The same-origin policy is one of the cornerstones in the web application security model. It allows Javascript running in a webpage to access data inside an iframe (or another browser window that it opened) only if the URL loaded in that iframe/window is part of the same origin. Strictly speaking, an “origin” in this context is defined as the triple of scheme/protocol, hostname and port number. If all three are the same among two URIs; then those two URIs belong to the same origin.

If Javascript was able to access data inside an iframe from another origin, then a malicious webpage could just load in an iframe and then reach into the DOM of that iframe to steal the session cookie, using:


This also assumes that the cookie wasn’t marked with the HttpOnly flag, of course. Even if all cookies were HttpOnly, it would still be unacceptable that a malicious site could load your Internet-bank page in an iframe and then read out your account balance, VISA-card number etc from the iframe DOM (assuming said Internet-bank had a liberal “Keep me logged in” feature). The same origin policy is the security mechanism that prevents the above by making sure that the iframe DOM can only be accessed if the iframe URL points to the same origin as the parent page.

The same origin policy also imposes limits on the fetch() (or XMLHttpRequest) operations when they target an URL on another origin; for these cases the browser will make the HTTP request but it will not give the response data back to the Javascript that triggered the operation.

If you think this sounds odd, think about how to browser works outside of XHR. For example, when the HTML at loads an image from, the browser automatically attaches any cookies to the request. This means that if an attacker can trick a victim into viewing HTML that the attacker wrote, then regardless of what origin hosts that HTML code, the attacker will be able to do authenticated HTTP GET requests using the victims credentials. The same is true for HTTP POST requests because HTML forms can be created and submitted by Javascript, and cookies are automatically attached to such cross-origin POST requests as well. This type of attack is called Cross-Site Request Forgery (CSRF).

The above alone (without involving XHR) is already a huge problem, which takes a lot of effort to guard against (using CSRF tokens etc). However, it would be even worse if XHR did the same thing and also provided access to the response text because then we would be back to the iframe case in the beginning: the attacker could write HTML that loaded using XHR and then read the account balance and VISA-card number etc from the response text. While XHR and fetch does not automatically attach cookies, the attacker can simply set myReq.withCredentials = true; on the XHR before submitting it, or equivalently pass { credentials: 'include' } in the init options parameter to fetch(). So for all intents are purposes, any third party can make authenticated (in the cookie-only sense) XHR requests to your website whenever one of your customers visits the third-party website; but the browser will prevent them from reading the response.

This type of same origin checks (i.e. not allowing the XHR response to be read when the XHR is cross-origin, but allowing the response to be read if the XHR does target the same origin) have to be done in every part of the web platform. For example, imagine a system administration dashboard running at that has a dynamically generated image (generated server-side) that shows the physical location of all their users as dots on a map. Now, assume that an attacker tricks the system administrator into opening and that this page:

  • creates an IMG element with src set to (the browser will make an HTTP GET request for this image and it will include the regular dashboard session cookie proving that the user is currently logged-in)
  • creates an HTML5 canvas and draws the IMG element onto it using .drawImage()
  • calls toDataURL() on the canvas to convert its pixel data into string representation imgData
  • makes an XHR call to so that the user location data can be stored on the attacker server

The reason why the above doesn’t work is because whenever a cross-origin image is drawn into a canvas, the browser marks that canvas as “tainted”, and whenever you call toDataURL() on a tainted canvas, it will throw a security exception. Corresponding mechanisms exists in many other parts of the browser, for example the CSS Object Model (CSSOM) that allows Javascript to read CSS rules programmatically (Houdini CSS even lets you access the unparsed CSS to make CSS polyfills possible and at that point the data might as well be HTML). The bottom line is that Javascript must not be able to read data coming from another origin.

Ironically, in the real world, it happens very often that web developers must do exactly the above, namely to read data from another origin and have access to the data in Javascript. It might be because their webapp is so large that it spans multiple domains, or it might be as simple as wanting to use XHR to call third party APIs. Before 2009, there was no good way to do this, so instead people had to resort to workarounds like for example proxying the XHR through server-side on the same-origin alternatively this little hack called JSONP. The latter takes advantage of the fact that Javascript files is loaded cross-origin with cookies automatically attached to the request, so you can use them to return data if you instead of returning { ... json data here ... }, simply return something like foo = { ... json data here ... }. Any website can then do:

<script src=""></script>

Of course asking everybody to execute untrusted thirdparty Javascript to be able to get the data is undesirable. Also, can also put the above code on their site and instead of console.log(foo) they might have an XHR that stores the foo data on their server-side. Once they have set that up they can just trick customers from to visit, thus giving the attack access to the foo data. So basically all data loaded via jsonp (or any SCRIPT element for that matter) can be read by anyone on the Internet unless CSRF protection is put in place.

It should be noted that users who have ticked the “Block third-party cookies” checkbox in their browser settings cannot be phished using this type of basic CSRF attack, because in this configuration the browser will not automatically attach cookies to cross-origin requests. Another hugely important feature that helps here is SameSite=strict cookies, support for which is available in Chrome but unfortunately not Firefox or Edge yet (see caniuse).

Because of the many security pitfalls around JSONP, a new specification for cross-origin requests was created; namely Cross-origin Resource Sharing (CORS). Thinking around such a specification started as early as 2005, but it wasn’t until 2009 (when Firefox 3.5 came out) that it was implemented anywhere and it took until 2013 before the other browsers (IE) had fully implemented the new spec (including the tainted canvas parts etc). It become a W3C recommendation in 2014.

The idea behind CORS is that when you need to load data from another origin, instead of using a SCRIPT elment to load data in Javascript/JSONP format, you use a regular XHR/fetch call but you modify the server-side so that it attaches a “Access-Control-Allow-Origin” (ACAO) header in the reply that describes which origins are allowed to make cross-origin requests. The browser will then return the response to the Javascript code that triggered the XHR/fetch, only if the origin was listed in the Access-Control-Allow-Origin header. Similar headers exists that allows you to opt-in to cross-origin XHR only for specific HTTP verbs etc.

So basically CORS is a way for a web-based API to declare which websites that are allowed to make cross-origin calls to it. Of course, these restricts only apply for Javascript running inside a web browser, anyone can still curl the API or make calls to it from Python or whatever. If what you’re after is the opposite, namely to control which servers/services that your web application may talk to, then Content Security Policy is what you need instead.

Finally, WebSockets are always allowed to connect cross-origin while simultaneously allowing Javascript to access the data they return, but they always send an Origin: header when they connect so that the server can reject connections from undesired origins. Having the security check on the server-side is simple and elegant, and it works this way because websockets is a comparatively modern standard. CORS is also fairly modern, but there the security check is done client-side largely for compatibility reasons.