6.3 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 exception will be raised.


The origin of a resource and the effective script origin of a resource are both either opaque identifiers or tuples consisting of a scheme component, a host component, a port component, and optionally extra data.

The extra data could include the certificate of the site when using encrypted connections, to ensure that if the site's secure certificate changes, the origin is considered to change as well.

6.3.1 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 effective script origin to allow pages on other subdomains of the same domain (if they do the same thing) to access each other.

The domain attribute is used to enable pages on different hosts of a domain to access each others' 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.

6.4 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 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, the window.open() method, or the showModalDialog() method.

The sandboxed top-level navigation browsing context flag

This flag prevents content from navigating their top-level browsing context and prevents content from closing their top-level browsing context.

When the sandboxed top-level navigation 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 plugins browsing context flag

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

The sandboxed seamless iframes flag

This flag prevents content from using the seamless attribute on descendant iframe elements.

This prevents a page inserted using the allow-same-origin keyword from using a CSS-selector-based method of probing the DOM of other pages on the same site (in particular, pages that contain user-sensitive information).

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. [WEBSTORAGE]

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 fullscreen browsing context flag

This flag prevents content from using the requestFullscreen() method.

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

  1. Split input on spaces, 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 given a browsing context name.

Every 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 the Content Security Policy specification. [CSP]


When a user agent is to implement the sandboxing for a Document, it must populate Document's active sandboxing flag set with the union of the flags that are present in the following sandboxing flag sets at the time the Document object is created: