HTML: The Living Standard

Edition for Web Developers — Last Updated 21 November 2024

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
   4. 12.4 Security
      1. 12.4.1 DNS spoofing attacks
      2. 12.4.2 Cross-directory attacks

12 Web storage

12.1 Introduction

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 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 agent clusters 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

storage.length

Returns the number of key/value pairs.

storage.key (n)

Returns the name of the nth 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.

12.2.2 The sessionStorage getter

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).

12.2.3 The localStorage getter

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).

12.2.4 The StorageEvent interface

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.

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 traversable, for instance denying access to the API for pages from other domains running in iframes.

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 navigables 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.

12.4 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.