HTML

Living Standard — Last Updated 30 November 2020

1. 12 Web storage
   1. 12.1 Introduction
   2. 12.2 The API
      1. 12.2.1 The Storage interface
      2. 12.2.2 The sessionStorage getter
      3. 12.2.3 The localStorage getter
      4. 12.2.4 The StorageEvent interface
   3. 12.3 Privacy
      1. 12.3.1 User tracking
      2. 12.3.2 Sensitivity of data
   4. 12.4 Security
      1. 12.4.1 DNS spoofing attacks
      2. 12.4.2 Cross-directory attacks
      3. 12.4.3 Implementation risks

12 Web storage

12.1 Introduction

This section is non-normative.

This specification introduces two related mechanisms, similar to HTTP session cookies, for storing name-value pairs on the client side. [COOKIES]

The first is designed for scenarios where the user is carrying out a single transaction, but could be carrying out multiple transactions in different windows at the same time.

Cookies don't really handle this case well. For example, a user could be buying plane tickets in two different windows, using the same site. If the site used cookies to keep track of which ticket the user was buying, then as the user clicked from page to page in both windows, the ticket currently being purchased would "leak" from one window to the other, potentially causing the user to buy two tickets for the same flight without really noticing.

To address this, this specification introduces the sessionStorage getter. Sites can add data to the session storage, and it will be accessible to any page from the same site opened in that window.

<label>
 <input type="checkbox" onchange="sessionStorage.insurance = checked ? 'true' : ''">
  I want insurance on this trip.
</label>

if
(sessionStorage.insurance)
{
...
}

The second storage mechanism is designed for storage that spans multiple windows, and lasts beyond the current session. In particular, web applications might wish to store megabytes of user data, such as entire user-authored documents or a user's mailbox, on the client side for performance reasons.

Again, cookies do not handle this case well, because they are transmitted with every request.

The localStorage getter is used to access a page's local storage area.

<p>
  You have viewed this page
  <span id="count">an untold number of</span>
  time(s).
</p>
<script>
  if (!localStorage.pageLoadCount)
    localStorage.pageLoadCount = 0;
  localStorage.pageLoadCount = parseInt(localStorage.pageLoadCount) + 1;
  document.getElementById('count').textContent = localStorage.pageLoadCount;
</script>

Each site has its own separate storage area.

The localStorage getter provides access to shared state. This specification does not define the interaction with other browsing contexts in a multiprocess user agent, and authors are encouraged to assume that there is no locking mechanism. A site could, for instance, try to read the value of a key, increment its value, then write it back out, using the new value as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects.

12.2 The API

12.2.1 The Storage interface

[Exposed=Window]
interface Storage {
  readonly attribute unsigned long length;
  DOMString? key(unsigned long index);
  getter DOMString? getItem(DOMString key);
  setter undefined setItem(DOMString key, DOMString value);
  deleter undefined removeItem(DOMString key);
  undefined clear();
};

storage . length

Returns the number of key/value pairs.

storage . key ( n )

Returns the name of the n th key, or null if n is greater than or equal to the number of key/value pairs.

value = storage . getItem ( key )

value = storage [ key ]

Returns the current value associated with the given key , or null if the given key does not exist.

storage . setItem ( key , value )

storage [ key ] = value

Sets the value of the pair identified by key to value , creating a new key/value pair if none existed for key previously.

Throws a " QuotaExceededError " DOMException exception if the new value couldn't be set. (Setting could fail if, e.g., the user has disabled storage for the site, or if the quota has been exceeded.)

Dispatches a storage event on Window objects holding an equivalent Storage object.

storage . removeItem ( key )

delete storage [ key ]

Removes the key/value pair with the given key , if a key/value pair with the given key exists.

Dispatches a storage event on Window objects holding an equivalent Storage object.

storage . clear ()

Removes all key/value pairs, if there are any.

Dispatches a storage event on Window objects holding an equivalent Storage object.

A Storage object has an associated:

map

A storage proxy map .

type

" local " or " session ".

To reorder a Storage object storage , reorder storage 's map 's entries in an implementation-defined manner.

Unfortunate as it is, iteration order is not defined and can change upon most mutations.

To broadcast a Storage object storage , given a key , oldValue , and newValue , run these steps:

1. Let url be storage 's relevant global object 's associated Document 's URL .

2. Let remoteStorages be all Storage objects excluding storage whose:

   • type is storage 's type
   • relevant settings object 's origin is same origin with storage 's relevant settings object 's origin .

   and, if type is " session ", whose relevant settings object 's browsing session is storage 's relevant settings object 's browsing session .

