. However that service worker effectively translates navigation fetches into subresource fetches, and only top-level navigations discover new sets. We resolve this by moving Sec-Fetch-Party-Set header processing to the navigation logic. If the header is present, whether it came from the network directly or the service worker, we attempt to validate the set. This is fine because the header is not directly trusted.

UI Treatment

In order to provide transparency to users regarding the First-Party Set that a web page’s top-level domain belongs to, browsers may choose to present UI with information about the First-Party Set owner and the members list. One potential location in Chrome is the Origin/Page Info Bubble - this provides requisite information to discerning users, while avoiding the use of valuable screen real-estate or presenting confusing permission prompts. However, browsers are free to choose different presentation based on their UI patterns, or adjust as informed by user research.

Note that First-Party Sets also gives browsers the opportunity to group per-site controls (such as those at chrome://settings/content/all) by the “first-party” boundary instead of eTLD+1, which is not always the correct site boundary.

Alternative designs

Origins instead of registrable domains

A first-party set is a collection of origins, but it is specified by registrable domains, which carries a dependency on the public suffix list. While this is consistent with the various proposed privacy models as well as cookie handling, the security boundary on the web is the origin, not registrable domain.

An alternate design would be to instead specify sets by origins directly. In this model, any https origin would be a possible first-party set owner, and each origin must individually join a set, rather than relying on the root as we do here. For continuity with the existing behavior, we would then define the registrable domain as the default first-party set for each origin. That is, by default, https://foo.example.com, https://bar.example.com, and https://example.com:444 would all be in a set owned by https://example.com. Defining a set explicitly would override this default set.

This would reduce the web's dependency on the public suffix list, which would mitigate various problems. For instance, a university may allow students to register arbitrary subdomains at https://foo.university.example, but did not place university.example on the public suffix list, either due to compatibility concerns or oversight. With an origin-specified first-party set, individual origins could then detach themselves from the default set to avoid security problems with non-origin-based features such as cookies. (Note the __Host- cookie prefix also addresses this issue.)

This origin-defined approach has additional complications to resolve:

• There are a handful of features (cookies, document.domain) which are scoped to registrable domains, not origins. Those features should not transitively join two different sets. For instance, we must account for one set containing https://foo.bar.example.com and https://example.com, but not https://bar.example.com. For cookies, we can say that cookies remember the set which created them and we match both the Domain attribute and the first-party set. Thus if https://foo.bar.example.com sets a Domain=example.com cookie, https://example.com can read it, but not https://bar.example.com. Other features would need similar updates.
• The implicit state should be expressible explicitly, to simplify rollback and deployment, which means first-party set manifests must describe patterns of origins, rather than a simple bounded list of domains. In particular, we should support subtree patterns.
• https://foo.example.com's implicit owner is https://example.com. If https://example.com then forms an explicit set which does not include https://foo.example.com, we need to change https://foo.example.com's implicit state, perhaps to a singleton set.
• This complex set of patterns and implicit behaviors must be reevaluated against existing origins every time a first-party set is updated.
• Certificate wildcards (which themselves depend on the public suffix list) don't match an entire subtree. This conflicts with wanting to express implicit states above.

These complexities are likely solvable while keeping most of this design, should browsers believe this is worthwhile.

Prior Art

• Firefox's entity list
• draft-sullivan-dbound-problem-statement-02
• Single Trust and Same-Origin Policy v2 and affiliated domains from John Wilander to public-webappsec