Sandboxing JavaScript via Libraries and Wrappers Phu H. Phung University of Gothenburg, Sweden, and University of Illinois at Chicago
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Sandboxing JavaScript via Libraries and Wrappers
Phu H. Phung
University of Gothenburg, Sweden, and
University of Illinois at Chicago
About Me
• Receipt of international postdoc grant (3 years) by Swedish Research Council (VR), employed by Univ. of Gothenburg.
• Research Associate at UIC.• PhD in Computer Science in 2011 from
Chalmers University, Sweden.
Hosted by OWASP & the NYC Chapter
• Selected research projects– European WebSand (complete)
• End-to-end secure web framework
– Secure Web Advertisements, funded by NSF (on-going)
– Defensive Optimizing Compiler, funded by DARPA (on-going)
Hosted by OWASP & the NYC Chapter
This talk
• Based on the two published papers:– PH Phung, L Desmet. A
two-tier sandbox architecture for untrusted JavaScript, invited paper, JSTools’12.
– P Agten, S Van Acker, Y Brondsema, PH Phung, L Desmet, F Piessens. JSand: complete client-side sandboxing of third-party JavaScript without browser modifications, ACSAC’12.
92% of all websites use JavaScript[w3techs.com]
“88.45% of the Alexa top 10,000 web sites included at least one remote
JavaScript library”CCS’12
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Third-party JavaScript is everywhere
• Advertisements– Adhese ad network
• Social web– Facebook Connect– Google+– Twitter– Feedsburner
• Tracking– Scorecardresearch
• Web Analytics– Yahoo! Web Analytics– Google Analytics
• …
Two basic composition techniques
Iframe integration
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<html><body>…<iframe src=“http://3rdparty.com/frame.html”></iframe>…</body></html>
3rd party
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Two basic composition techniques
<html><body>…<script src=“http://3rdparty.com/script.js”></script>…</body></html>
3rd party
Script inclusion
Third-party JavaScript issues
• Third-party script inclusion run with the same privilege of the hosting page.
• Security issues:– Malicious third-party code– Trusted third-party is compromised – Confidentiality, integrity, and other security risks
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Difficult issues with JavaScript
• JavaScript is a powerful language, but the language design is bad for security, e.g.:– Dynamic scripts: document.write, eval, ...– Encapsulation leakage– ...
<script> document.write(‘<scr’);document.write(‘ipt> malic’);var i= 1;document.write(‘ious code; </sc’);document.write(‘ript>’);</script>
<script> malicious code; </script>
A lot of attacks were launched in
practice
Malicious third-party JavaScript example
The most reliable, cost effective method to inject evil code is to buy an ad.
Principles of Security. Douglas Crockfordhttp://fromonesrc.com/blog/page/2/
An attack scenario
Million Browser Botnet (July 2013)
– Leverage Advertising Networks using JavaScript to launch Application-Level DDoS
– Paid on 2 ad networks for displaying treacherous advertisements on pages visited by hundreds of thousands of people
– One day, got 13.6 million views of the ads, just spent less than $100
Jeremiah Grossman & Matt JohansenWhiteHat SECURITY
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State-of-the-art
• Limit third-party code to safe subset of JavaScript– Facebook JS, ADSafe, ADSafety, ...
• Browser-based sandboxing solutions– ConScript, WebJail, Contego, ...
• Server-side transformations of scripts to be included– Google Caja, BrowserShield, ...
No compatibility with existing scripts
Browser modifications imply short-term deployment issues
No direct script delivery to browserGreat runtime overhead
Our approach
• A sandbox model for third-party JavaScript– Using only JS libraries and wrappers– Whitelist (least-privilege) implementation approach
• Only properties and objects defined in policies are available to the untrusted code
– No browser modification is required – The third-party code is keep in original– Easily dealing with dynamic features of JavaScript
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“Lightweight Self-Protecting JavaScript”, ASIACCS’09
Two-tier sandbox architecture
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Sandbox running untrusted code,
defined in a separate file e.g.
