## 11 Web storage

### 11.1 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` IDL attribute. Sites can add data to the session storage, and it will be accessible to any page from the same site opened in that window.

For example, a page could have a checkbox that the user ticks to indicate that he wants insurance:

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

A later page could then check, from script, whether the user had checked the checkbox or not:

    if (sessionStorage.insurance) { ... }

If the user had multiple windows opened on the site, each one would have its own individual copy of the session storage object.

The second storage mechanism is designed for storage that spans multiple windows, and lasts beyond the current session. In particular, Web applications may 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` IDL attribute is used to access a page's local storage area.

The site at example.com can display a count of how many times the user has loaded its page by putting the following at the bottom of its page:

    <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.

### 11.2 The API

#### 11.2.1 The `Storage` interface

    interface Storage {
      readonly attribute unsigned long length;
      DOMString? key(unsigned long index);
      getter DOMString? getItem(DOMString key);
      setter creator void setItem(DOMString key, DOMString value);
      deleter void removeItem(DOMString key);
      void clear();
    };

Each `Storage` object provides access to a list of key/value pairs, which are sometimes called items. Keys are strings. Any string (including the empty string) is a valid key. Values are similarly strings.

Each `Storage` object is associated with a list of key/value pairs when it is created, as defined in the sections on the `sessionStorage` and `localStorage` attributes. Multiple separate objects implementing the `Storage` interface can all be associated with the same list of key/value pairs simultaneously.

The `length` attribute must return the number of key/value pairs currently present in the list associated with the object.

The `key(n)` method must return the name of the nth key in the list. The order of keys is user-agent defined, but must be consistent within an object so long as the number of keys doesn't change. (Thus, adding or removing a key may change the order of the keys, but merely changing the value of an existing key must not.) If n is greater than or equal to the number of key/value pairs in the object, then this method must return null.

The supported property names on a `Storage` object are the keys of each key/value pair currently present in the list associated with the object, in the order that the keys were last added to the storage area.

The `getItem(key)` method must return the current value associated with the given key. If the given key does not exist in the list associated with the object then this method must return null.

The `setItem(key, value)` method must first check if a key/value pair with the given key already exists in the list associated with the object.

If it does not, then a new key/value pair must be added to the list, with the given key and with its value set to value.

If the given key does exist in the list, and its value is not equal to value, then it must have its value updated to value. If its previous value is equal to value, then the method must do nothing.

If it couldn't set the new value, the method must throw a `QuotaExceededError` exception. (Setting could fail if, e.g., the user has disabled storage for the site, or if the quota has been exceeded.)

The `removeItem(key)` method must cause the key/value pair with the given key to be removed from the list associated with the object, if it exists. If no item with that key exists, the method must do nothing.

The `setItem()` and `removeItem()` methods must be atomic with respect to failure. In the case of failure, the method does nothing. That is, changes to the data storage area must either be successful, or the data storage area must not be changed at all.

The `clear()` method must atomically cause the list associated with the object to be emptied of all key/value pairs, if there are any. If there are none, then the method must do nothing.

When the `setItem()`, `removeItem()`, and `clear()` methods are invoked, events are fired on the `Window` objects of other `Document`s that can access the newly stored or removed data, as defined in the sections on the `sessionStorage` and `localStorage` attributes.

This specification does not require that the above methods wait until the data has been physically written to disk. Only consistency in what different scripts accessing the same underlying list of key/value pairs see is required.

#### 11.2.2 The `sessionStorage` attribute

    [NoInterfaceObject]
    interface WindowSessionStorage {
      readonly attribute Storage sessionStorage;
    };
    Window implements WindowSessionStorage;

The `sessionStorage` attribute represents the set of storage areas specific to the current top-level browsing context.

Each top-level browsing context has a unique set of session storage areas, one for each origin.

User agents should not expire data from a browsing context's session storage areas, but may do so when the user requests that such data be deleted, or when the UA detects that it has limited storage space, or for security reasons. User agents should always avoid deleting data while a script that could access that data is running. When a top-level browsing context is destroyed (and therefore permanently inaccessible to the user) the data stored in its session storage areas can be discarded with it, as the API described in this specification provides no way for that data to ever be subsequently retrieved.

The lifetime of a browsing context can be unrelated to the lifetime of the actual user agent process itself, as the user agent may support resuming sessions after a restart.

When a new `Document` is created in a browsing context which has a top-level browsing context, the user agent must check to see if that top-level browsing context has a session storage area for that document's origin. If it does, then that is the `Document`'s assigned session storage area. If it does not, a new storage area for that document's origin must be created, and then that is the `Document`'s assigned session storage area. A `Document`'s assigned storage area does not change during the lifetime of a `Document`.

In the case of an `iframe` being moved to another `Document`, the nested browsing context is destroyed and a new one created.

The `sessionStorage` attribute must return a `Storage` object associated with the `Document`'s assigned session storage area, if any, or null if there isn't one. Each `Document` object must have a separate object for its `Window`'s `sessionStorage` attribute.

When a new top-level browsing context is created by cloning an existing browsing context, the new browsing context must start with the same session storage areas as the original, but the two sets must from that point on be considered separate, not affecting each other in any way.

When a new top-level browsing context is created by a script in an existing browsing context, or by the user following a link in an existing browsing context, or in some other way related to a specific `Document`, and the creation is not a new start for session storage, then the session storage area of the origin of that `Document` must be copied into the new browsing context when it is created. From that point on, however, the two session storage areas must be considered separate, not affecting each other in any way.

When the `setItem()`, `removeItem()`, and `clear()` methods are called on a `Storage` object x that is associated with a session storage area, if the methods did not throw an exception or "do nothing" as defined above, then for every `Document` object whose `Window` object's `sessionStorage` attribute's `Storage` object is associated with the same storage area, other than x, send a storage notification.

#### 11.2.3 The `localStorage` attribute

    [NoInterfaceObject]
    interface WindowLocalStorage {
      readonly attribute Storage localStorage;
    };
    Window implements WindowLocalStorage;

The `localStorage` object provides a `Storage` object for an origin.

User agents must have a set of local storage areas, one for each origin.

User agents should expire data from the local storage areas only for security reasons or when requested to do so by the user. User agents should always avoid deleting data while a script that could access that data is running.

When the `localStorage` attribute is accessed, the user agent must run the following steps, which are known as the `Storage` object initialisation steps:

1. The user agent may throw a `SecurityError` exception and abort these steps instead of returning a `Storage` object if the request violates a policy decision (e.g. if the user agent is configured to not allow the page to persist data).

2. If the `Document`'s origin is not a scheme/host/port tuple, then throw a `SecurityError` exception and abort these steps.

3. Check to see if the user agent has allocated a local storage area for the origin of the `Document` of the `Window` object on which the attribute was accessed. If it has not, create a new storage area for that origin.

4. Return the `Storage` object associated with that origin's local storage area. Each `Document` object must have a separate object for its `Window`'s `localStorage` attribute.

When the `setItem()`, `removeItem()`, and `clear()` methods are called on a `Storage` object x that is associated with a local storage area, if the methods did not throw an exception or "do nothing" as defined above, then for every `Document` object whose `Window` object's `localStorage` attribute's `Storage` object is associated with the same storage area, other than x, send a storage notification.

Whenever the properties of a `localStorage` attribute's `Storage` object are to be examined, returned, set, or deleted, whether as part of a direct property access, when checking for the presence of a property, during property enumeration, when determining the number of properties present, or as part of the execution of any of the methods or attributes defined on the `Storage` interface, the user agent must first obtain the storage mutex.

#### 11.2.4 The `storage` event

The `storage` event is fired on a `Document`'s `Window` object when a storage area changes, as described in the previous two sections (for session storage, for local storage).

When a user agent is to send a storage notification for a `Document`, the user agent must queue a task to fire a trusted event with the name `storage`, which does not bubble and is not cancelable, and which uses the `StorageEvent` interface, at the `Document` object's `Window` object.

Such a `Document` object 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 task source for these tasks is the DOM manipulation task source.

If the event is being fired due to an invocation of the `setItem()` or `removeItem()` methods, the event must have its `key` attribute initialised to the name of the key in question, its `oldValue` attribute initialised to the old value of the key in question, or null if the key is newly added, and its `newValue` attribute initialised to the new value of the key in question, or null if the key was removed.

Otherwise, if the event is being fired due to an invocation of the `clear()` method, the event must have its `key`, `oldValue`, and `newValue` attributes initialised to null.

In addition, the event must have its `url` attribute initialised to the address of the document whose `Storage` object was affected; and its `storageArea` attribute initialised to the `Storage` object from the `Window` object of the target `Document` that represents the same kind of `Storage` area as was affected (i.e. session or local).

##### 11.2.4.1 The `StorageEvent` interface

    [Constructor(DOMString type, optional StorageEventInit eventInitDict)]
    interface StorageEvent : Event {
      readonly attribute DOMString? key;
      readonly attribute DOMString? oldValue;
      readonly attribute DOMString? newValue;
      readonly attribute DOMString url;
      readonly attribute Storage? storageArea;
    };

    dictionary StorageEventInit : EventInit {
      DOMString? key;
      DOMString? oldValue;
      DOMString? newValue;
      DOMString url;
      Storage? storageArea;
    };

The `key` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the key being changed.

The `oldValue` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the old value of the key being changed.

The `newValue` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the new value of the key being changed.

The `url` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to the empty string. It represents the address of the document whose key changed.

The `storageArea` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the `Storage` object that was affected.

#### 11.2.5 Threads

Because of the use of the storage mutex, multiple browsing contexts will be able to access the local storage areas simultaneously in such a manner that scripts cannot detect any concurrent script execution.

Thus, the `length` attribute of a `Storage` object, and the value of the various properties of that object, cannot change while a script is executing, other than in a way that is predictable by the script itself.

### 11.3 Disk space

User agents should limit the total amount of space allowed for storage areas, because hostile authors could otherwise use this feature to exhaust the user's available disk space.

User agents should guard against sites storing data under their origin's other affiliated sites, e.g. storing up to the limit in a1.example.com, a2.example.com, a3.example.com, etc, circumventing the main example.com storage limit.

User agents may prompt the user when quotas are reached, allowing the user to grant a site more space. This enables sites to store many user-created documents on the user's computer, for instance.

User agents should allow users to see how much space each domain is using.

A mostly arbitrary limit of five megabytes per origin is suggested. Implementation feedback is welcome and will be used to update this suggestion in the future.

For predictability, quotas should be based on the uncompressed size of data stored.

### 11.4 Privacy

#### 11.4.1 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 he authenticates 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 white-listing 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 authorise 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 blacklist ("delete this data and prevent this domain from ever storing data again"), the user can restrict the use of persistent storage to sites that he trusts.

Shared blacklists

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

  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.

#### 11.4.2 Sensitivity of data

User agents should treat persistently stored data as potentially sensitive; it's quite possible for e-mails, 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.

### 11.5 Security

#### 11.5.1 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.

#### 11.5.2 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.

#### 11.5.3 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.