3. For each remoteStorage of remoteStorages : queue a global task on the DOM manipulation task source given remoteStorage 's relevant global object to fire an event named storage at remoteStorage 's relevant global object , using StorageEvent , with key initialized to key , oldValue initialized to oldValue , newValue initialized to newValue , url initialized to url , and storageArea initialized to remoteStorage .

   The Document object associated with the resulting task is not necessarily fully active , but events fired on such objects are ignored by the event loop until the Document becomes fully active again.

The length getter steps are to return this 's map 's size .

The key( n ) method steps are:

1. If n is greater than or equal to this 's map 's size , then return null.

2. Let keys be the result of running get the keys on this 's map .

3. Return keys [ n ].

The supported property names on a Storage object storage are the result of running get the keys on storage 's map .

The getItem( key ) method steps are:

1. If this 's map [ key ] does not exist , then return null.

2. Return this 's map [ key ].

The setItem( key , value ) method are:

1. Let oldValue be null.

2. Let reorder be true.

3. If this 's map [ key ] exists :

   1. Set oldValue to this 's map [ key ].

   2. If oldValue is value , then return.

   3. Set reorder to false.

4. If value cannot be stored, then throw a " QuotaExceededError " DOMException exception.

5. Set this 's map [ key ] to value .

6. If reorder is true, then reorder this .

7. Broadcast this with key , oldValue , and value .

The removeItem( key ) method steps are:

1. If this 's map [ key ] does not exist , then return null.

2. Set oldValue to this 's map [ key ].

3. Remove this 's map [ key ].

4. Reorder this .

5. Broadcast this with key , oldValue , and null.

The clear() method steps are:

1. Clear this 's map .

2. Broadcast this with null, null, and null.

12.2.2 The sessionStorage getter

interface mixin WindowSessionStorage {
  readonly attribute Storage sessionStorage;
};

Window

includes

WindowSessionStorage

;

window . sessionStorage

Returns the Storage object associated with that window 's origin's session storage area.

Throws a " SecurityError " DOMException if the Document 's origin is an opaque origin or if the request violates a policy decision (e.g., if the user agent is configured to not allow the page to persist data).

A Document object has an associated session storage holder , which is null or a Storage object. It is initially null.

The sessionStorage getter steps are:

1. If this 's associated Document 's session storage holder is non-null, then return this 's associated Document 's session storage holder .

2. Let map be the result of running obtain a session storage bottle map with this 's relevant settings object and " sessionStorage ".

3. If map is failure, then throw a " SecurityError " DOMException .

4. Let storage be a new Storage object whose map is map .

5. Set this 's associated Document 's session storage holder to storage .

6. Return storage .

While creating a new auxiliary browsing context , the session storage is copied over.

12.2.3 The localStorage getter

interface mixin WindowLocalStorage {
  readonly attribute Storage localStorage;
};

Window

includes

WindowLocalStorage

;

window . localStorage

Returns the Storage object associated with window 's origin's local storage area.

Throws a " SecurityError " DOMException if the Document 's origin is an opaque origin or if the request violates a policy decision (e.g., if the user agent is configured to not allow the page to persist data).

A Document object has an associated local storage holder , which is null or a Storage object. It is initially null.

The localStorage getter steps are:

1. If this 's associated Document 's local storage holder is non-null, then return this 's associated Document 's local storage holder .

2. Let map be the result of running obtain a local storage bottle map with this 's relevant settings object and " localStorage ".

3. If map is failure, then throw a " SecurityError " DOMException .

4. Let storage be a new Storage object whose map is map .

5. Set this 's associated Document 's local storage holder to storage .

6. Return storage .

12.2.4 The StorageEvent interface

[Exposed=Window]
interface StorageEvent : Event {
  constructor(DOMString type, optional StorageEventInit eventInitDict = {});
  readonly attribute DOMString? key;
  readonly attribute DOMString? oldValue;
  readonly attribute DOMString? newValue;
  readonly attribute USVString url;
  readonly attribute Storage? storageArea;
  undefined initStorageEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional DOMString? key = null, optional DOMString? oldValue = null, optional DOMString? newValue = null, optional USVString url = "", optional Storage? storageArea = null);
};
dictionary StorageEventInit : EventInit {
  DOMString? key = null;
  DOMString? oldValue = null;
  DOMString? newValue = null;
  USVString url = "";
  Storage? storageArea = null;
};

event . key

Returns the key of the storage item being changed.

event . oldValue

Returns the old value of the key of the storage item whose value is being changed.

event . newValue

Returns the new value of the key of the storage item whose value is being changed.

event . url

Returns the URL of the document whose storage item changed.

event . storageArea

Returns the Storage object that was affected.

The key , oldValue , newValue , url , and storageArea attributes must return the values they were initialized to.

