Same origin policy and CORS

I have been recently focused a lot on Web application security especially web vulnerabilities such as Cross Site Scripting (XSS) and Cross Site Request Forgery (CSRF)

One of the interesting protection mechanism around these attacks is Same Origin Policy which I believe every application developer should understand in deep.

So what is cross origin policy?

Reference: Mozilla docs

When a browser loads the web page, the web page elements might refer to other origins (basically the combination of protocol, host and port). These elements can be CSS links <link href>, JS links <script src>, <img> tags,

What is not covered under same origin policy?

This is most important. Its very important for developers to understand what is excluded under same origin policy because that’s where the web application becomes vulnerable.

1) cross domain form posting is perfectly acceptable in web application. i.e. a form loaded at http://localhost:8080 , can execute the following code without any issues:

    <body onload='document.getElementById("csrfform").submit()'>
        <form method='post' id='csrfform' action=''>
            <input type='hidden' name='token' value =''/>
            <input type='hidden' name='real_name' value ='attacker'/>
            <input type='hidden' name='bio' value ='test'/>
            <input type='hidden' name='status' value ='active'/>
            <input type='hidden' name='role' value ='administrator'/>
            <input type='hidden' name='username' value ='evilattacker2'/>
            <input type='hidden' name='password' value ='pwnd1111'/>
            <input type='hidden' name='email' value ='[email protected]'/>
            <input type='submit' value ='submit'/>

The above is a classic case of CSRF attack where a forged page is tricking the user to submit a request to other site without his knowledge. If the user is already logged onto , the browser would also automatically include the cookies appropriate for to automatically authenticate the request.

2) The script tags do not come under same origin policy. i.e. it is perfectly legal for a site at to have the following script tag in its html

        <script src="">

3) The <img> tag is allowed to retrieve images from the cross origin. This might seem very innocuous but there have been some attacks because of this.

4) Just like images, in iframe the contents may be loaded but scripts in the outer framing page are not allowed to access the framed page contents.

Web Storage, IndexedDB and LocalStorage

Each origin gets its own dedicated Web Storage, IndexedDb and LocalStorage. The site in one origin cannot access the storage data of other origin. For example, the site running at http://localhost:8081 cannot access the localstorage of http://localhost:8082

How to circumvent Same origin policy?

The Cross origin request sharing (CORS) specification permits the cross origin sharing under special circumstances.

Protections against these attacks

The protection against these attacks is really a big topic. But primarily two headers are helpful: