Same-origin policy

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This mechanism bears a particular significance for modern web applications that extensively depend on HTTP cookies[1] to maintain authenticated user sessions, as servers act based on the HTTP cookie information to reveal sensitive information or take state-changing actions. A strict separation between content provided by unrelated sites must be maintained on the client-side to prevent the loss of data confidentiality or integrity.

The same-origin policy mainly applies to data access from scripts; embedding resources across origins, such as images, CSS and scripts via the corresponding HTML tags is not restricted[2] (with fonts being a notable exception[3]).


The concept of same-origin policy dates back to Netscape Navigator 2 in 1995. The policy was originally designed to protect access to the Document Object Model, but has since been broadened to protect sensitive parts of the global JavaScript object.[4]


All modern browsers implement some form of the Same-Origin Policy as it is an important security cornerstone.[5] The policies are not required to match an exact specification [6] but are often extended to define roughly compatible security boundaries for other web technologies, such as Microsoft Silverlight, Adobe Flash, or Adobe Acrobat, or for mechanisms other than direct DOM manipulation, such as XMLHttpRequest.

Origin determination rules[edit]

The algorithm used to calculate the "origin" of a URI is specified in RFC 6454, Section 4. For absolute URIs, the origin is the triple {protocol, host, port}. If the URI does not use a hierarchical element as a naming authority (see RFC 3986, Section 3.2) or if the URI is not an absolute URI, then a globally unique identifier is used. Two resources are considered to be of the same origin if and only if all these values are exactly the same.

To illustrate, the following table gives an overview of typical outcomes for checks against the URL "".

Compared URL Outcome Reason Success Same protocol, host and port Success Same protocol, host and port Success Same protocol, host and port Failure Same protocol and host but different port Failure Different protocol Failure Different host Failure Different host (exact match required) Failure Different host (exact match required) Depends Port explicit. Depends on implementation in browser.

Unlike other browsers, Internet Explorer does not include the port in the calculation of the origin, using the Security Zone in its place.[7]

Security Applications[edit]

The same-origin policy helps protect sites that use authenticated sessions. The following example illustrates a potential security risk that could arise without the same-origin policy. Assume that a user is visiting a banking website and doesn't log out. Then, the user goes to another site that has some malicious JavaScript code running in the background that requests data from the banking site. Because the user is still logged in on the banking site, the malicious code could do anything the user could do on the banking site. For example, it could get a list of the user's last transactions, create a new transaction, etc. This is because the browser can send and receive session cookies to the banking site based on the domain of the banking site.

The user visiting the malicious site would expect that the site he or she is visiting has no access to the banking session cookie. While it is true that the JavaScript has no direct access to the banking session cookie, it could still send and receive requests to the banking site with the banking site's session cookie. Because the script can essentially do the same as the user would do, even CSRF protections by the banking site would not be effective.

Relaxing the same-origin policy[edit]

In some circumstances, the same-origin policy is too restrictive, posing problems for large websites that use multiple subdomains. At first, a number of workarounds such as using the fragment identifier or the property were used to pass data between documents residing in different domains. Modern browsers support multiple techniques for relaxing the same-origin policy in a controlled manner:

document.domain property[edit]

If two windows (or frames) contain scripts that set domain to the same value, the same-origin policy is relaxed for these two windows, and each window can interact with the other. For example, cooperating scripts in documents loaded from and might set their document.domain properties to “”, thereby making the documents appear to have the same origin and enabling each document to read properties of the other. Setting this property implicitly sets the port to null, which most browsers will interpret differently from port 80 or even an unspecified port. To assure that access will be allowed by the browser, set the document.domain property of both pages.[8]

Cross-Origin Resource Sharing[edit]

The second technique for relaxing the same-origin policy is standardized under the name Cross-Origin Resource Sharing. This standard extends HTTP with a new Origin request header and a new Access-Control-Allow-Origin response header.[9] It allows servers to use a header to explicitly list origins that may request a file or to use a wildcard and allow a file to be requested by any site. Browsers such as Firefox 3.5, Safari 4 and Internet Explorer 10 use this header to allow the cross-origin HTTP requests with XMLHttpRequest that would otherwise have been forbidden by the same-origin policy.

Cross-document messaging[edit]

Another technique, cross-document messaging allows a script from one page to pass textual messages to a script on another page regardless of the script origins. Calling the postMessage() method on a Window object asynchronously fires an "onmessage" event in that window, triggering any user-defined event handlers. A script in one page still cannot directly access methods or variables in the other page, but they can communicate safely through this message-passing technique.


JSONP allows a page to receive JSON data from a different domain by adding a <script> element to the page which loads a JSON response with a callback from different domain.


Modern browsers will permit a script to connect to a WebSocket address without applying the same-origin policy. However, they recognize when a WebSocket URI is used, and insert an Origin: header into the request that indicates the origin of the script requesting the connection. To ensure cross-site security, the WebSocket server must compare the header data against a whitelist of origins permitted to receive a reply.

Corner cases[edit]

The behavior of same-origin checks and related mechanisms is not well-defined in a number of corner cases such as for pseudo-protocols that do not have a clearly defined host name or port associated with their URLs (file:, data:, etc.). This historically caused a fair number of security problems, such as the generally undesirable ability of any locally stored HTML file to access all other files on the disk, or communicate with any site on the Internet.

Lastly, certain types of attacks, such as DNS rebinding or server-side proxies, permit the host name check to be partly subverted, and make it possible for rogue web pages to directly interact with sites through addresses other than their "true", canonical origin. The impact of such attacks is limited to very specific scenarios, since the browser still believes that it is interacting with the attacker's site, and therefore does not disclose third-party cookies or other sensitive information to the attacker.

See also[edit]


  1. ^ IETF HTTP State Management Mechanism, Apr, 2011
  2. ^ Kemp, John (2011-02-04). "Security on the Web". Retrieved 2018-07-24. The same-origin policy states that a document from one unique origin may only load resources from the origin from which the document was loaded. In particular this applies to XMLHttpRequest calls made from within a document. Images, CSS and dynamically-loaded scripts are not subject to same-origin policy. 
  3. ^ "@font-face". MDN Web Docs. Retrieved 2018-07-24. Web fonts are subject to the same domain restriction (font files must be on the same domain as the page using them), unless HTTP access controls are used to relax this restriction. 
  4. ^ "The Tangled Web". Network Security. 2012 (4). doi:10.1016/s1353-4858(12)70022-3. 
  5. ^ "Browser Security Handbook, part 2". Retrieved 31 January 2014. 
  6. ^ "Same Origin Policy". W3C. Retrieved 31 January 2014. 
  7. ^ Lawrence, Eric. "IEInternals - Same Origin Policy Part 1". Retrieved 22 October 2013. 
  8. ^ LePera, Scott. "Cross-domain security woes". The Strange Zen Of JavaScript. Retrieved 4 April 2014. 
  9. ^ Creating WSGI Middleware

External links[edit]