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Re: [caplet] Re: ADsafe validation

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  • David-Sarah Hopwood
    ... @ does not appear anywhere in the ES3 grammar outside string literals, regexp literals, and comments, right? Isn t ADsafe defined to be a subset of ES3?
    Message 1 of 16 , Mar 21, 2008
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      Mike Samuel wrote:
      > On 21/03/2008, Kris Zyp <kris@...> wrote:
      >>
      >>> Do we have a regression test suite of tricky examples?
      >> That would be awesome.
      >>
      >>> For instance, I don't see the string "cc_on" in Kris' validator, but that feature
      >>> tripped up ADsafe a few months ago.
      >> Thanks for the heads, fixed it.
      >
      > Can you disallow @ outside of string literals entirely?
      >
      > What if ADSafe code is included in a container that has @cc_on, and
      > does an @set that overrides a variable defined in the container?

      '@' does not appear anywhere in the ES3 grammar outside string literals,
      regexp literals, and comments, right? Isn't ADsafe defined to be a subset
      of ES3?

      (See ECMA-262 section 7.6; note that '@' is 'Punctuation' but not
      'Connector punctuation' in Unicode 2.1 [insert grumble about using such
      an old Unicode version], so it is not valid in an identifier.)

      --
      David-Sarah Hopwood
    • Mike Samuel
      On 21/03/2008, David-Sarah Hopwood ... Yep. @ often appears in JSDoc style comments: http://jsdoc.sourceforge.net/#tagref so banning @ in comments might make
      Message 2 of 16 , Mar 21, 2008
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        On 21/03/2008, David-Sarah Hopwood
        <david.hopwood@...> wrote:
        >
        >
        >
        >
        >
        >
        >
        > Mike Samuel wrote:
        > > On 21/03/2008, Kris Zyp <kris@...> wrote:
        > >>
        > >>> Do we have a regression test suite of tricky examples?
        > >> That would be awesome.
        > >>
        > >>> For instance, I don't see the string "cc_on" in Kris' validator, but that feature
        > >>> tripped up ADsafe a few months ago.
        > >> Thanks for the heads, fixed it.
        > >
        > > Can you disallow @ outside of string literals entirely?
        > >
        > > What if ADSafe code is included in a container that has @cc_on, and
        > > does an @set that overrides a variable defined in the container?
        >
        > '@' does not appear anywhere in the ES3 grammar outside string literals,
        > regexp literals, and comments, right? Isn't ADsafe defined to be a subset
        > of ES3?
        >
        > (See ECMA-262 section 7.6; note that '@' is 'Punctuation' but not
        > 'Connector punctuation' in Unicode 2.1 [insert grumble about using such
        > an old Unicode version], so it is not valid in an identifier.)

        Yep. @ often appears in JSDoc style comments:
        http://jsdoc.sourceforge.net/#tagref so banning @ in comments might
        make some programmers grumble.

        It also indicates a conditional compilation directive in IE and I
        don't have a specific exploit in mind but I don't know whether
        blacklisting @cc_on is sufficient to avoid using conditional
        compilation to split or join tokens that otherwise appear separate.

        Perhaps either ban @ outside string/regexp contexts, or recommend that
        containers not allow @cc_on.




        >
        >
        >
        >
        > --
        > David-Sarah Hopwood
        >
        >
      • Kris Zyp
        ... Certainly seems reasonable to insist that containers don t do the eval inside a @cc_on. Kris
        Message 3 of 16 , Mar 21, 2008
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          > Perhaps either ban @ outside string/regexp contexts, or recommend
          that
          > containers not allow @cc_on.
          Certainly seems reasonable to insist that containers don't do the eval inside a @cc_on.
          Kris
        • David-Sarah Hopwood
          ... I meant my point a bit more generally: Assume that any extension to strict ES3 is designed by an evil genius trying to break ADsafe (or Caja, or whatever),
          Message 4 of 16 , Mar 21, 2008
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            Mike Samuel wrote:
            > On 21/03/2008, David-Sarah Hopwood
            > <david.hopwood@...> wrote:
            >> Mike Samuel wrote:
            >> > On 21/03/2008, Kris Zyp <kris@...> wrote:
            >> >>
            >> >>> Do we have a regression test suite of tricky examples?
            >> >> That would be awesome.
            >> >>
            >> >>> For instance, I don't see the string "cc_on" in Kris' validator, but that feature
            >> >>> tripped up ADsafe a few months ago.
            >> >> Thanks for the heads, fixed it.
            >> >
            >> > Can you disallow @ outside of string literals entirely?
            >> >
            >> > What if ADSafe code is included in a container that has @cc_on, and
            >> > does an @set that overrides a variable defined in the container?
            >>
            >> '@' does not appear anywhere in the ES3 grammar outside string literals,
            >> regexp literals, and comments, right? Isn't ADsafe defined to be a subset
            >> of ES3?
            >>
            >> (See ECMA-262 section 7.6; note that '@' is 'Punctuation' but not
            >> 'Connector punctuation' in Unicode 2.1 [insert grumble about using such
            >> an old Unicode version], so it is not valid in an identifier.)
            >
            > Yep. @ often appears in JSDoc style comments:
            > http://jsdoc.sourceforge.net/#tagref so banning @ in comments might
            > make some programmers grumble.
            >
            > It also indicates a conditional compilation directive in IE and I
            > don't have a specific exploit in mind but I don't know whether
            > blacklisting @cc_on is sufficient to avoid using conditional
            > compilation to split or join tokens that otherwise appear separate.
            >
            > Perhaps either ban @ outside string/regexp contexts, or recommend that
            > containers not allow @cc_on.

