Cookie Store API

1. Introduction

This section is non-normative.

This is a proposal to bring an asynchronous cookie API to scripts running in HTML documents and service workers.

HTTP cookies have, since their origins at Netscape (documentation preserved by archive.org), provided a valuable state-management mechanism for the web.

The synchronous single-threaded script-level document.cookie interface to cookies has been a source of complexity and performance woes further exacerbated by the move in many browsers from:

• a single browser process,

• a single-threaded event loop model, and

• no general expectation of responsiveness for scripted event handling while processing cookie operations

... to the modern web which strives for smoothly responsive high performance:

• in multiple browser processes,

• with a multithreaded, multiple-event loop model, and

• with an expectation of responsiveness on human-reflex time scales.

On the modern web a cookie operation in one part of a web application cannot block:

• the rest of the web application,

• the rest of the web origin, or

• the browser as a whole.

Newer parts of the web built in service workers need access to cookies too but cannot use the synchronous, blocking document.cookie interface at all as they both have no document and also cannot block the event loop as that would interfere with handling of unrelated events.

1.1. A Taste of the Proposed Change

Although it is tempting to rethink cookies entirely, web sites today continue to rely heavily on them, and the script APIs for using them are largely unchanged over their first decades of usage.

Today writing a cookie means blocking your event loop while waiting for the browser to synchronously update the cookie jar with a carefully-crafted cookie string in Set-Cookie format:

document.cookie =
  '__Secure-COOKIENAME=cookie-value' +
  '; Path=/' +
  '; expires=Fri, 12 Aug 2016 23:05:17 GMT' +
  '; Secure' +
  '; Domain=example.org';
// now we could assume the write succeeded, but since
// failure is silent it is difficult to tell, so we
// read to see whether the write succeeded
var successRegExp =
  /(^|; ?)__Secure-COOKIENAME=cookie-value(;|$)/;
if (String(document.cookie).match(successRegExp)) {
  console.log('It worked!');
} else {
  console.error('It did not work, and we do not know why');
}

What if you could instead write:

const one_day_ms = 24 * 60 * 60 * 1000;
cookieStore.set(
  {
    name: '__Secure-COOKIENAME',
    value: 'cookie-value',
    expires: Date.now() + one_day_ms,
    domain: 'example.org'
  }).then(function() {
    console.log('It worked!');
  }, function(reason) {
    console.error(
      'It did not work, and this is why:',
      reason);
  });
// Meanwhile we can do other things while waiting for
// the cookie store to process the write...

This also has the advantage of not relying on document and not blocking, which together make it usable from Service Workers, which otherwise do not have cookie access from script.

This proposal also includes a power-efficient monitoring API to replace setTimeout-based polling cookie monitors with cookie change observers.

1.2. Summary

This proposal outlines an asynchronous API using Promises/async functions for the following cookie operations:

• write (or "set") and delete (or "expire") cookies

• read (or "get") script-visible cookies

  • ... including for specified in-scope request paths in service worker contexts

• monitor script-visible cookies for changes using CookieChangeEvent

  • ... in long-running script contexts (e.g. document)

  • ... for script-supplied in-scope request paths in service worker contexts

1.3. Querying Cookies

Both documents and service workers access the same query API, via the cookieStore property on the global object.

The get() and getAll() methods on CookieStore are used to query cookies. Both methods return Promises. Both methods take the same arguments, which can be either:

• a name, or

• a dictionary of options (optional for getAll())

The get() method is essentially a form of getAll() that only returns the first result.

try {
  const cookie = await cookieStore.get('session_id');
  if (cookie) {
    console.log(`Found ${cookie.name} cookie: ${cookie.value}`);
  } else {
    console.log('Cookie not found');
  }
} catch (e) {
  console.error(`Cookie store error: ${e}`);
}

try {
  const cookies = await cookieStore.getAll('session_id'});
  for (const cookie of cookies)
    console.log(`Result: ${cookie.name} = ${cookie.value}`);
} catch (e) {
  console.error(`Cookie store error: ${e}`);
}

Service workers can obtain the list of cookies that would be sent by a fetch to any URL under their scope.

await cookieStore.getAll({url: '/admin'});

Documents can only obtain the cookies at their current URL. In other words, the only valid url value in Document contexts is the document’s URL.

