Krishna Chaitanya T

Infosys Labs

Microsoft MVP, Internet Explorer

Content Isolation with

Same Origin Policy

You know this is possible… (why?)

Why not this?

Why?

Why not?

The big (small) picture

• WHO can access WHAT from WHERE, HOW and WHY? Any IFs and BUTs? ;)

Site A

Browsing context of

Site A

Site B

Browsing context of

Site B

The questions…

• Can A get resources from B.com?

• Can A execute resources from B.com?

• Can A post content to B.com?

• Can A interfere with the DOM of B?

• Can A redirect a browsing context of B?

More questions…

• Can A read cookies/localStorage of B?

• What about http/https protocols

• How about different port numbers?

• Can chat.A.com communicate with A.com?

• Can blog.com/user1 talk to blog.com/user2?

Ok. Now enough of questions.

Let’s clear the confusion!

Same Origin Policy (SOP)

• Browser has to isolate different origins

• Origin = scheme://host:port• https://mysite.com

• http://chat.mysite.com

• http://mysite.com:81/

• Privileges within origin• Full network access, storage, read/write access

to DOM

SOP facts…

• Script requests are not subjected to SOP!

• Frames have separate security contexts for each origin.

• Frame Navigation Policy: Script in Frame A can navigate Frame B (This is not SOP!)

• Access to HTML5 LocalStorage, Cookies* is by SOP.

SOP facts…

• Browsers do not prevent cross domain content inclusion!

• Examples:

<iframe src=“…”/><img src=“…”/><link rel=“stylesheet” href=“…”/>

• Information about user’s interaction can be collected using events onload, onerror etc.

So how is cross origin communication feasible with Same Origin Policy in place?

HACKS / SOP bypass

SOP Hacks

• JSONP – JSON with Padding

• Domain relaxation – document.domain

• Server side proxies

• JavaScript window.name hack

• Iframe hacks-Fragment Identifier Messaging (FIM), Subspace etc.

Understanding JSONP

1. Create a JavaScript function (callback)

2. Pass valid JSON data & execute it

3. Move the code in step 2 to external JS file (Idea is to simulate server’s response). So far it’s good.

function processData(data){ console.log('Hello '+data.firstName+'

'+data.lastName); }

processData({firstName:'Krishna', lastName:'Chaitanya'});

Understanding JSONP

4. Configure server side code to respond to the query string

5. Script loading is exempted from SOP, so the code so far still works.

6. Wrap JSON data with function name.

<script src=“http://mysite.com/index.aspx?callback=processData”/>

processData({firstName:'Krishna', lastName:'Chaitanya'});

Domain relaxation

• Cooperating websites sharing common TLDs can relax their origins

• “a.site.com” & “site.com” - different origins

• Both parties should set document.domain

• Now sub domain enjoys same origin benefits!

document.domain=“site.com”

Surprisingly, there wasn’t a standard for cross origin communication till recently. Only few

clever hacks.

Here comes HTML5!

Genuine Cross Origin Access

• Client side - HTML5 PostMessage API

• Secure communication between frames

otherwindow.postMessage(message, targetOrigin);

//Posting message to a cross domain partner.frames[0].postMessage(“Hello Partner!”, "http://localhost:81/");

//Retrieving message from the senderwindow.onmessage = function (e) { if (e.origin == 'http://localhost') { //sanitize and accept data }};

Genuine Cross Origin Access

• Server side – HTML5 CORS

• XHR enhanced for secure cross origin sharing

• Server just needs to send this new header:

More about these in future events

Access-Control-Allow-Origin: http://mysite.com (or) *

var xhr = new XMLHttpRequest();if ("withCredentials" in xhr) {

xhr.open("GET", "http://mysite.com", true);xhr.send();

} else {// Fallback behavior

}

A better picture

Site A

Browsing context of

Site A

Site B

Browsing context of

Site B

AJAX

Cross Origin Resource Sharing (HTML5)

Server side proxy

PostMessage (HTML5)

If (!sleepy && !confused){

GoTo slide 2;

print(“Answer all questions till slide 8 correctly”);

}

else {

GoTo slide 9;

print(“Repeat”);

}

Litmus Test ;)

Thank You!

Twitter: @novogeek

Blog: http://novogeek.com