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GRADUAL TYPING EMBEDDED SECURELY IN JAVASCRIPT

Aseem RastogiUniversity of Maryland, College Park

Joint Work With:

Nikhil Swamy, Cédric Fournet, Karthikeyan Bhargavan, Juan Chen, Pierre-Yves Strub, Gavin Bierman

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Architecture of JavaScript Applications

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Application

Libraries(e.g. JQuery)

Untrusted(e.g. ads)

Shared Global State(e.g. Object.prototype, String.prototype, Array.prototype)

All scripts execute in the same environment

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At Least It’s Dynamically Type Safe

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var x = 0; x(17); ~>* TypeError /* cannot apply a non-function */

Provides some useful security propertiesvar x = 0x1234567; x.f(); ~>* TypeError /* cannot forge an address */

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function protect(rawSend) { var whitelist = { “www.microsoft.com/mail” : true, “www.microsoft.com/owa” : true }; return function(url, msg) { if(whitelist[url]) rawSend(msg); }}

Or Is It ?

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function send(url, msg){ /* e.g. XMLHttpRequest */ …}

Object.prototype[“evil.com”] = true;

• Goal : Protect the send message function to restrict malicious URLs

send(“evil.com”, “gotcha”);

Attacker Succeeds !

Also looks up in Object.prototypewindow.send = protect(send);

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Type Errors ≈ Security Vulnerabilities

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• Attacker can exploit missing property accesses• Can execute arbitrary JavaScript

Need a stronger notion of type safety !

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Stronger Type Safety for JavaScript ?DJS (Chugh et. al.), DJS(Maffeis et. al.), JSVerify(Swamy et. al.), JSVerify(Gardner et. al.), Adsafety(Guha et. al.), SES-light(Taly et. al.), Moller et. al., …

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Handle only subsets of JavaScript

• Cannot ignore the adversary

• Lots of crazy stuff• eval• Proxies• Stack walking• Prototype poisoning• Global namespace corruption• …

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Attempts to Handle Full JavaScript ?• TypeScript, Closure• Great in increasing programmer productivity• But Not Type Safe

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We ask …• Can we provide stronger JS type safety

• While accounting for the full ECMAScript5 language• Unrestricted adversary

• And still retaining idiomatic JS programming interface

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TS★: Gradual Type System for All of JavaScriptStatically typed core

• number, bool, string• T1 T2 • { fi : Ti } (mutable, extensible) • ADTs

Dynamically typed fragment• any• JSON• Runtime type tests

Un typed adversary• arbitrary JavaScript• unmodified• unverified• unrestricted

Run time checks mediate interactions

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U

D S

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Key Invariants of TS★

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D S

Static Safety:Statically typed code is safe without any runtime checks

Dynamic Safety:Runtime types are always refinements of static types

Memory Isolation:No un-location referenced directly in static/any codeNo static/any reference leaked to un-code

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Key Idea: Gradual Security

ad.js

lib.js

app.js

function protect(rawSend) { var whitelist = { “www.microsoft.com/mail” : true, “www.microsoft.com/owa” : true }; return function(url, msg) { if(whitelist[url]) rawSend(msg); }}

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• Identify security critical code

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Key Idea: Gradual Security

ad.js

lib.js

app.js

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function protect(rawSend)function protect(rawSend:(string,string)=>any){ var whitelist = { “www.microsoft.com/mail” : true, “www.microsoft.com/owa” : true }; return function(url:string, msg:string) { if(whitelist[url]) rawSend(msg); }}

• Identify security critical code

• Port to TS★

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Key Idea: Gradual Security

ad.js

lib.js

app.js

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function protect(rawSend)function protect(rawSend:(string,string)=>any){ var whitelist = { “www.microsoft.com/mail” : true, “www.microsoft.com/owa” : true }; return function(url:string, msg:string) { if(whitelist[url]) rawSend(msg); }}

• Identify security critical code

• Port to TS★

function protected() { function protect(rawSend) { … } return wrap<Un>(protect);}window.send = protected();

TS★

• Compile

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Key Idea: Gradual Security

ad.js

lib.js

app.js

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function protect(rawSend)

• Identify security critical code

• Port to TS★

• Compilefunction protected() { function protect(rawSend) { … } return wrap<Un>(protect);}window.send = protected();