The objects returned by get() and getAll() contain all the relevant information in the cookie store, not just the name and the value as in the older document.cookie API.

await cookie = cookieStore.get('session_id');
console.log(`Cookie scope - Domain: ${cookie.domain} Path: ${cookie.path}`);
if (cookie.expires === null) {
  console.log('Cookie expires at the end of the session');
} else {
  console.log(`Cookie expires at: ${cookie.expires}`);
}
if (cookie.secure)
  console.log('The cookie is restricted to secure origins');

1.4. Modifying Cookies

Both documents and service workers access the same modification API, via the cookieStore property on the global object.

Cookies are created or modified (written) using the set() method.

try {
  await cookieStore.set('opted_out', '1');
} catch (e) {
  console.error(`Failed to set cookie: ${e}`);
}

await cookieStore.set({
  name: 'opted_out',
  value: '1',
  expires: null,  // session cookie

  // By default, domain is set to null which means the scope is locked at the current domain.
  domain: null,
  path: '/'
});

Cookies are deleted (expired) using the delete() method.

try {
  await cookieStore.delete('session_id');
} catch (e) {
  console.error(`Failed to delete cookie: ${e}`);
}

Under the hood, deleting a cookie is done by changing the cookie’s expiration date to the past, which still works.

try {
  const one_day_ms = 24 * 60 * 60 * 1000;
  await cookieStore.set({
    name: 'session_id',
    value: 'value will be ignored',
    expires: Date.now() - one_day_ms });
} catch (e) {
  console.error(`Failed to delete cookie: ${e}`);
}

1.5. Monitoring Cookies

To avoid polling, it is possible to observe changes to cookies.

In documents, change events are fired for all relevant cookie changes.

cookieStore.addEventListener('change', event => {
  console.log(`${event.changed.length} changed cookies`);
  for (const cookie in event.changed)
    console.log(`Cookie ${cookie.name} changed to ${cookie.value}`);

  console.log(`${event.deleted.length} deleted cookies`);
  for (const cookie in event.deleted)
    console.log(`Cookie ${cookie.name} deleted`);
});

In service workers, cookiechange events are fired against the global scope, but an explicit subscription is required, associated with the service worker’s registration.

self.addEventListener('activate', (event) => {
  event.waitUntil(async () => {
    // Snapshot current state of subscriptions.
    const subscriptions = await self.registration.cookies.getSubscriptions();

    // Clear any existing subscriptions.
    await self.registration.cookies.unsubscribe(subscriptions);

    await self.registration.cookies.subscribe([
      {
        name: 'session_id',  // Get change events for cookies named session_id.
      }
    ]);
  });
});

self.addEventListener('cookiechange', event => {
  // The event has |changed| and |deleted| properties with
  // the same semantics as the Document events.
  console.log(`${event.changed.length} changed cookies`);
  console.log(`${event.deleted.length} deleted cookies`);
});

Calls to subscribe() are cumulative, so that independently maintained modules or libraries can set up their own subscriptions. As expected, a service worker's subscriptions are persisted for with the service worker registration.

Subscriptions can use the same options as get() and getAll(). The complexity of fine-grained subscriptions is justified by the cost of dispatching an irrelevant cookie change event to a service worker, which is is much higher than the cost of dispatching an equivalent event to a Window. Specifically, dispatching an event to a service worker might require waking up the worker, which has a significant impact on battery life.

The getSubscriptions() allows a service worker to introspect the subscriptions that have been made.

const subscriptions = await self.registration.cookies.getSubscriptions();
for (const sub of subscriptions) {
  console.log(sub.name, sub.url);
}

2. Concepts

2.1. Cookie

A cookie is normatively defined for user agents by Cookies § User Agent Requirements.

A cookie is script-visible when it is in-scope and does not have the HttpOnly cookie flag. This is more formally enforced in the processing model, which consults Cookies § Retrieval Model at appropriate points.

A cookie is also subject to certain size limits. Per Cookies § Storage Model:

• The combined lengths of the name and value fields must not be greater than 4096 bytes (the maximum name/value pair size).

• The length of every field except the name and value fields must not be greater than 1024 bytes (the maximum attribute value size).

NOTE: Cookie attribute-values are stored as byte sequences, not strings.

2.2. Cookie Store

A cookie store is normatively defined for user agents by Cookies § User Agent Requirements.

When any of the following conditions occur for a cookie store, perform the steps to process cookie changes.

• A newly-created cookie is inserted into the cookie store.

• A user agent evicts expired cookies from the cookie store.

• A user agent removes excess cookies from the cookie store.

2.3. Extensions to Service Worker

[Service-Workers] defines service worker registration, which this specification extends.

A service worker registration has an associated cookie change subscription list which is a list; each member is a cookie change subscription. A cookie change subscription is a tuple of name and url.

3. The CookieStore Interface

[Exposed=(ServiceWorker,Window),
 SecureContext]
interface CookieStore : EventTarget {
  Promise<CookieListItem?> get(USVString name);
  Promise<CookieListItem?> get(optional CookieStoreGetOptions options = {});

  Promise<CookieList> getAll(USVString name);
  Promise<CookieList> getAll(optional CookieStoreGetOptions options = {});

  Promise<undefined> set(USVString name, USVString value);
  Promise<undefined> set(CookieInit options);

  Promise<undefined> delete(USVString name);
  Promise<undefined> delete(CookieStoreDeleteOptions options);

  [Exposed=Window]
  attribute EventHandler onchange;
};

dictionary CookieStoreGetOptions {
  USVString name;
  USVString url;
};

enum CookieSameSite {
  "strict",
  "lax",
  "none"
};

dictionary CookieInit {
  required USVString name;
  required USVString value;
  DOMHighResTimeStamp? expires = null;
  USVString? domain = null;
  USVString path = "/";
  CookieSameSite sameSite = "strict";
  boolean partitioned = false;
};

dictionary CookieStoreDeleteOptions {
  required USVString name;
  USVString? domain = null;
  USVString path = "/";
  boolean partitioned = false;
};

dictionary CookieListItem {
  USVString name;
  USVString value;
  USVString? domain;
  USVString path;
  DOMHighResTimeStamp? expires;
  boolean secure;
  CookieSameSite sameSite;
  boolean partitioned;
};

typedef sequence<CookieListItem> CookieList;

3.1. The get() method

3.2. The getAll() method

3.3. The set() method

3.4. The delete() method

4. The CookieStoreManager Interface

A CookieStoreManager has an associated registration which is a service worker registration.

The CookieStoreManager interface allows Service Workers to subscribe to events for cookie changes. Using the subscribe() method is necessary to indicate that a particular service worker registration is interested in change events.

[Exposed=(ServiceWorker,Window),
 SecureContext]
interface CookieStoreManager {
  Promise<undefined> subscribe(sequence<CookieStoreGetOptions> subscriptions);
  Promise<sequence<CookieStoreGetOptions>> getSubscriptions();
  Promise<undefined> unsubscribe(sequence<CookieStoreGetOptions> subscriptions);
};

4.1. The subscribe() method

4.2. The getSubscriptions() method

4.3. The unsubscribe() method

4.4. The ServiceWorkerRegistration interface

The ServiceWorkerRegistration interface is extended to give access to a CookieStoreManager via cookies which provides the interface for subscribing to cookie changes.

[Exposed=(ServiceWorker,Window)]
partial interface ServiceWorkerRegistration {
  [SameObject] readonly attribute CookieStoreManager cookies;
};

Each ServiceWorkerRegistration has an associated CookieStoreManager object. The CookieStoreManager's registration is equal to the ServiceWorkerRegistration's service worker registration.

