ConScript

ConScriptSpecifying and Enforcing Fine-Grained Security Policies

for JavaScript in the Browser

Leo MeyerovichUC Berkeley

Benjamin LivshitsMicrosoft Research

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Complications

Benign but buggy:

who is to blame? Code constantly evolving

How do we maintain quality?

Downright malicious

Prototype hijacking

Developer’s Dilemma

4

5

Only Allow eval of JSON

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) { … }

heap

Advising Calls is Trickywindow.eval = function allowJSON() { … }

windowobject

documentwindow

xyz…

frames[0]

stackfunction

allowJSONeval

frameobject

eval

eval

function eval

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

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. } );">

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Contributions of ConScript

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ImplementationA case for aspects in browser

Correctness checking

Expressiveness

Real-world Evaluation

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heap

Advising JavaScript Functions in IE8

fish.........dog

stack

function withBoundChecks

function paint

around(paint, withBoundChecks);dog.draw();fish.display();

draw

display

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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 browser

Type systemCorrectness checking

Expressiveness

Real-world Evaluation

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Policies are Easy to Get Wrongvar 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); }});

1.2.3.4.5.6.7.8.9.

toString redefinition!

Function.prototype poisoning!

Object.prototype poisoning!

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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

Reference 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 poisoned

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A case for aspects in browser

Correctness checking

PoliciesExpressiveness

Real-world Evaluation

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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

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

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Generating Intrusion Detection Policies

ConScript instrumentation

ConScript enforcement

evalnew Function(“string”)postMessageXDomainRequestxmlHttpRequest…

Observed method calls

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Enforcing C# Access Modifiers

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

C# JavaScript

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

Script#compiler

policygenerator

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

ConScript

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A case for aspects in browser

Correctness checking

Expressiveness

EvaluationReal-world Evaluation

Experimental Evaluation

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DoCoMo Policy Enforcement Overhead

Google Maps (183ms) MSN (439ms) GMail (736ms)0%

10%

20%

30%

40%

50%

60%

70%

80%

7%1% 0%

30%

73%63%

ConScript DoCoMo (JavaScript rewriting)

Runti

me

over

head

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

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File Size Increase for Blacklisting Policy

ConScript Docomo Caja Sandbox1.0

4.0

7.0

10.0

13.0

1.01.7

4.8

1.21.0 1.5

3.9

10.4

1.0 1.5

4.4

1.5

MSN GMail Google Maps

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Conclusions

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QUESTIONS?

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Mediating DOM Functionswindow.postMessage

frame2.postMessage

JavaScript interpreter

IE8 libraries(HTML, Networking, …)

postMessage

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

call advice

around(window.postMessage,

off

0xff34e5 off

);

advice dispatch

[not found]

0xff34e5

deep aspects

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

function foo () { }

Resuming Calls

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

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 foo () { }

advice on

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

function foo () { }

advice offadvice on

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