• Drop-in in the app

function protect(rawSend:(string,string)=>any){ var whitelist = { “www.microsoft.com/mail” : true, “www.microsoft.com/owa” : true }; return function(url:string, msg:string) { if(whitelist[url]) rawSend(msg); }}

TS★

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Gradual Security – Initial Experience• OWASP CSRFGuard and Facebook API

• Reported many attacks• Both widely used and security critical libraries• Ported critical fragments to TS★

• Easy to argue correctness in the presence of memory isolation

• Secure, High Integrity, and Efficient HTML5 localStorage

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(http://rise4fun.com/FStar/tutorial/tsStar)

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TS★ Gradual Typing Overview

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D S

Based on runtime type information (RTTI)

Point { x = 2, y = 3 }

type Point = { x:number; y:number }

Compiled as is

Compiled with runtime checks to respect RTTI tags

Library provided wrappers ensure memory isolation

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

TS★ Tour with Example

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

TS★

JS

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

Compilation of Statically Typed Code

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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JS

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function diag(p){ bar(p); p.x = p.y; return p;}

(Statically typed code is safe as is)

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

RTTI Instrumentation

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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JS

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diag.rtti = [[Point Point]]

function diag(p){ bar(p); p.x = p.y; return p;}

(Statically typed code is safe as is)

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

RTTI Instrumentation

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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JS

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diag.rtti = [[Point Point]]

function diag(p){ bar(p); p.x = p.y; return p;}

(Statically typed code is safe as is)

(Compiled with runtime type checks)

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Runtime Checks on RTTI (Dynamic Safety)

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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any { x = true }

o:

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Runtime Checks on RTTI (Dynamic Safety)

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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any { x = true }

o:

Is o a record ? Does o.x = 2 respect o’s rtti ? ✔

any { x = 2 }

o:

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Runtime Checks on RTTI (Dynamic Safety)

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

TS★

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any { x = true }

o:

Is o a record ? Does o.y = 3 respect o’s rtti ? ✔

any { x = 2 }

o:

any { x = 2, y = 3 }

o:

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Dynamically Typed to Statically Typed

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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any { x = 2, y = 3 }

o:

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Attempt 1 : Use Higher Order Casts for Mutable Records

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

TS★

JS

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var o’ ={ get x() { if hasOwnProperty(o, “x”) … }; get y() { … }; set x(v) { … }; set y(v) { … };}diag(o’);

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Problems with Higher Order Casts

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

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var o’ ={ get x() { … }; get y() { … }; set x(v) { … }; set y(v) { … };}diag(o’);

1. Lazy failures in statically typed code• Undesirable for security critical applications• Performance penalty for casts reduction

2. Space inefficient• Might recover with fancy coercion reductions

3. Breaks object identity• o === o’ ?

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var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Gradual Typing with RTTI

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

TS★

JS

✔

any { x = 2, y = 3 }

o:

Does o look like a Point ? If so, tag it. (setTag)

Point { x = 2, y = 3 }

o, p:

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Monotonic Evolution of RTTI

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t0

v0t2

v2t1

v1tn

vn

v0:t0

v1:t1

v2:t2 vn:tn

…

RTTI is always a sound approximation of a runtime value

RTTI evolves monotonically w.r.t the subtyping relation

t0 :> t1 :> t2 :> … :> tn

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type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

var o = { x : true };o.x = 2; o.y = 3;

diag(o);

function bar(q){ q.x = true;}

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Seamless Transition from Statically Typed to Dynamically Typed

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Seamless via subtyping – Point <: any.

Point { x = 2, y = 3 }

o, p:

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function bar(q){ q.x = true;}

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

var o = { x : true };o.x = 2; o.y = 3;

diag(o);

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RTTI Violations Cause Runtime Failures

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Point { x = 2, y = 3 }

o, p, q:

Is q a record ? Does q.x = true respect q’s rtti ? ✗ Runtime failure

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function bar(q){ q.color = “red”;}

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

var o = { x : true };o.x = 2; o.y = 3;

diag(o);

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Runtime Checks on RTTI (Dynamic Safety)

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Point { x = 2, y = 3 }

o, p, q:

Is q a record ? Does q.color = “red” respect q’s rtti ? ✔

Point { x = 2, y = 3, color = “red” }

o, p, q:

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function bar(q){ q.color = “red”;}

type Point = { x:number; y:number }

function diag(p:Point) : Point{ bar(p); p.x = p.y; return p;}

var o = { x : true };o.x = 2; o.y = 3;

diag(o);

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Statically Typed Code Executes As Is

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Point { x = 2, y = 3, color = “red” }

o, p, q:

Executes as expected, without any checks.

