Same-origin policy

The same-origin policy is a critical security mechanism that restricts how a document or script loaded by one origin can interact with a resource from another origin.

It helps isolate potentially malicious documents, reducing possible attack vectors. For example, it prevents a malicious website on the Internet from running JS in a browser to read data from a third-party webmail service (which the user is signed into) or a company intranet (which is protected from direct access by the attacker by not having a public IP address) and relaying that data to the attacker.

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

URL Outcome Reason Same origin Only the path differs Same origin Only the path differs Failure Different protocol Failure Different port (http:// is port 80 by default) Failure Different host

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 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.

File origins

Modern browsers usually treat the origin of files loaded using the file:/// schema as opaque origins. What this means is that if a file includes other files from the same folder (say), they are not assumed to come from the same origin, and may trigger CORS errors.

Note that the URL specification states that the origin of files is implementation-dependent, and some browsers may treat files in the same directory or subdirectory as same-origin even though this has security implications.

Changing origin

Warning: The approach described here (using the document.domain setter) is deprecated because it undermines the security protections provided by the same origin policy, and complicates the origin model in browsers, leading to interoperability problems and security bugs.

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 executes the following:

document.domain = "";

Afterward, the page can pass the same-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 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 talk to by only setting document.domain = "" in the first. It has to be set in both so their port numbers are both null.

The mechanism has some limitations. For example, it will throw a "SecurityError" DOMException if the document-domain Permissions-Policy is enabled or the document is in a sandboxed <iframe>, and changing the origin in this way does not affect the origin checks used by many Web APIs (e.g. localStorage, indexedDB, BroadcastChannel, SharedWorker). A more exhaustive list of failure cases can be found in Document.domain > Failures.

Note: When using document.domain to allow a subdomain to access its parent, 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 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 fetch() 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. Browsers block stylesheet loads if it is a cross-origin load where the MIME type is incorrect and the resource does not start with a valid CSS construct.
  • Images displayed by <img>.
  • Media played by <video> and <audio>.
  • External resources embedded with <object> and <embed>.
  • Fonts applied with @font-face. Some browsers allow cross-origin fonts, others require same-origin.
  • Anything embedded by <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 any other hosts from which a browser should permit loading of content.

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,, 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.


The following cross-origin access to these 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 above.


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

location.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 Web Storage 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. When you set a cookie, you can limit its availability using the Domain, Path, Secure, and HttpOnly 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