The cookies getter steps are to return this's associated CookieStoreManager object.

self.registration.cookies.subscribe([{name:'session-id'}]);

navigator.serviceWorker.register('sw.js').then(registration => {
  registration.cookies.subscribe([{name:'session-id'}]);
});

5. Event Interfaces

5.1. The CookieChangeEvent interface

A CookieChangeEvent is dispatched against CookieStore objects in Window contexts when any script-visible cookie changes have occurred.

[Exposed=Window,
 SecureContext]
interface CookieChangeEvent : Event {
  constructor(DOMString type, optional CookieChangeEventInit eventInitDict = {});
  [SameObject] readonly attribute FrozenArray<CookieListItem> changed;
  [SameObject] readonly attribute FrozenArray<CookieListItem> deleted;
};

dictionary CookieChangeEventInit : EventInit {
  CookieList changed;
  CookieList deleted;
};

The changed and deleted attributes must return the value they were initialized to.

5.2. The ExtendableCookieChangeEvent interface

An ExtendableCookieChangeEvent is dispatched against ServiceWorkerGlobalScope objects when any script-visible cookie changes have occurred which match the Service Worker's cookie change subscription list.

Note: ExtendableEvent is used as the ancestor interface for all events in Service Workers so that the worker itself can be kept alive while the async operations are performed.

[Exposed=ServiceWorker]
interface ExtendableCookieChangeEvent : ExtendableEvent {
  constructor(DOMString type, optional ExtendableCookieChangeEventInit eventInitDict = {});
  [SameObject] readonly attribute FrozenArray<CookieListItem> changed;
  [SameObject] readonly attribute FrozenArray<CookieListItem> deleted;
};

dictionary ExtendableCookieChangeEventInit : ExtendableEventInit {
  CookieList changed;
  CookieList deleted;
};

The changed and deleted attributes must return the value they were initialized to.

6. Global Interfaces

A CookieStore is accessed by script using an attribute in the global scope in a Window or ServiceWorkerGlobalScope context.

6.1. The Window interface

[SecureContext]
partial interface Window {
  [SameObject] readonly attribute CookieStore cookieStore;
};

A Window has an associated CookieStore, which is a CookieStore.

The cookieStore getter steps are to return this's associated CookieStore.

6.2. The ServiceWorkerGlobalScope interface

partial interface ServiceWorkerGlobalScope {
  [SameObject] readonly attribute CookieStore cookieStore;

  attribute EventHandler oncookiechange;
};

A ServiceWorkerGlobalScope has an associated CookieStore, which is a CookieStore.

The cookieStore getter steps are to return this's associated CookieStore.

7. Algorithms

7.1. Query Cookies

7.2. Set a Cookie

7.3. Delete a Cookie

7.4. Process Changes

8. Security Considerations

Other than cookie access from service worker contexts, this API is not intended to expose any new capabilities to the web.

8.1. Gotcha!

Although browser cookie implementations are now evolving in the direction of better security and fewer surprising and error-prone defaults, there are at present few guarantees about cookie data security.

• unsecured origins can typically overwrite cookies used on secure origins

• superdomains can typically overwrite cookies seen by subdomains

• cross-site scripting attacks and other script and header injection attacks can be used to forge cookies too

• cookie read operations (both from script and on web servers) don’t give any indication of where the cookie came from

• browsers sometimes truncate, transform or evict cookie data in surprising and counterintuitive ways

  • ... due to reaching storage limits

  • ... due to character encoding differences

  • ... due to differing syntactic and semantic rules for cookies

For these reasons it is best to use caution when interpreting any cookie’s value, and never execute a cookie’s value as script, HTML, CSS, XML, PDF, or any other executable format.

8.2. Restrict?

This API may have the unintended side-effect of making cookies easier to use and consequently encouraging their further use. If it causes their further use in non-secure contexts this could result in a web less safe for users. For that reason this API has been restricted to secure contexts only.

8.3. Secure cookies

This section is non-normative.

This API only allows writes for Secure cookies to encourage better decisions around security. However the API will still allow reading non-Secure cookies in order to facilitate the migration to Secure cookies. As a side-effect, when fetching and modifying a non-Secure cookie with this API, the non-Secure cookie will automatically be modified to Secure.

8.4. Surprises

Some existing cookie behavior (especially domain-rather-than-origin orientation, non-secure contexts being able to set cookies readable in secure contexts, and script being able to set cookies unreadable from script contexts) may be quite surprising from a web security standpoint.

Other surprises are documented in Cookies § Introduction - for instance, a cookie may be set for a superdomain (e.g. app.example.com may set a cookie for the whole example.com domain), and a cookie may be readable across all port numbers on a given domain name.

Further complicating this are historical differences in cookie-handling across major browsers, although some of those (e.g. port number handling) are now handled with more consistency than they once were.

8.5. Prefixes

Where feasible the examples use the __Host- and __Secure- name prefixes which causes some current browsers to disallow overwriting from non-secure contexts, disallow overwriting with no Secure flag, and — in the case of __Host- — disallow overwriting with an explicit Domain or non-'/' Path attribute (effectively enforcing same-origin semantics.) These prefixes provide important security benefits in those browsers implementing Secure Cookies and degrade gracefully (i.e. the special semantics may not be enforced in other cookie APIs but the cookies work normally and the async cookies API enforces the secure semantics for write operations) in other browsers. A major goal of this API is interoperation with existing cookies, though, so a few examples have also been provided using cookie names lacking these prefixes.

Prefix rules are also enforced in write operations by this API, but may not be enforced in the same browser for other APIs. For this reason it is inadvisable to rely on their enforcement too heavily until and unless they are more broadly adopted.

8.6. URL scoping

Although a service worker script cannot directly access cookies today, it can already use controlled rendering of in-scope HTML and script resources to inject cookie-monitoring code under the remote control of the service worker script. This means that cookie access inside the scope of the service worker is technically possible already, it’s just not very convenient.

When the service worker is scoped more narrowly than / it may still be able to read path-scoped cookies from outside its scope’s path space by successfully guessing/constructing a 404 page URL which allows IFRAME-ing and then running script inside it the same technique could expand to the whole origin, but a carefully constructed site (one where no out-of-scope pages are IFRAME-able) can actually deny this capability to a path-scoped service worker today and I was reluctant to remove that restriction without further discussion of the implications.

8.7. Cookie aversion

To reduce complexity for developers and eliminate the need for ephemeral test cookies, this async cookies API will explicitly reject attempts to write or delete cookies when the operation would be ignored. Likewise it will explicitly reject attempts to read cookies when that operation would ignore actual cookie data and simulate an empty cookie jar. Attempts to observe cookie changes in these contexts will still "work", but won’t invoke the callback until and unless read access becomes allowed (due e.g. to changed site permissions.)

Today writing to document.cookie in contexts where script-initiated cookie-writing is disallowed typically is a no-op. However, many cookie-writing scripts and frameworks always write a test cookie and then check for its existence to determine whether script-initiated cookie-writing is possible.

Likewise, today reading document.cookie in contexts where script-initiated cookie-reading is disallowed typically returns an empty string. However, a cooperating web server can verify that server-initiated cookie-writing and cookie-reading work and report this to the script (which still sees empty string) and the script can use this information to infer that script-initiated cookie-reading is disallowed.

9. Privacy Considerations

9.1. Clear cookies

This section is non-normative.

When a user clears cookies for an origin, the user agent needs to wipe all storage for that origin; including service workers and DOM-accessible storage for that origin. This is to prevent websites from restoring any user identifiers in persistent storage after a user initiates the action.

10. Acknowledgements

Thanks to Benjamin Sittler, who created the initial proposal for this API.

Many thanks to Adam Barth, Alex Russell, Andrea Marchesini, Andrew Williams, Anne van Kesteren, Ben Kelly, Craig Francis, Daniel Appelquist, Daniel Murphy, Domenic Denicola, Elliott Sprehn, Fagner Brack, Jake Archibald, Joel Weinberger, Kenneth Rohde Christiansen, Lukasz Olejnik, Marijn Kruisselbrink, and Mike West for helping craft this proposal.

Special thanks to Tab Atkins, Jr. for creating and maintaining Bikeshed, the specification authoring tool used to create this document, and for his general authoring advice.