            I meant my point a bit more generally:

            Assume that any extension to strict ES3 is designed by an evil genius trying
            to break ADsafe (or Caja, or whatever), and you won't go far wrong. It's
            impossible to review all of the browser extensions: they aren't adequately
            documented, even if they were documented it would be too much work, and
            many of them display total cluelessness about programming language design.

            (Preprocessing features in client-side Javascript? What's the point of that?
            Just preprocess it on the server.)

            --
            David-Sarah Hopwood
          • Mike Samuel
            On 21/03/2008, David-Sarah Hopwood ... Or a committee of evil geniuses. ... Caja deals with many of these problems by rewriting. We can deal perfectly well
            Message 5 of 16 , Mar 21, 2008
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              On 21/03/2008, David-Sarah Hopwood
              <david.hopwood@...> wrote:
              >
              >
              >
              >
              >
              >
              >
              > Mike Samuel wrote:
              > > On 21/03/2008, David-Sarah Hopwood
              > > <david.hopwood@...> wrote:
              > >> Mike Samuel wrote:
              > >> > On 21/03/2008, Kris Zyp <kris@...> wrote:
              > >> >>
              > >> >>> Do we have a regression test suite of tricky examples?
              > >> >> That would be awesome.
              > >> >>
              > >> >>> For instance, I don't see the string "cc_on" in Kris' validator, but that feature
              > >> >>> tripped up ADsafe a few months ago.
              > >> >> Thanks for the heads, fixed it.
              > >> >
              > >> > Can you disallow @ outside of string literals entirely?
              > >> >
              > >> > What if ADSafe code is included in a container that has @cc_on, and
              > >> > does an @set that overrides a variable defined in the container?
              > >>
              > >> '@' does not appear anywhere in the ES3 grammar outside string literals,
              > >> regexp literals, and comments, right? Isn't ADsafe defined to be a subset
              > >> of ES3?
              > >>
              > >> (See ECMA-262 section 7.6; note that '@' is 'Punctuation' but not
              > >> 'Connector punctuation' in Unicode 2.1 [insert grumble about using such
              > >> an old Unicode version], so it is not valid in an identifier.)
              > >
              > > Yep. @ often appears in JSDoc style comments:
              > > http://jsdoc.sourceforge.net/#tagref so banning @ in comments might
              > > make some programmers grumble.
              > >
              > > It also indicates a conditional compilation directive in IE and I
              > > don't have a specific exploit in mind but I don't know whether
              > > blacklisting @cc_on is sufficient to avoid using conditional
              > > compilation to split or join tokens that otherwise appear separate.
              > >
              > > Perhaps either ban @ outside string/regexp contexts, or recommend that
              > > containers not allow @cc_on.
              >
              > I meant my point a bit more generally:
              >
              > Assume that any extension to strict ES3 is designed by an evil genius trying

              Or a committee of evil geniuses.


              > to break ADsafe (or Caja, or whatever), and you won't go far wrong. It's
              > impossible to review all of the browser extensions: they aren't adequately
              > documented, even if they were documented it would be too much work, and
              > many of them display total cluelessness about programming language design.

              Caja deals with many of these problems by rewriting. We can deal
              perfectly well with @ by stripping comments, and rewriting '@' to \x40
              in strings and regexps.

              ADSafe is a really elegant design, but my concern with the validation
              approach, besides blacklisting in general, is that it seems to accrete
              these rules which must seem arbitrary and hard to remember to coders.

              And if a vulnerability is discovered, a rewriter can add a new rewrite
              rule, and apply that to existing programs, whereas a validator might
              have to reject previously valid programs, which won't work until their
              creator actually looks at the program, learns the new rule, and
              applies it.


              > (Preprocessing features in client-side Javascript? What's the point of that?
              > Just preprocess it on the server.)

              This presupposes the existence of a server. If you're using a
              cut-rate hosting service that only serves of static files, then you
              need to do everything on the client.

              I agree though that preprocessing on the client is a little weird,
              since one of the typical goals of preprocessing is to avoid wasting
              bandwidth on code the client doesn't need -- the logic could otherwise
              just use the language's conditional constructs.


              > --
              > David-Sarah Hopwood
            • Kris Zyp
              ... Also, because with the new cross-site XHR and XDR capabilities, web sites can directly request the scripts from other sites, which can potentially be
              Message 6 of 16 , Mar 21, 2008
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                > (Preprocessing features in client-side Javascript? What's the point of that?
                > Just preprocess it on the server.)
                Also, because with the new cross-site XHR and XDR capabilities, web sites can directly request the scripts from other sites, which can potentially be significantly faster than sending them through your proxy (and incurring queuing against your own connection limit on the client). This could also reduces the burden on the server, always nice to offload to the infinitely scalable client (a new cpu for each user).
                IMO, performance is going to be a critical part of the acceptance of secured JavaScript. I would bet that a large percentage of potential users won't find the benefit of secure JavaScript compelling enough if there site takes twice as long to load. Of course this belief is one of the reasons for wanting to create a small fast 5K-ish library.
                Even without the new cross-site XHR and XDR capabilities, I think ADsafe client side validation could have a use, since we do have cross-site requesting mechanisms that are safe (like CrossSafe/Subspace). However, CrossSafe can't safely bring scripts into the container, only data. With ADsafe, CrossSafe could bring those scripts across using existing widespread browser technology.
                However, I do certainly agree that many users will prefer the server-side validation. Has anyone created a server-side ADsafe validator yet, or is that another project waiting to be undertaken?
                Kris
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