The same-origin policy is a critical security mechanism that restricts how a document or script loaded from one origin can interact with a resource from another origin. It helps to isolate potentially malicious documents, reducing possible attack vectors.

Definition of an origin

Two pages have the same origin if the protocol, port (if one is specified), and host are the same for both pages. You'll see this referred to as the "scheme/host/port tuple" or just "tuple" at times (where a "tuple" is a set of three components that together comprise a whole).

The following table gives examples of origin comparisons to the URL

URL Outcome Reason Success Success Failure Different protocol Failure Different port Failure Different host

See also origin definition for file: URLs.

Inherited origins

Scripts executed from pages with an about:blank or javascript: URL inherit the origin of the document that opened that URL, since these types of URLs do not explicitly contain information about the server of origin. For example, about:blank is often used as a URL of a new, empty popup window into which the parent script writes content (e.g. via the mechanism). If this popup window were to also contain code, that code would inherit the same origin as the script that created it. data: URLs get a new, empty, security context.

Exceptions in Internet Explorer

Internet Explorer has two major exceptions when it comes to same origin policy

  • Trust Zones: if both domains are in highly trusted zone e.g, corporate domains, then the same origin limitations are not applied
  • Port: IE doesn't include port into Same Origin components, therefore and are considered from the same origin and no restrictions are applied.

These exceptions are non-standard and not supported in any other browser but would be helpful if developing an app for Windows RT (or) IE based web application.

Changing origin

A page may change its own origin with some limitations. A script can set the value of document.domain to its current domain or a superdomain of its current domain. If it sets it to a superdomain of its current domain, the shorter domain is used for subsequent origin checks. For example, assume a script in the document at executes the following statement:

document.domain = "";

After that statement executes, the page can pass the origin check with (assuming sets its document.domain to "" to indicate that it wishes to allow that - see document.domain for more). However, could not set document.domain to since that is not a superdomain of

The port number is kept separately by the browser. Any call to the setter, including document.domain = document.domain causes the port number to be overwritten with null. Therefore one cannot make talk to by only setting document.domain = "" in the first. It has to be set in both so that port numbers are both null.

Note: When using document.domain to allow a subdomain to access its parent securely, you need to set document.domain to the same value in both the parent domain and the subdomain. This is necessary even if doing so is simply setting the parent domain back to its original value. Failure to do this may result in permission errors.

Cross-origin network access

The same-origin policy controls interactions between two different origins, such as when you use XMLHttpRequest or an <img> element. These interactions are typically placed into three categories:

  • Cross-origin writes are typically allowed. Examples are links, redirects and form submissions. Certain rarely used HTTP requests require preflight.
  • Cross-origin embedding is typically allowed. Examples are listed below.
  • Cross-origin reads are typically not allowed, but read access is often leaked by embedding. For example, you can read the width and height of an embedded image, the actions of an embedded script, or the availability of an embedded resource.

Here are some examples of resources which may be embedded cross-origin:

  • JavaScript with <script src="..."></script>. Error messages for syntax errors are only available for same-origin scripts.
  • CSS with <link rel="stylesheet" href="...">. Due to the relaxed syntax rules of CSS, cross-origin CSS requires a correct Content-Type header. Restrictions vary by browser: IE, Firefox, Chrome, Safari (scroll down to CVE-2010-0051) and Opera.
  • Images with <img>. Supported image formats include PNG, JPEG, GIF, BMP, SVG, ...
  • Media files with <video> and <audio>.
  • Plug-ins with <object>, <embed> and <applet>.
  • Fonts with @font-face. Some browsers allow cross-origin fonts, others require same-origin fonts.
  • Anything with <frame> and <iframe>. A site can use the X-Frame-Options header to prevent this form of cross-origin interaction.

How to allow cross-origin access

Use CORS to allow cross-origin access. CORS is part of HTTP that lets servers share with clients information about what hosts are permitted to load within content loaded from that server.

How to block cross-origin access

  • To prevent cross-origin writes, check for an unguessable token in the request, known as a Cross-Site Request Forgery (CSRF) token. You must prevent cross-origin reads of pages that know this token.
  • To prevent cross-origin reads of a resource, ensure that it is not embeddable. It is often necessary to prevent embedding because embedding a resource always leaks some information about it.
  • To prevent cross-origin embedding, ensure that your resource cannot be interpreted as one of the embeddable formats listed above. The browser does not respect the Content-Type in most cases. For example, if you point a <script> tag at an HTML document, the browser will try to parse the HTML as JavaScript. When your resource is not an entry point to your site, you can also use a CSRF token to prevent embedding.

Cross-origin script API access

JavaScript APIs such as iframe.contentWindow, window.parent, and window.opener allow documents to directly reference each other. When the two documents do not have the same origin, these references provide very limited access to Window and Location objects, as described in the next two sections.

To communicate further between documents from different origins, use window.postMessage.



The following cross-origin access to Window properties is allowed:

window.closed Read only.
window.frames Read only.
window.length Read only.
window.location Read/write.
window.opener Read only.
window.parent Read only.
window.self Read only. Read only.
window.window Read only.

Some browsers allow access to more properties than the specification allows.


The following cross-origin access to Location properties is allowed:

URLUtils.href Write only.

Some browsers allow access to more properties than the specification allows.

Cross-origin data storage access

Access to data stored in the browser such as localStorage and IndexedDB are separated by origin. Each origin gets its own separate storage, and JavaScript in one origin cannot read from or write to the storage belonging to another origin.

Cookies use a separate definition of origins. A page can set a cookie for its own domain or any parent domain, as long as the parent domain is not a public suffix. Firefox and Chrome use the Public Suffix List to determine if a domain is a public suffix. Internet Explorer uses its own internal method to determine if a domain is a public suffix. The browser will make a cookie available to the given domain including any sub-domains, no matter which protocol (HTTP/HTTPS) or port is used. When you set a cookie, you can limit its availability using the Domain, Path, Secure and Http-Only flags. When you read a cookie, you cannot see from where it was set. Even if you use only secure https connections, any cookie you see may have been set using an insecure connection.

See also

Original Document Information

  • Author(s): Jesse Ruderman