About this specification

This specification is like no other — it has been processed with you, the humble web developer, in mind.

The focus of this specification is readability and ease of access. Unlike the full HTML Standard, this "developer's edition" removes information that only browser vendors need know. It is automatically produced from the full specification by our build tooling, and thus always in sync with the latest developments in HTML.

To read about its conception, construction, and future, read the original press release, and the blog post about its relaunch.

Finally, feel free to contribute on GitHub to make this edition better for everyone!

    1. 7.3 Origin
      1. 7.3.1 Relaxing the same-origin restriction
    2. 7.4 Sandboxing

7.3 Origin

Origins are the fundamental currency of the Web's security model. Two actors in the Web platform that share an origin are assumed to trust each other and to have the same authority. Actors with differing origins are considered potentially hostile versus each other, and are isolated from each other to varying degrees.

For example, if Example Bank's Web site, hosted at bank.example.com, tries to examine the DOM of Example Charity's Web site, hosted at charity.example.org, a "SecurityError" DOMException will be raised.

An origin is one of the following:

An opaque origin

An internal value, with no serialization it can be recreated from (it is serialized as "null" per serialization of an origin), for which the only meaningful operation is testing for equality.

A tuple origin

A tuple consists of:

Origins can be shared, e.g., among multiple Document objects. Furthermore, origins are generally immutable. Only the domain of a tuple origin can be changed, and only through the document.domain API.

The effective domain of an origin origin is computed as follows:

  1. If origin is an opaque origin, then return null.

  2. If origin's domain is non-null, then return origin's domain.

  3. Return origin's host.

7.3.1 Relaxing the same-origin restriction

document . domain [ = domain ]

Returns the current domain used for security checks.

Can be set to a value that removes subdomains, to change the origin's domain to allow pages on other subdomains of the same domain (if they do the same thing) to access each other. (Can't be set in sandboxed iframes.)

The document.domain attribute is used to enable pages on different hosts of a domain to access each other's DOMs.

Do not use the document.domain attribute when using shared hosting. If an untrusted third party is able to host an HTTP server at the same IP address but on a different port, then the same-origin protection that normally protects two different sites on the same host will fail, as the ports are ignored when comparing origins after the document.domain attribute has been used.

7.4 Sandboxing

A sandboxing flag set is a set of zero or more of the following flags, which are used to restrict the abilities that potentially untrusted resources have:

The sandboxed navigation browsing context flag

This flag prevents content from navigating browsing contexts other than the sandboxed browsing context itself (or browsing contexts further nested inside it), auxiliary browsing contexts (which are protected by the sandboxed auxiliary navigation browsing context flag defined next), and the top-level browsing context (which is protected by the sandboxed top-level navigation without user activation browsing context flag and sandboxed top-level navigation with user activation browsing context flag defined below).

If the sandboxed auxiliary navigation browsing context flag is not set, then in certain cases the restrictions nonetheless allow popups (new top-level browsing contexts) to be opened. These browsing contexts always have one permitted sandboxed navigator, set when the browsing context is created, which allows the browsing context that created them to actually navigate them. (Otherwise, the sandboxed navigation browsing context flag would prevent them from being navigated even if they were opened.)

The sandboxed auxiliary navigation browsing context flag

This flag prevents content from creating new auxiliary browsing contexts, e.g. using the target attribute or the window.open() method.

The sandboxed top-level navigation without user activation browsing context flag

This flag prevents content from navigating their top-level browsing context and prevents content from closing their top-level browsing context. It is consulted only from algorithms that are not triggered by user activation.

When the sandboxed top-level navigation without user activation browsing context flag is not set, content can navigate its top-level browsing context, but other browsing contexts are still protected by the sandboxed navigation browsing context flag and possibly the sandboxed auxiliary navigation browsing context flag.

The sandboxed top-level navigation with user activation browsing context flag

This flag prevents content from navigating their top-level browsing context and prevents content from closing their top-level browsing context. It is consulted only from algorithms that are triggered by user activation.

As with the sandboxed top-level navigation without user activation browsing context flag, this flag only affects the top-level browsing context; if it is not set, other browsing contexts might still be protected by other flags.

The sandboxed plugins browsing context flag

This flag prevents content from instantiating plugins, whether using the embed element, the object element, or through navigation of a nested browsing context, unless those plugins can be secured.

The sandboxed origin browsing context flag

This flag forces content into a unique origin, thus preventing it from accessing other content from the same origin.

This flag also prevents script from reading from or writing to the document.cookie IDL attribute, and blocks access to localStorage.

The sandboxed forms browsing context flag

This flag blocks form submission.

The sandboxed pointer lock browsing context flag

This flag disables the Pointer Lock API. [POINTERLOCK]

The sandboxed scripts browsing context flag

This flag blocks script execution.

The sandboxed automatic features browsing context flag

This flag blocks features that trigger automatically, such as automatically playing a video or automatically focusing a form control.

The sandboxed storage area URLs flag

This flag prevents URL schemes that use storage areas from being able to access the origin's data.

The sandboxed document.domain browsing context flag

This flag prevents content from using the document.domain setter.

The sandbox propagates to auxiliary browsing contexts flag

This flag prevents content from escaping the sandbox by ensuring that any auxiliary browsing context it creates inherits the content's active sandboxing flag set.

The sandboxed modals flag

This flag prevents content from using any of the following features to produce modal dialogs:

The sandboxed orientation lock browsing context flag

This flag disables the ability to lock the screen orientation. [SCREENORIENTATION]

The sandboxed presentation browsing context flag

This flag disables the Presentation API. [PRESENTATION]

When the user agent is to parse a sandboxing directive, given a string input, a sandboxing flag set output, it must run the following steps:

  1. Split input on ASCII whitespace, to obtain tokens.

  2. Let output be empty.

  3. Add the following flags to output:

Every top-level browsing context has a popup sandboxing flag set, which is a sandboxing flag set. When a browsing context is created, its popup sandboxing flag set must be empty. It is populated by the rules for choosing a browsing context.

Every browsing context that is a nested browsing context has an iframe sandboxing flag set, which is a sandboxing flag set. Which flags in a nested browsing context's iframe sandboxing flag set are set at any particular time is determined by the iframe element's sandbox attribute.

Every Document has an active sandboxing flag set, which is a sandboxing flag set. When the Document is created, its active sandboxing flag set must be empty. It is populated by the navigation algorithm.

Every resource that is obtained by the navigation algorithm has a forced sandboxing flag set, which is a sandboxing flag set. A resource by default has no flags set in its forced sandboxing flag set, but other specifications can define that certain flags are set.

In particular, the forced sandboxing flag set is used by Content Security Policy. [CSP]

To implement the sandboxing for a Document object document, populate document's active sandboxing flag set with the union of the flags that are present in the following sandboxing flag sets: