Subresource Integrity

Subresource IntegrityPhilippe De Ryck

@PhilippeDeRyck

Who Uses Code Like This?

2

<script src="https://code.jquery.com/jquery-2.1.3.min.js"></script>

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet" >

https://blog.jquery.com/2014/01/14/jquerys-content-delivery-network-you-got-served/

Maybe It Also Puts You to Shame?

3http://arstechnica.com/security/2015/06/us-army-website-defaced-by-syrian-electronic-army/

And If You Thought That Was Bad …

4

And If You Thought That Was Bad …

5http://www.cbc.ca/news/trending/anonymous-rickrolls-isis-on-twitter-after-losing-cyber-war-1.3331934

And the Problem Is Much Worse …

6https://blog.cloudflare.com/an-introduction-to-javascript-based-ddos/

But Not So Easy To Comprehend

7https://xkcd.com/932/

Remote Content Inclusion Is Dangerous

8

§ You give third party content full control over your context§ Governed by the Same-Origin Policy of the browser§ No separation between your code and external code§ Access to same privileges, data, code …

§ External scripts can§ Read and manipulate your DOM§ Access JS APIs with the privileges granted to your origin§ Send requests to your origin without limitations§ Send requests to other origins on behalf of your origin

Large-scale Study of Remote JS Inclusions

9

“88.45% of the Alexa top 10,000 web sites included at least one remote

JavaScript library”

https://seclab.cs.ucsb.edu/media/uploads/papers/jsinclusions.pdf

Large-scale Study of Remote JS Inclusions

10https://seclab.cs.ucsb.edu/media/uploads/papers/jsinclusions.pdf

Subresource Integrity

11

§ There is nothing you can do against these attacks§ Either you trust a CDN, or you host it yourself

§ Welcome Subresource Integrity (SRI)§ W3C Candidate Recommendation since November 12, 2015

<script src="https://code.jquery.com/jquery-2.1.3.min.js"integrity=“sha256-TXuiaAJuML3…uMLTXuiaAJ3”crossorigin=“anonymous”></script>

Subresource Integrity

12

§ Allows you to specify a hash of an external resource§ Using the integrity attribute on script or link tags

§ Browsers verify this hash before loading the file§ Refuse to load the file if the hash does not match

§ SRI supports the specification of multiple hashes§ The strongest one available will be used by the browser

<script src=”myapplication.js” integrity=“sha256-… sha512-… ”>

</script>

<link href=“myapp.css” type=“text/css”integrity=“sha384-… sha512-…” />

Unfortunately, SRI Enables Data Leakage

13

§ You can check the presence of specific external resources§ Requests from the user’s browser, with credentials

<script src=https://some-shop.com/accountbalance integrity=“…”></script>

{“balance”: 1234.00} dPdFnnWdXY6eHXiK+3O/OSi3OeLFHlLch1qZ3iqD3MGNXck+Oz4LETv8lnsoNyFI

{“balance”: 1235.00} RasWnvVTFAiT+6NeqIJFRDDDSklMaljV0FxUQysJqUB65TGm/lFqKJkrGif2wzYj

{“balance”: 1236.00} uSCKm1yloPZ7VexjyLQ+sUvakZKycl3CsblGH/9XpGV09ymyf1nKAzU5tXTFH5oi

{“balance”: 1237.00} 4SI2gcfIFhX2NRE5KPbeXR87PaiCSAan6PL2mxKWndBp8wvE2Dfcn7HenpNXD0oJ

SRI Prevents Data Leakage with CORS

14

§ Not needed for resources from your own origin§ The browser allows access to these anyway

§ The crossorigin attribute controls CORS behavior§ Anonymous does not attach cookies to the request§ Use-credentials means that that cookies will be present


Simple CORS Example

Load page

XHR: load user’s profile from websec.be

www.example.com

www.websec.be

Origin: http://www.example.com

Access-Control-Allow-Origin: http://www.example.com

CORS Protects Legacy Servers by Design

16

Load page


www.example.com

www.websec.be


No CORS headers present

Handling Credentials

17

§ Requests can be anonymous or authenticated§ By default, credentials (i.e. cookies) are not sent§ Can be enabled by setting the withCredentials flag

§ When credentials are used, the server must acknowledge this§ By sending the Access-Control-Allow-Credentials response header

§ Aim is to prevent illegitimate use of the user’s credentials§ Not intended to protect the server from malicious requests

Simple CORS Example with Credentials

18

Load page


www.example.com

www.websec.be

Origin: http://www.example.comCookie: PHPSESSID=1a2b3c4d5e6f


Access-Control-Allow-Credentials: true

var xhr = new XMLHttpRequest();xhr.open('GET', 'http://www.websec.be/profile', false);xhr.withCredentials = true;xhr.send();

Subresource Integrity with CORS

Load page

Include script with integrity check

www.example.com

www.websec.be



SRI Prevents Data Leakage with CORS

20

§ CORS protects legacy servers§ CORS response headers need to be present§ If they are missing, the resource will not be loaded

§ The only reason is the integrity check§ Simply including the script without integrity check would still work


SRI Error Recovery

21

§ Failure to load a resource triggers an error event§ Clean way to load an alternative resource yourself

§ Last-resort recovery from an integrity error§ Host a backup version of the script on your own server§ Load when the main version was not loaded

<script>window.jQuery|| document.write('<script src="/jquery.min.js"><\/script>');</script>

Subresource Integrity in Practice

22http://caniuse.com/#search=subres


23

openssl dgst -sha384 -binary bootstrap.min.css| openssl enc -base64 -A

https://www.srihash.org/


24https://www.bootstrapcdn.com/

EmberJS and Subresource Integrity

25

§ Ember-cli-sri enables SRI for your application files§ Hashes are automatically added during production build process§ Included by default for a few months already

§ Assumes your application serves files from the same origin§ Can be configured to use a different origin or domain

Conclusion

26

§ SRI gives you more control over what you include§ Of course this only makes sense over an HTTPS channel§ But you should be running everything over HTTPS anyway

§ If you host public libraries, enable CORS for them§ This allows other people to use SRI for these files

§ If you build apps with external resources§ Include an integrity attribute if CORS is supported§ Bug the admins if CORS is not supported J

About Me – Philippe De Ryck§ Postdoctoral Researcher @ DistriNet (KU Leuven)

§ Focus on (client-side) Web security

§ Responsible for the Web Security training program§ Dissemination of knowledge and research results§ Target audiences include industry and researchers

§ Main author of the Primer on Client-Side Web Security§ 7 attacker models, broken down in 10 capabilities§ 13 attacks and their countermeasures§ Overview of security best practices

Subresource IntegrityPhilippe De Ryck

[email protected]

/in/philippederyck

https://distrinet.cs.kuleuven.be/events/websecurity/

@PhilippeDeRyck

Technology

Subresource Integrity