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Re: ADsafe banned list

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  • Douglas Crockford
    ... I do not understand the value in preventing information leaks here. What is the hazard? I am considering the blocking of try/catch in ADsafe. I am
    Message 1 of 5 , May 24, 2009
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      > So, I suggest that you consider adding 'stack', and possibly
      > 'message', 'stacktrace' and 'toSource', to the banned list.

      I do not understand the value in preventing information leaks here.
      What is the hazard?

      I am considering the blocking of try/catch in ADsafe. I am concerned about the potential of using exceptions to deliver capabilities between isolated widgets.
    • Tyler Close
      On Sun, May 24, 2009 at 7:49 AM, Douglas Crockford ... Javascript s catch is also problematic since it enables catching of stack overflow and out of memory
      Message 2 of 5 , May 25, 2009
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        On Sun, May 24, 2009 at 7:49 AM, Douglas Crockford
        <douglas@...> wrote:
        >> So, I suggest that you consider adding 'stack', and possibly
        >> 'message', 'stacktrace' and 'toSource', to the banned list.
        >
        > I do not understand the value in preventing information leaks here.
        > What is the hazard?
        >
        > I am considering the blocking of try/catch in ADsafe. I am concerned about the
        > potential of using exceptions to deliver capabilities between isolated widgets.

        Javascript's catch is also problematic since it enables catching of
        stack overflow and out of memory errors. A widget could use this
        ability to put another object, or perhaps even the browser, in an
        inconsistent state. For example, the widget could use up all but one
        stack frame and then make a call to a browser object which mutates
        part of its state and then attempts a function call before making
        additional mutations. The victim object would make the first mutation,
        but suffer a stack overflow error before being able to complete the
        rest of the mutations. The widget code could catch the Error, leaving
        the victim object in the inconsistent state.

        Note that the widget doesn't need to guess the size of the stack, but
        can measure it at runtime before engaging in the attack.

        --Tyler

        --
        "Waterken News: Capability security on the Web"
        http://waterken.sourceforge.net/recent.html
      • Brendan Eich
        ... I d like to know too -- you can throw an object that you could return, so that s not it. Is it the ES3 spec bug, not implemented by many browsers, where
        Message 3 of 5 , May 25, 2009
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          On May 25, 2009, at 2:56 PM, Tyler Close wrote:
          > On Sun, May 24, 2009 at 7:49 AM, Douglas Crockford
          > <douglas@...> wrote:
          > >> So, I suggest that you consider adding 'stack', and possibly
          > >> 'message', 'stacktrace' and 'toSource', to the banned list.
          > >
          > > I do not understand the value in preventing information leaks here.
          > > What is the hazard?
          >
          I'd like to know too -- you can throw an object that you could return,
          so that's not it.

          Is it the ES3 spec bug, not implemented by many browsers, where the
          scope of the catch variable is a new Object (and so can be attacked by
          Object.prototype setters or throwing a function that's called to
          capture |this|)? What browsers still do that?

          > > I am considering the blocking of try/catch in ADsafe. I am
          > concerned about the
          > > potential of using exceptions to deliver capabilities between
          > isolated widgets.
          >
          > Javascript's catch is also problematic since it enables catching of
          > stack overflow and out of memory errors.
          >

          Out of memory is not catchable in SpiderMonkey.

          What browsers did you test?

          > A widget could use this
          > ability to put another object, or perhaps even the browser, in an
          > inconsistent state. For example, the widget could use up all but one
          > stack frame and then make a call to a browser object which mutates
          > part of its state and then attempts a function call before making
          > additional mutations. The victim object would make the first mutation,
          > but suffer a stack overflow error before being able to complete the
          > rest of the mutations. The widget code could catch the Error, leaving
          > the victim object in the inconsistent state.
          >

          Sounds like a bug in the victim object. Why didn't it catch and clean
          up?

          Really, there are lots of potential bugs where an inconsistent state
          could result from errors. Making the errors fatal to the currently
          exeucting script only increases consistency in that particular script
          or event handler's control flow. The next script or event can still
          the inconsistency.

          If you poison the whole well, meaning both fail-stop the script and
          make the entire reachable object graph inaccessible or error-tainted,
          then you can limit the leak. But there's still a termination channel.

          /be
        • Tyler Close
          ... What about stack overflow? ... I did the testing during the caja security review and I believe I got an exploit working in both IE 6 and Firefox 2 on
          Message 4 of 5 , May 25, 2009
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            On Mon, May 25, 2009 at 3:37 PM, Brendan Eich <brendan@...> wrote:
            > On May 25, 2009, at 2:56 PM, Tyler Close wrote:
            >> On Sun, May 24, 2009 at 7:49 AM, Douglas Crockford
            >> > I am considering the blocking of try/catch in ADsafe. I am
            >> concerned about the
            >> > potential of using exceptions to deliver capabilities between
            >> isolated widgets.
            >>
            >> Javascript's catch is also problematic since it enables catching of
            >> stack overflow and out of memory errors.
            >>
            >
            > Out of memory is not catchable in SpiderMonkey.

            What about stack overflow?

            > What browsers did you test?

            I did the testing during the caja security review and I believe I got
            an exploit working in both IE 6 and Firefox 2 on Windows using the
            stack overflow Error. The stack overflow Error was easier to work with
            than the out of memory Error, since it's more predictable.

            >> A widget could use this
            >> ability to put another object, or perhaps even the browser, in an
            >> inconsistent state. For example, the widget could use up all but one
            >> stack frame and then make a call to a browser object which mutates
            >> part of its state and then attempts a function call before making
            >> additional mutations. The victim object would make the first mutation,
            >> but suffer a stack overflow error before being able to complete the
            >> rest of the mutations. The widget code could catch the Error, leaving
            >> the victim object in the inconsistent state.
            >>
            >
            > Sounds like a bug in the victim object. Why didn't it catch and clean
            > up?

            Because it wasn't expecting the function to throw. For example, the
            called function may have been one implemented in the same block of
            code as the calling function. Seeing that the called function didn't
            throw, the programmer of the victim object didn't write a try catch
            block. In general, protecting yourself against this kind of attack by
            defensively writing try catch blocks is too cumbersome and error prone
            to be feasible.

            > Really, there are lots of potential bugs where an inconsistent state
            > could result from errors. Making the errors fatal to the currently
            > exeucting script only increases consistency in that particular script
            > or event handler's control flow. The next script or event can still
            > the inconsistency.

            Sounds like an excellent argument for making errors fail-stop. ADsafe
            is in a good position to implement this, since it can wrap all event
            handlers created by a widget.

            > If you poison the whole well, meaning both fail-stop the script and
            > make the entire reachable object graph inaccessible or error-tainted,
            > then you can limit the leak. But there's still a termination channel.

            Allowing the widget to explicitly terminate the page is fine, since it
            implicitly has this authority anyways, since it could always just
            enter an infinite loop, or use other DOS techniques.

            --Tyler

            --
            "Waterken News: Capability security on the Web"
            http://waterken.sourceforge.net/recent.html
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