Same-origin policy

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.

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.

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.

The algorithm used to calculate the "origin" of a URI is specified in , 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 , 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.

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