Take the 2-minute tour ×
Information Security Stack Exchange is a question and answer site for Information security professionals. It's 100% free, no registration required.

I can't really fully understand what same origin domain means. I know it means that when getting a resource from another domain (say a JS file) it will run from the context of the domain that serves it (like google analytics code), which means it can't modify the data or read the data on the domain that "includes the resource".

So if domain a.com is embedding a js file from google.com in it's source, that js will run from google.com and it can't access the DOM\cookies\any other element on a.com -- am I right?

Here is a definition for the same origin policy which I can't really understand:

The same-origin policy is a key mechanism implemented within browsers that is designed to keep content that came from different origins from interfering with each other. Basically, content received from one website is allowed to read and modify other content received from the same site but is not allowed to access content received from other sites.

What does that really mean? Can you please give me a real life example?

Another question is: what is the purpose of Origin header and how do cross domain requests still exist? Why doesn't it influence the security or the same origin policy?

share|improve this question
add comment

2 Answers

up vote 23 down vote accepted

Why is the same origin policy important?

Assume you are logged into Facebook and visit a malicious website in another browser tab. Without the same origin policy JavaScript on that website could do anything to your Facebook account that you are allowed to do. For example read private messages, post status updates, analyse the HTML DOM-tree after you entered your password before submitting the form.

But of course Facebook wants to use JavaScript to enhance the user experience. So it is important that the browser can detect that this JavaScript is trusted to access Facebook resources. That's where the same origin policy comes into play: If the JavaScript is included from a HTML page on facebook.com, it may access facebook.com resources.

Now replace Facebook with your online banking website, and it will be obvious that this is an issue.

What is the origin?

I can't really fully understand what same origin domain means. I know it means that when getting a resource from another domain (say a JS file) it will run from the context of the domain that serves it (like google analytics code), which means it can't modify the data or read the data on the domain that "includes the resource".

This is not correct: The origin of a JavaScript file is defined by the domain of the HTML page which includes it. So if you include the Google Analytics code with a <script>-tag, it can do anything to your website but does not have same origin permissions on the Google website.

How does cross domain communication work?

The same origin policy is not enforced for all requests. Among others the <script>- and <img>-tags may fetch resources from any domain. Posting forms and linking to other domains is possible, too. Frames and iframes way display information from other domains but interaction with that content is limited.

There are some approaches to allow XMLHttpRequest (ajax) calls to other domains in a secure way, but they are not well supported by common browsers. The common way to enable communication with another domain is JSONP:

It is based on a <script>-tag. The information, which shall be sent to another domain, is encoded in the URL as parameters. The returned JavaScript consists of a function call with the requested information as parameter:

<script type="text/javascript" src="http://example.com/
     ?some-variable=some-data&jsonp=parseResponse">
</script>

The dynamically generated JavaScript from example.com may look like:

parseResponse({"variable": "value", "variable2": "value2"})

What is Cross Site Scripting?

Cross Site Scripting is a vulnerability that allows an attacker to inject JavaScript code into a website, so that it originates from the attacked website from the browser point of view.

This can happen if user input is not sufficiently sanitised. For example a search function may display the string "Your search results for [userinput]". If [userinput] is not escaped an attacker may search for:

<script>alert(document.cookie)</script>

The browser has no way to detect that this code was not provided by the website owner, so it will execute it. Nowadays cross site scripting is a major issue, so there is work done to prevent this vulnerability. Most notable is the Content Security Policy approach.

share|improve this answer
    
This was incredibly helpful to me. Can you elaborate on the iFrame case? Can a parent access child variables and vice versa? (sameorigin, ignoring sandbox and restricted configurations) –  makerofthings7 Mar 29 '13 at 16:12
    
In my opinion Same Origin Policy is very useless. It only gave me trouble when I want to do cross domain GET requests, and it can't even block many XSS attacks because there are so many other ways to get around it. –  Derek 朕會功夫 Nov 25 '13 at 16:33
add comment

Don't worry. I found it tricky to wrap my head around too.

It turns out that Google Analytics can, in theory, do anything they want to your users. (<script> tags create an exception to same-origin policy restrictions)

That's one of the biggest reasons XSS attacks are a Bad Thing™ and one reason Mozilla designed Content Security Policy which is now on its way to being standardized. (WebKit's implementation of the HTML5 iframe sandbox attribute can also be helpful)

Because they couldn't get away with breaking backward compatibility, the same-origin policy is more about preventing things like <iframe> and XMLHttpRequest from tricks like using your "Remember Me" cookies or lying login dialogs to access your accounts. (The X-Frame-Options header allows you to lock frames down even further to protect against things like Clickjacking.)

The Origin header is used by a mechanism named "Cross-Origin Resource Sharing" which allows sites to grant limited exceptions to same-origin policy for safe cross-site interaction. (supported fully in all current browsers except Opera and Internet Explorer and partially in IE8+ using the proprietary XDomainRequest object which omits cookies)

Basically, when you try to make an XMLHttpRequest to a different domain, the browser will do one of two things:

  1. If it's a GET or POST request which meets certain limits (which the makers of the standard have determined to add no extra risk for CSRF attacks) then the browser just passes the request through.

  2. Otherwise, it does what's called a "preflighted request", where it first sends an OPTIONS request instead and only does what you requested if the checks pass for the OPTIONS request.

In either case, the browser appends an Origin header which tells the target site who's calling. (It's sort of like the Referer header, but required and more strictly specified to ensure proper security)

The server on the receiving end (the one which relies heavily on the same-origin policy for protection) can then authorize the request by including an Access-Control-Allow-Origin header which contains either * or the value of the request's Origin header.

If it doesn't, the browser simply discards the response and returns an error to the Javascript callback. MDN goes into great detail (1) (2) on what goes on under the hood in both scenarios as well as what other headers the target system can set to further relax the security in a controlled fashion. (eg. Allowing access to custom HTTP headers you're setting)

GET requests and the allowed subset of POST requests can already be used for CSRF attacks via other mechanisms unless the target web app is properly protected, so it was decided that there was no benefit to doubling the number of HTTP requests involved in operating services like the Google Font Library.

share|improve this answer
    
It's worth noting that CORS wasn't supported by MSFT and Opera because they believed the complexity of CORS was best solved by an authentication token between sites, and not embedded in HTTP headers. Not to mention that CORS allows editing of the header which may cause complications with CSRF token validation. –  makerofthings7 Mar 30 '13 at 4:22
    
+1 You're the only one who mentioned`XDomainRequest` in this site. Have you heard anything about UMP (It's simple like XDomainRequest, and from W3C) –  makerofthings7 Mar 30 '13 at 4:25
    
No. I tend to focus on things that are or may eventually be of practical use to me and UMP never reached critical mass for it to come to my attention until you mentioned it. As far as I can tell, the widespread implementation of CORS seems to have stolen all its inertia. –  ssokolow Mar 31 '13 at 6:36
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.