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 isolate potentially malicious documents, reducing possible attack vectors.

Definition of an origin

Two URLs have the same origin if the protocol, port (if specified), and host are the same for both. You may see this referenced as the "scheme/host/port tuple", or just "tuple". (A "tuple" is a set of items that together comprise a whole — a generic form for double/triple/quadruple/quintuple/etc.)

The following table gives examples of origin comparisons with the URL http://store.company.com/dir/page.html:

URL Outcome Reason
http://store.company.com/dir2/other.html Same origin Only the path differs
http://store.company.com/dir/inner/another.html Same origin Only the path differs
https://store.company.com/page.html Failure Different protocol
http://store.company.com:81/dir/page.html Failure Different port (http:// is port 80 by default)
http://news.company.com/dir/page.html Failure Different host

See also origin definition for file: URLs, because their comparison is more complicated.

Inherited origins

Scripts executed from pages with an about:blank or javascript: URL inherit the origin of the document containing that URL, since these types of URLs do not contain information about an origin server.

For example, about:blank is often used as a URL of new, empty popup windows into which the parent script writes content (e.g. via the Window.open() mechanism). If this popup also contains JavaScript, that script 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 to the same-origin policy:

Trust Zones
If both domains are in the highly trusted zone (e.g. corporate intranet domains), then the same-origin limitations are not applied.
Port
IE doesn't include port into same-origin checks. Therefore, https://company.com:81/index.html and https://company.com/index.html are considered the same origin and no restrictions are applied.

These exceptions are nonstandard and unsupported in any other browser.

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 set to a superdomain of the current domain, the shorter superdomain is used for same-origin checks.

For example, assume a script from the document at http://store.company.com/dir/other.html executes the following:

document.domain = "company.com";

Afterward, the page can pass the same-origin check with http://company.com/dir/page.html (assuming http://company.com/dir/page.html sets its document.domain to "company.com" to indicate that it wishes to allow that - see document.domain for more). However, company.com could not set document.domain to othercompany.com, since that is not a superdomain of company.com.

The port number is checked separately by the browser. Any call to document.domain, including document.domain = document.domain, causes the port number to be overwritten with null. Therefore, one cannot make company.com:8080 talk to company.com by only setting document.domain = "company.com" in the first. It has to be set in both so their 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. Some HTTP requests require preflight.
  • Cross-origin embedding is typically allowed. (Examples are listed below.)
  • Cross-origin reads are typically disallowed, but read access is often leaked by embedding. For example, you can read the dimensions 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 details for syntax errors are only available for same-origin scripts.
  • CSS applied 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 displayed by <img>.
  • Media played by <video> and <audio>.
  • Plugins embedded with <object>, <embed>, and <applet>.
  • Fonts applied with @font-face. Some browsers allow cross-origin fonts, others require same-origin.
  • Anything embedded by <frame> and <iframe>. Sites can use the X-Frame-Options header to prevent cross-origin framing.

How to allow cross-origin access

Use CORS to allow cross-origin access. CORS is a part of HTTP that lets servers specify what hosts are permitted to load content from that server.

How to block cross-origin access

  • To prevent cross-origin writes, check an unguessable token in the request — known as a Cross-Site Request Forgery (CSRF) token. You must prevent cross-origin reads of pages that require 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 embeds, ensure that your resource cannot be interpreted as one of the embeddable formats listed above. Browsers may not respect the Content-Type header. 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 like iframe.contentWindow, window.parent, window.open, and window.opener allow documents to directly reference each other. When 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 between documents from different origins, use window.postMessage.

Specification: HTML Living Standard § Cross-origin objects.

Window

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

Methods
window.blur
window.close
window.focus
window.postMessage
Attributes  
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.
window.top Read only.
window.window Read only.

Some browsers allow access to more properties than the above.

Location

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

Methods
location.replace
Attributes  
URLUtils.href Write-only.

Some browsers allow access to more properties than the above.

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