What is same origin policy

Please note that they do not also permit cross-origin read. The difference between embedding and reading a resource is that when embedded, the resource is copied from the external origin and rendered locally, while reading the resource means their origin is preserved. Although same-origin policy is an important browser security feature that provides significant protection against malicious scripts, it is far from perfect.

In other cases the policy is too restrictive and tangles the workflow of the web application. Engineers introduced a standard called Cross-Origin Resource Sharing as a way to relax the same-origin policy's restrictions.

In this case, the browser adds an Origin: header describing the origin from where the request has been initiated. Once the request is received the server tells the browser if the CORS request is valid by appending the Access-Control-Allow-Origin header to the response. Here is an example to make things clear. It should look like this:. If the check fails, the response is blocked immediately.

Also, one important note is that the Access-Control-Allow-Origin header supports only a single origin. This means you cannot specify multiple websites as the value of this header e. This is a bad idea unless you want anyone to consume your restful API. Any request that is not considered Simple i. This is because the browser sends a preflight request before the original request to make sure that the original request is acceptable to the server.

Below is an example:. Once the server receives the preflight request it responds with headers which tell if the preflight request has been accepted:. Cross-Origin Resource Sharing , or CORS for short, is a mechanism for a website to partially opt-out of the same-origin policy in a controlled way.

You can also use a wildcard as the origin, but then Access-Control-Allow-Credentials below cannot be true. It's all or nothing, a complete wildcard or an exact origin. By default, CORS doesn't allow credentialed requests that include the browser user's cookies.

After all, credentialed CORS requests effectively give the websites to whom the privilege is granted full read and write control of the browser user's data in the application. If you still want to enable it, you can use the Access-Control-Allow-Credentials header like so:. In fact, even if you only want to allow, e. Simple requests with the whitelisted HTTP verbs, headers, and content-type are always sent.

Still, the website is forbidden access to the response data if the response doesn't contain the appropriate Access-Control-Allow-Origin header. But how does the browser know whether it is allowed to send a PUT request or not?

If the answer to the question "can I send a PUT request" is in response to the PUT request, doesn't this create a chicken and egg problem?

That's a great question, and the answer is simple: we send two requests. The same-origin policy is there to protect you, so think carefully before opting out of it. The same-origin policy is at the root of the web browser security model.

It's old, and it's not perfect. As such, developers must understand the risks and implement the proper defense measures in their applications. Generally, writing is allowed e. This means that without CSRF protection, websites are in trouble. This can have surprising and unpleasant effects if you're not careful when implementing them. Failure to do this may result in permission errors. These interactions are typically placed into three categories:. Use CORS to allow cross-origin access.

JavaScript APIs like iframe. 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. 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.

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. Skip to main content Skip to search Skip to select language Web technology for developers Web security Same-origin policy Change language.

Internet Explorer has two major exceptions to the same-origin policy: Trust Zones If both domains are in the highly trusted zone e. Port IE doesn't include port into same-origin checks. These exceptions are nonstandard and unsupported in any other browser. These interactions are typically placed into three categories: Cross-origin writes are typically allowed. Examples are links, redirects, and form submissions.