`untrusted.js’
Sandbox running policy code, defined in a separate JS e.g. `policy.js’
Base-line API implementation,in e.g. `api.js’ file
JavaScript environment, e.g. the DOM
The policy code can only access the base-line API and provided
wrapper functions (ensuring no leaks to global)
The untrusted code can only access objects returned by the outer sandbox
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Two-tier sandbox architecture
var api = loadAPI(…);
var outerSandbox = cajaVM.compileModule(policyCode);
var enforcedAPI = outerSandbox(api);
var innerSandbox = cajaVM.compileModule(untrustedCode);
innerSandbox(enforcedAPI);
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The architecture in multiple-principal untrusted code
Policy 2Policy 1
untrusted
Policy 3
untrusted
untrusted
Base-line API implementation,in e.g. `api.js’ file
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Sandboxing untrusted code
• Use Secure ECMAScript (SES) library developed by Google Caja team– Load a piece of code to execute within an isolated
environment• The code can only interact with the outside world via provided
APIs
var api = {...}; //constructingvar makeSandbox = cajaVM.compileModule(untrustedCodeSrc);var sandboxed = makeSandbox(api);
Isolation technique: The SES library
Object-capability environment• Scripts can access
– Objects they create themselves– Objects explicitly handed to them
APIGlobal context
untrustedCodesandbox
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Isolation technique: The SES library
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Base-line APIs implementation
• Create a Virtual DOM– Intercepting wrapper around real DOM– Use Harmony Proxies to generically intercept property
accesses on objects
• Virtual DOM implementation uses the Membrane Pattern– Wrap any object passed from DOM to sandbox (return
values)– Unwrap any object passed from sandbox to DOM
(arguments)21
Wrapper example
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Policy definition
• Base-line APIs implementation– Can enforce coarse-grained, generic policies, e.g.:
• Sanitize HTML• Ensure complete mediation
• Fine-grained policies for multiple untrusted JavaScript code– Modular, principal-specific, e.g.: script1 is allowed to
read/write elemt_A, script2 is allowed to read elemt_A – Stafeful, e.g.: limit the number of popups to 3 – Cross-principal stateful policies, e.g: after script1 write to
elemt_A, disallow access from script2 to elemt_A
Deployment model
• Untrusted code is loaded into a string variable– Using server-side proxy + XMLHttpRequest (to
overcome same origin policy)– CORS (Cross-Origin Resource Sharing)
/UMP(Uniform Messaging Policy) headers set by the script provider
<script src=“http://3rdparty.com/script.js”></script>
<script src=“ses.js”></script><script src=“api.js”></script><script src=“policy0.js”></script><script>var script = get(“http://3rdparty.com/script.js”);ses.execute(script,policy0);</script>before
after 24
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Secure dynamic script evaluation
• Special handlers to intercept all methods that allow script tags to be added– node.appendChild, node.insertBefore,
node.replaceChild, node.insertAfter– document.write, …– Event handlers in HTML, e.g. <…onclick=“javascript:xyz(…)”>
1. Parse partial DOM tree/HTML2. Execute scripts in the sandbox environment
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Dynamic script loading in JavaScript
• Example from Google Maps
Different parsing techniques
• Via a sandboxed iframe1. Create sandbox iframe2. Set content via srcdoc attribute– Better performance– Parsed exactly as will be interpreted by browser– Executed asynchronously
• (Alternative) Via a HTML parsing library in JavaScript
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Loading additional code in the sandbox
• External code needs to be executed in a previously set up sandbox– Loading API + glue code– Dynamic script loading
• Two new operations:– innerEval(code)– innerLoadScript(url)
Case studies
• Single principal code
• Multiple-principal code– Context-aware ads
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Implementation challenges
• Legacy scripts need additional pre-processing to be compatible with the framework– Secure ECMAScript restrictions
• A subset of ECMAScritp strict mode• Global variables aliased as window
properties• No ‘this’ auto coercion
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JS transformation examples
Summary
– A client-side JavaScript architecture for untrusted JavaScript• Only using libraries and wrappers
– Complete mediation using Secure ECMAScript• DOM node operations• JavaScript APIs
– Backward compatibility• No browser modifications• Direct script delivery to the browser• Support for legacy scripts
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Thank you!
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