User:Kebrey/sandbox

In computing, the same-origin policy is an important concept in the web application security model. Under the policy, a web browser permits scripts contained in a first web page to access data in a second web page, but only if both web pages have the same origin. An origin is defined as a combination of URI scheme, hostname, and port number. This policy prevents a malicious script on one page from obtaining access to sensitive data on another web page through that page's Document Object Model.

This mechanism bears a particular significance for modern web applications that extensively depend on HTTP cookies 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.

History

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 root JavaScript object .

Implementation

All modern browsers implement some form of the Same-Origin Policy as it is an important security cornerstone. The policies are not required to match an exact specification 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

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.

URLs are only one type of URI. To illustrate, the following table gives an overview of typical outcomes for checks against the URL "http://www.example.com/dir/page.html".

Compared URL	Outcome	Reason
http://www.example.com/dir/page2.html	Success	Same protocol, host and port
http://www.example.com/dir2/other.html	Success	Same protocol, host and port
http://username:password@www.example.com/dir2/other.html	Success	Same protocol, host and port
http://www.example.com:81/dir/other.html	Failure	Same protocol and host but different port
https://www.example.com/dir/other.html	Failure	Different protocol
http://en.example.com/dir/other.html	Failure	Different host
http://example.com/dir/other.html	Failure	Different host (exact match required)
http://v2.www.example.com/dir/other.html	Failure	Different host (exact match required)
http://www.example.com:80/dir/other.html	Depends	Port explicit. Depends on implementation in browser.

Most objects (also known as application programming interfaces or APIs) exposed by the user agent are available only to the same origin. Specifically, content retrieved from one URI can access objects associated with content retrieved from another URI if, and only if, the two URIs belong to the same origin, e.g., have the same scheme, host, and port. There are some exceptions to this general rule. For example, some parts of HTML's Location interface are available across origins (e.g., to allow for navigating other browsing contexts). As another example, HTML's postMessage interface is visible across origins explicitly to facilitate cross-origin communication. Exposing objects to foreign origins is dangerous and should be done only with great care because doing so exposes these objects to potential attackers.

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

IE Exceptions

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 http://company.com:81/index.html and http://company.com/index.html are considered from 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.

Inherited origins

Content from about:blank and javascript: URLs inherits the origin from the document that loaded the URL, since the URL itself does not give any information about the origin. data: URLs get a new, empty, security context.

Security Applications

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

In some circumstances the same-origin policy is too restrictive, posing problems for large websites that use multiple subdomains. Here are some techniques for relaxing it:

document.domain property

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 orders.example.com and catalog.example.com might set their document.domain properties to “example.com”, thereby making the documents appear to have the same origin and enabling each document to read properties of the other. This might not always work as the port stored in the internal representation can become marked as null. In other words, example.com port 80 will become example.com port null because we update document.domain. Port null might not be treated as 80 (depending on your browser) and hence might fail or succeed depending on your browser.

Cross-Origin Resource Sharing

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. 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

Another new 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

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.

WebSockets

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 and exceptions

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.

In addition, many legacy cross-domain operations predating JavaScript are not subjected to same-origin checks; one such example is the ability to include scripts across domains, or submit POST forms.

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.

Workarounds

To enable developers to, in a controlled manner, circumvent the same-origin policy, a number of "hacks" such as using the fragment identifier or the window.name property have been used to pass data between documents residing in different domains. With the HTML5 standard, a method was formalized for this: the postMessage interface, which is only available on recent browsers. JSONP can also be used to enable Ajax-like calls to other domains.

See also

• Cross-origin resource sharing (CORS)
• Cross-site scripting (XSS)
• Cross-site request forgery (CSRF)
• Cross-document messaging
• Content Security Policy

External links

• A detailed comparison of several flavors of same-origin policies
• A review of deficiencies in same-origin policies and their implication for web security at the Wayback Machine (archived February 11, 2007)
• Sample vendor-provided same-origin policy specification
• The HTML5 spec's definition of Origin
• W3C Article on the Same Origin Policy
• RFC 6454 on The Web Origin Concept
• Blog post: The Cookie Same Origin Policy