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Key Invariants of TS★

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D S

Static Safety:Statically typed code is safe without any runtime checks

Dynamic Safety:Runtime types are always refinements of static types

Memory Isolation:No un-location referenced directly in static/any codeNo static/any reference leaked to un-code

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type Point = { x:number; y:number }

function diag(p:Point) : Point{ baz(p); p.x = p.y; return p;}

function baz(q){ …}

Memory Isolation from Un

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Unmodified, unverified, unrestricted.

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type Point = { x:number; y:number }

function diag(p:Point) : Point{ baz(p); p.x = p.y; return p;}

function baz(q){ delete q.x; }

Memory Isolation from Un

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Unmodified, unverified, unrestricted.

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function baz(q){ delete q.rtti; }

function baz(q){ q.rtti = “junk”; }

How to protect invariants ?

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type Point = { x:number; y:number }

baz : Un

function diag(p:Point) : Point{ baz(p); p.x = p.y; return p;}

function baz(q){ …}

Memory Isolation from Un

TS★

• A second dynamic type Un

• Abstract type: not related to any other type

• Point <: any <\: Un

• { f : number; g : Un } <: { g : Un } <\: { }

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type Point = { x:number; y:number }

baz : Un

function diag(p:Point) : Point{ baz(p); p.x = p.y; return p;}

function baz(q){ …}

Memory Isolation from Un

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Compile error: Cannot apply an Un typed term

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type Point = { x:number; y:number }

baz : Un

function diag(p:Point) : Point{ wrap<Un, Point any>(baz)(p); p.x = p.y; return p;}

function baz(q){ …}

Memory Isolation from Un

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Library provided wrappers, ensure memory isolation

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Wrappers Enforce Heap Shape Invariant

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un fragmentStatic and any-typed DMZ(stubs)

• Non-Un values completely independent of untrusted global state (prototypes etc.) – thus send/protect example is secure in TS★

• TS★ runtime system needs “first starter privileges” on the page

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Facebook API Example

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Untrusted web page Facebook API

Iframe

Retrieves user’s access token

Gives access token to the untrusted page if it’s authorized by user

Wants to connect to Facebook on current user’s credentials

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Facebook API Sample Code

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function decode(s){ var res = { }; if(s === “”) return res; var p = String.split(s,“&”); for(var k in p) { var kv = String.split(p[k],“=“); res[kv[“0”]] = kv[“1”]; } return res;}

function checkOrigins(g, e){ for(var k in e) { if(g === e[k]) return true; } return false;}

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Example Vulnerabilities in Facebook API

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function checkOrigins(g, e){ for(var k in e) { if(g === e[k]) return true; } return false;}

Attacks similar toprotect/send(Using Object.prototype)

function decode(s){ var res = { }; if(s === “”) return res; var p = String.split(s,“&”); for(var k in p) { var kv = String.split(p[k],“=“); res[kv[“0”]] = kv[“1”]; } return res;}

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function decode(s:string):any{ var res = { }; if(s === “”) return res; var p = String.split(s,“&”); for(var k in p) { var kv = String.split(p[k],“=“); res[kv[“0”]] = kv[“1”]; } return res;}

Porting Facebook API to TS★

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function checkOrigins(g:string, e:array string):bool{ for(var k in e) { if(g === e[k]) return true; } return false;}

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Also in the paper …• More details on the wrappers• Formal translation from TS★ to JavaScript• Formalization of TS★ in JSVerify†

• Type soundness theorem and proof sketch• A standards based mechanism for first starter privileges• More examples

†Swamy et. al. PLDI’ 13

See our paper !

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TS★:The First JavaScript Type System To

• Provide strong type safety in a modular way• While accounting for ALL of JavaScript

http://research.microsoft.com/en-us/um/people/nswamy/Playground/TSSecure/index.html

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