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LICENSE Revised First-Party Sets Explainer. Sep 5, 2019 Fix formatting Oct 29, 2019

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.

Table of Contents


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,,, and are owned by the same entity, as are and, or and

We may wish to include these kinds of related names, 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 dynamic mechanism for defining these lists, which trades off the costs of a static list with other considerations.


  • Allow related domain names to declare themselves as the same first-party.
  • Provide a scalable and maintainable web platform mechanism to achieve the above, and thus avoid a hard-coded list.


  • 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 owner and each secondary domain in a first-party set hosts a first-party set manifest at https://<domain>/.well-known/first-party-set, containing a JSON dictionary. The secondary domains point to the owning domain while the owning domain lists the members of the set, as well as a version number to trigger updates.

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:

{ "owner": "a.example",
  "version": 1,
  "members": ["b.example", "c.example"] }

{ "owner": "a.example" }

{ "owner": "a.example" }

We then impose additional constraints on the owner's manifest:

  • Entries in members that are not registrable domains are ignored.
  • To mitigate unacceptable sets, if the number of entries in members must not exceed some limit, reject the entire manifest. As to the size of the limit, the largest entry in the Firefox entity list is around 200 domains (due to ccTLDs), although a tighter limit below 20-30 would much more effectively limit the scope. Per Chromium's document, one of the criteria is that "the resulting identity scope is not too large".
  • All domains in the set must be covered by the same X.509 certificate.

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.


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, draft-west-cookie-samesite-firstparty-01 describes a SameSite cookie attribute that sites may use to opt individual cookies in to relaxed variants of SameSite=Strict and SameSite=Lax: SameSite=FirstPartySetStrict and SameSite=FirstPartySetLax. 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 make one origin's state directly accessible to another origin in the set. It 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

Acceptable and unacceptable sets

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.

Exactly how to define this is an open question to be discussed. For an initial set of principles, we can look to how the various browser proposals say the following about 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."

This definition should also consider scenarios such as otherwise unrelated sites forming a consortium in order to expand the scope of their site identities.

Mitigating unacceptable sets

This proposal includes technical measures which limit which first-party sets may be formed. First, first-party sets require opt-in from all parties. This means one origin cannot form a set unilaterally without opt-in from other members. Second, manifest sizes are bounded, which limits the number of unrelated domains which may be in an unacceptable first-party set. Finally, we apply a certificate constraint, which correlates first-party sets with technical control of the domain names.

However, it is important to emphasize that these technical measures are not sufficient to exclude unacceptable sets. While we have not defined a criteria above, the initial principles above are tighter than the technical measures. First, while we bound first-party sets sizes, there are many ccTLDs. If we decide,, etc., are in scope, the limit may end up fairly generous. Second, certificates only validate technical control of a domain name. CDNs and hosting providers often legitimately acquire a single certificate covering multiple names that they host. The names may not be operated by the same organization or have a relationship meaningful to the user.

Thus these technical measures are only a first-pass filter on unacceptable sets. The browser still must apply interventions to unacceptable sets. This may be done by detecting and maintaining a list of blocked first-party owners, as in Google Safe Browsing. All first-party sets whose owners appear on the list are ignored and, if already present, cleared. This will change the set owner and trigger state clearing. This repairs the inconsistency with the privacy model, as well as disincentivizes sites from participating in unacceptable first-party sets. Note also this state clearing means sites cannot cycle between different sets to get around size limitations.

Detecting unacceptable sets

Maintaining a blocklist requires the browser monitor and detect unacceptable sets. Some possible strategies:

  • Such sets will likely center on popular sites, which simplifies the monitoring.
  • The certificate constraint causes sets to leave evidence in certificate transparency logs, which provides some degree of auditability. Note this is imperfect because giant CDN certificates may contain many names but not be used for a first-party set.
  • First-party set ownership can be monitored by monitoring the /.well-known/first-party-set resource for various sites. Servers could attempt to defeat this by using cloaking techniques to serve different sets to different monitors and users, but this can be helped by general measures to reduce fingerprinting, and by first-party-set-specific measures to avoid personalized sets (credentialless fetches, partitioning network state, etc.).

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 SameSite=FirstPartyLax 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 is partially addressed by mitigating personalized first-party sets (if not personalized, the sites must coordinate via a global signal like time). Beyond that, this can be addressed by mitigations against unacceptable sets in general (a domain that can be part of two sets is clearly unacceptable). See further discussion below.

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 contents of its manifest file. 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

Using a static list

The immediate alternate design is to use a static list, such as Firefox's entity list. A static list has several advantages. It is much simpler: it does not require mechanisms for the set changing, and there is no need to monitor unacceptable sets. A browser can more directly impose its policy on what kinds of sets are and are not acceptable.

At the same time, hardcoded lists can develop availability and deployment issues. First, each change must be propagated to each user's browser via an update. This complicates sites' ability to deploy new related domains. In comparison, the HSTS preload list is only a hardening measure around a dynamic HTTP header, so sites can deploy HSTS unilaterally. Relatedly, each browser also becomes a gatekeeper for these new domains. This produces an internet choke point. Finally, as preload lists grow, they can also develop scalability issues, as in the HSTS preload list.

This dynamic design avoids these concerns. It is worth noting, however, that the design does not remove the need for browsers to apply policies. Browsers still must mitigate unacceptable sets. The design only removes the browser from the critical path in deploying new entries.

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,,, and would all be in a set owned by 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, 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 and, but not 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 sets a cookie, can read it, but not 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.
  •'s implicit owner is If then forms an explicit set which does not include, we need to change'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.
  • The patterns also make the meaning of the size limit unclear.
  • 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

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