The initStorageEvent() method must initialize the event in a manner analogous to the similarly-named initEvent() method. [DOM]

12.3 Privacy

12.3.1 User tracking

A third-party advertiser (or any entity capable of getting content distributed to multiple sites) could use a unique identifier stored in its local storage area to track a user across multiple sessions, building a profile of the user's interests to allow for highly targeted advertising. In conjunction with a site that is aware of the user's real identity (for example an e-commerce site that requires authenticated credentials), this could allow oppressive groups to target individuals with greater accuracy than in a world with purely anonymous web usage.

There are a number of techniques that can be used to mitigate the risk of user tracking:

Blocking third-party storage

User agents may restrict access to the localStorage objects to scripts originating at the domain of the active document of the top-level browsing context , for instance denying access to the API for pages from other domains running in iframe s.

Expiring stored data

User agents may, possibly in a manner configured by the user, automatically delete stored data after a period of time.

For example, a user agent could be configured to treat third-party local storage areas as session-only storage, deleting the data once the user had closed all the browsing contexts that could access it.

This can restrict the ability of a site to track a user, as the site would then only be able to track the user across multiple sessions when they authenticate with the site itself (e.g. by making a purchase or logging in to a service).

However, this also reduces the usefulness of the API as a long-term storage mechanism. It can also put the user's data at risk, if the user does not fully understand the implications of data expiration.

Treating persistent storage as cookies

If users attempt to protect their privacy by clearing cookies without also clearing data stored in the local storage area, sites can defeat those attempts by using the two features as redundant backup for each other. User agents should present the interfaces for clearing these in a way that helps users to understand this possibility and enables them to delete data in all persistent storage features simultaneously. [COOKIES]

Site-specific safelisting of access to local storage areas

User agents may allow sites to access session storage areas in an unrestricted manner, but require the user to authorize access to local storage areas.

Origin-tracking of stored data

User agents may record the origins of sites that contained content from third-party origins that caused data to be stored.

If this information is then used to present the view of data currently in persistent storage, it would allow the user to make informed decisions about which parts of the persistent storage to prune. Combined with a blocklist ("delete this data and prevent this domain from ever storing data again"), the user can restrict the use of persistent storage to sites that they trust.

Shared blocklists

User agents may allow users to share their persistent storage domain blocklists.

This would allow communities to act together to protect their privacy.

While these suggestions prevent trivial use of this API for user tracking, they do not block it altogether. Within a single domain, a site can continue to track the user during a session, and can then pass all this information to the third party along with any identifying information (names, credit card numbers, addresses) obtained by the site. If a third party cooperates with multiple sites to obtain such information, a profile can still be created.

However, user tracking is to some extent possible even with no cooperation from the user agent whatsoever, for instance by using session identifiers in URLs, a technique already commonly used for innocuous purposes but easily repurposed for user tracking (even retroactively). This information can then be shared with other sites, using visitors' IP addresses and other user-specific data (e.g. user-agent headers and configuration settings) to combine separate sessions into coherent user profiles.

12.3.2 Sensitivity of data

User agents should treat persistently stored data as potentially sensitive; it's quite possible for emails, calendar appointments, health records, or other confidential documents to be stored in this mechanism.

To this end, user agents should ensure that when deleting data, it is promptly deleted from the underlying storage.

12.4 Security

12.4.1 DNS spoofing attacks

Because of the potential for DNS spoofing attacks, one cannot guarantee that a host claiming to be in a certain domain really is from that domain. To mitigate this, pages can use TLS. Pages using TLS can be sure that only the user, software working on behalf of the user, and other pages using TLS that have certificates identifying them as being from the same domain, can access their storage areas.

12.4.2 Cross-directory attacks

Different authors sharing one host name, for example users hosting content on the now defunct geocities.com , all share one local storage object. There is no feature to restrict the access by pathname. Authors on shared hosts are therefore urged to avoid using these features, as it would be trivial for other authors to read the data and overwrite it.

Even if a path-restriction feature was made available, the usual DOM scripting security model would make it trivial to bypass this protection and access the data from any path.

12.4.3 Implementation risks

The two primary risks when implementing these persistent storage features are letting hostile sites read information from other domains, and letting hostile sites write information that is then read from other domains.

Letting third-party sites read data that is not supposed to be read from their domain causes information leakage , For example, a user's shopping wishlist on one domain could be used by another domain for targeted advertising; or a user's work-in-progress confidential documents stored by a word-processing site could be examined by the site of a competing company.

Letting third-party sites write data to the persistent storage of other domains can result in information spoofing , which is equally dangerous. For example, a hostile site could add items to a user's wishlist; or a hostile site could set a user's session identifier to a known ID that the hostile site can then use to track the user's actions on the victim site.

Thus, strictly following the origin model described in this specification is important for user security.