"Remaining Hazards and Mitigating Patterns of Secure Mashups in ECMAScript 5"

Aymeric Vitte vitteaymeric at gmail.com
Tue May 15 15:26:38 PDT 2012


Finally I could hear the talk (++ "too bad that the end is missing")

Then I took a look mainly at domado.js, startSES.js, repairES5.js

Quite clever, but quite complicate...

I did focus on cajaVM.eval and tried to theorically follow its process 
with an example as described below, with some small simplifications 
since security is one thing but the final result of VM processing 
(whether security is involved or not) is interesting too

Maybe it will be undigest for other readers since it is difficult to 
summarize if you do not have all the code, but again it's quite smart as 
js allows (even if I have some doubts about performances with all the 
getters and setters, and even with proxies being added later to help 
this), I hope my understanding is correct, the exercise is not just a 
"vue de l'esprit", I have something in mind since some time related to 
this and other things that I might submit, maybe to simplify a little bit.

function compileExpr(exprSrc, opt_sourcePosition) {
   var wrapperSrc = securableWrapperSrc(exprSrc, opt_sourcePosition);
   var wrapper = unsafeEval(wrapperSrc); //eval
   var freeNames = atLeastFreeVarNames(exprSrc);
   var result = makeCompiledExpr(wrapper, freeNames);
   return freeze(result);
}

sharedImports : copy of the global object with original properties 
frozen (parseInt, etc);

imports : clone of sharedImports;      //this is now our virtual global 
for code execution
                                                                 //ex : 
imports.parseInt --> parseInt

window : imports; //let's call it window for better understanding, see 
it as a gadget's own window, not the usual global window

window.window=window; //Assign window property to window refering to 
itself. I did not see it in the code but probably it is somewhere

window.d="d"; //define it not frozen

window.f="f"; //define it frozen

src='var a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g"';

wrapperSrc='(function() {
       with (this) {
         return function() {
           "use strict";
           return (
             var 
a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g";
           );
         };
       }
     })';

wrapper=function() {
       with (this) {
         return function() {
           "use strict";
           return (
             var 
a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g";
           );
         };
       }
     }

freeNames=['a','c','parseInt','d','f'];

scopeObject= { //frozen
     a: {get:scopedGet, set:scopedSet}, //set and get window.a
     c: {get:scopedGet, set:scopedSet}, //set and get window.c
     d: {get:scopedGet, set:scopedSet}, //set and get window.d
     parseInt: window.parseInt, //initial parseInt, not evil
     f: "f"
}

function() {
       with (this) {
         return function() {
           "use strict";
           return (
             var 
a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g";
           );
         };
       }
     }.call(scopeObject)

returns :

function() {
     "use strict";
       return (
         var 
a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g";
       );

}

whose's scope is still under the |with| statement where  :
     a: {get:scopedGet, set:scopedSet}, //set and get window.a
     c: {get:scopedGet, set:scopedSet}, //set and get window.c
     d: {get:scopedGet, set:scopedSet}, //set and get window.d
     parseInt: window.parseInt; //initial parseInt, not evil
     f: "f" //frozen initial value

Then :

function() {
     "use strict";
       return (
         var 
a="a";window.b="b";c="c";parseInt="evil";d="D";f="F";this.g="g";
       );

}.call(window)

//|this| does refer to window for code execution

window.a==="a"; //scopedGet result
window.b==="b"; //assignment by window property of window
window.c==="c"; //scopedGet result
window.parseInt===parseInt; //parseInt reassignment fails (frozen), 
scopedGet result
window.d==="D"; //scopedGet result
window.f==="f"; //frozen initial value
window.g==="g"; //|this| is window


Le 20/03/2012 06:27, Mark S. Miller a écrit :
> On Mon, Mar 19, 2012 at 3:10 PM, Aymeric Vitte <vitteaymeric at gmail.com 
> <mailto:vitteaymeric at gmail.com>> wrote:
>
>     'Avoid “this”. Use closures rather than prototypes'
>
>     Probably the public was stunned by that one... (technical problems
>     too, could not hear the video, just saw the slides)
>
>
> Not really. I expected more resistance than I got. During the 20 
> minutes of lively Q&A, this came up again. I clarified then something 
> I should have said earlier in the talk. The objects that need to be 
> defensive are those that might be exposed across a trust boundary, 
> such as the counter in the first example. For objects purely inside 
> one trust domain, given that we really are confident they cannot 
> escape, they do not need to be defensive since their clients are all 
> presumably intimately cooperative.
>
>
>     Technically for the purpose of your presentation, it is correct,
>     but I am coming back again to real life, you are using strict mode
>     and other means (such as questionnable setTimeout(xxx,0)) to
>     secure Bob.
>
>
> Sorry, but we're using these techniques in real life. And what's 
> questionable about setTimeout? (or better, 
> <http://dbaron.org/log/20100309-faster-timeouts>)
>
>
>     Then what is the use of Bob if he can not do anything outside of
>     himself ?
>
>
> Please do make an effort to surmount whatever technical difficulties 
> you encountered, so that you can listen to the audio of the 
> presentation. The slides were not constructed to be self explanatory, 
> and the talk was clear on this point.
>
>
>     A much more trivial security leak could be that the calling
>     context does somewhere unexpectedly (or not) something like
>     counter.x.y.z=window (Ex : like passing a node to Bob since it
>     seems that Bob has to do some stuff with the dom to be usefull)
>
>
> If Alice does not trust Bob, Alice should generally never give Bob 
> direct unmediated access to one of her dom nodes. Instead, she gives 
> him access to a virtual dom tree that wraps the real dom tree, 
> allowing Bob to manipulate a subtree of Alice's dom tree. We 
> constructed the Domado library 
> <http://code.google.com/p/google-caja/source/browse/trunk/src/com/google/caja/plugin/domado.js> 
> for exactly this purpose.
>
> The difficultly of emulating the dom faithfully in JS was also the 
> original impetus for the proxy work. The Domado library above does not 
> rely on proxies, as they are not yet as available as ES5.
>
>
>     It's the same issue as multiple globals (if the concept of globals
>     still exist in the future) I believe : how to separate completely
>     several contexts while using objects between each others ? Looks
>     very difficult
>
>     I might be wrong, but on what today's examples the demonstration
>     here could apply without Bob being useless or just returning
>     something like a mathematical calculation or such not touching
>     anything in the page ?
>
>
> I'm sorry, I didn't understand these last two paragraphs. Could you 
> clarify?
>
> You might also want to try some of the scenarios you have in mind at 
> <https://caja.appspot.com/>.
>
>
> -- 
>     Cheers,
>     --MarkM

-- 
jCore
Email :  avitte at jcore.fr
Web :    www.jcore.fr
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