ConScript

ConScriptSpecifying and Enforcing Fine-Grained Security Policies

for JavaScript in the Browser

Leo MeyerovichUC Berkeley

Benjamin LivshitsMicrosoft Research

2

Complications

3

Benign but buggy:

who is to blame?

Benign but buggy:

who is to blame? Code constantly evolving

How do we maintain quality?

Code constantly evolving

How do we maintain quality?

Downright malicious

Prototype hijacking

Downright malicious

Prototype hijacking

Developer’s Dilemma

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Only Allow eval of JSON

5

eval(“([{‘hello’: ‘Oakland’}, 2010])”)

eval(“(xhr.open(‘evil.com’);)”)

• Idea for a policy: – Parse input strings instead of running them– Use ConScript to advise eval calls

• AspectJ advice for Java

• How to do advice in JavaScript?– No classes to speak of

void around call Window::eval (String s) { … }void around call Window::eval (String s) { … }

heap

Advising Calls is Tricky

window.eval = function allowJSON() { … }

windowobjectwindow

object

stack

function allowJSON

function allowJSON

eval

frameobjectframeobject

eval

eval

function eval

function eval

ConScript approach– Deep advice for complete mediation– Implemented within the browser for

efficiency and reliability

ConScript approach– Deep advice for complete mediation– Implemented within the browser for

efficiency and reliability

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Example of Applying Advice in ConScript

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1. <SCRIPT SRC=”facebook.js" POLICY="

2. var substr = String.prototype.substring;

3. var parse = JSON.parse;

4. around(window.eval,

5. function(oldEval, str) {

6. var str2 = uCall(str, substr, 1,

7. str.length - 1);

8. var res = parse(str2);

9. if (res) return res;

10. else throw "eval only for JSON";

11. } );">

Contributions of ConScript

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ImplementationA case for aspects in browserA case for aspects in browser

Correctness checkingCorrectness checking

ExpressivenessExpressiveness

Real-world EvaluationReal-world Evaluation

heap

Advising JavaScript Functions in IE8

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stack

function withBoundChecks

function withBoundChecks

function paint

function paint

around(paint, withBoundChecks);

dog.draw();

fish.display();

draw

display

This is Just the Beginning…• Not just JavaScript functions

– native JavaScript calls: Math.round, …– DOM calls: document.getElementById, …

• Not just functions…– script introduction– …

• Optimizations– Blessing – Auto-blessing

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A case for aspects in browserA case for aspects in browser

Type systemCorrectness checkingCorrectness checking

ExpressivenessExpressiveness

Real-world EvaluationReal-world Evaluation

Policies are Easy to Get Wrong

var okOrigin={"http://www.google.com":true};

around(window.postMessage,

function (post, msg, target) {

if (!okOrigin[target]) {

throw ’err’;

} else {

return post.call(this, msg, target);

}

});

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1.

2.

3.

4.

5.

6.

7.

8.

9.

toString redefinition!toString redefinition!

Function.prototype poisoning!

Function.prototype poisoning!

Object.prototype poisoning!

Object.prototype poisoning!

How Do We Enforce Policy Correctness?

Application code

• Unperturbed usage of legacy code

• Disallow arguments.caller to avoid stack inspection

(disallowed by ES5’s strict mode)

Policy code

• Modify the JavaScript interpreter– introduce uCall, hasProp,

and toPrimitive– disable eval

• Propose a type system to enforce correct use of these primitives– disable with, …

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Policy Type System

• ML-like type system• Uses security labels to denote privilege levels• Enforces access path integrity and reference isolation

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Reference isolation•o does not leak through poisoning if f is a fieldReference isolation•o does not leak through poisoning if f is a field

Access path integrity for function calls•o.f remains unpoisoned if T in v : T is not poisonedAccess path integrity for function calls•o.f remains unpoisoned if T in v : T is not poisoned

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A case for aspects in browserA case for aspects in browser

Correctness checkingCorrectness checking

PoliciesExpressivenessExpressiveness

Real-world EvaluationReal-world Evaluation

ConScript Policies

• 17 hand-written policies

– Diverse: based on literature, bugs, and anti-patterns

– Short: wrote new HTML tags with only a few lines of code

• 2 automatic policy generators

– Using runtime analysis

– Using static analysis

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Paper presents

17 ConScript

Policies

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around(document.createElement, function (c : K, tag : U) { var elt : U = uCall(document, c, tag); if (elt.nodeName == "IFRAME") throw ’err’; else return elt; });

around(document.createElement, function (c : K, tag : U) { var elt : U = uCall(document, c, tag); if (elt.nodeName == "IFRAME") throw ’err’; else return elt; });

Generating Intrusion Detection Policies

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ConScript instrumentationConScript instrumentation

ConScript enforcementConScript enforcement

evalnew Function(“string”)postMessageXDomainRequestxmlHttpRequest…

evalnew Function(“string”)postMessageXDomainRequestxmlHttpRequest…

Observed method calls

Observed method calls

Enforcing C# Access Modifiers

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class File { public File () { … } private open () { … } …

C#C# JavaScriptJavaScript

function File () { … }File.construct = …File.open = ……

Script#compilerScript#

compiler

policygenerator

policygenerator

around(File, pubEntryPoint);around(File.construct, pubEntryPoint);around(File.open, privCall);

ConScriptConScript

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A case for aspects in browserA case for aspects in browser

Correctness checkingCorrectness checking

ExpressivenessExpressiveness

EvaluationReal-world EvaluationReal-world Evaluation

Experimental Evaluation

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DoCoMo Policy Enforcement Overhead

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H. Kikuchi, D. Yu, A. Chander, H. Inamura, and I. Serikov, “JavaScript instrumentation in practice,” 2008

File Size Increase for Blacklisting Policy

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Conclusions

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QUESTIONS?

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Mediating DOM Functions

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window.postMessage

frame2.postMessage

JavaScript interpreterJavaScript interpreter

IE8 libraries(HTML, Networking, …)

IE8 libraries(HTML, Networking, …)

postMessagepostMessage

0xff34e5arguments: “hello”, “evil.com”

call advice

around(window.postMessage,

offoff

0xff34e5 offoff

);

advice dispatch

[not found]

0xff34e5

deep aspects

function advice1 (foo2) { if (ok()) { foo2(); } else throw ‘exn’; }

function advice1 (foo2) { if (ok()) { foo2(); } else throw ‘exn’; }

function foo () { }function foo () { }

Resuming Calls

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function advice2 (foo2) { if (ok()) { bless(); foo2(); } else throw ‘exn’; }

function advice2 (foo2) { if (ok()) { bless(); foo2(); } else throw ‘exn’; }

function foo () { }function foo () { }

advice onadvice off

bless() temporarily disables advice for next call

Optimizing the Critical Path

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function advice2 (foo2) { if (ok()) { bless(); foo2(); } else throw ‘exn’; }

function advice2 (foo2) { if (ok()) { bless(); foo2(); } else throw ‘exn’; }

function foo () { }function foo () { }

advice on

function advice3 (foo2) { if (ok()) foo2(); else { curse(); throw ‘exn’; } }

function advice3 (foo2) { if (ok()) foo2(); else { curse(); throw ‘exn’; } }

function foo () { }function foo () { }

advice offadvice on

• calling advice turns advice off for next call• curse() enables advice for next call