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Re: [caplet] Re: ADsafe, Take 3

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  • Mike Samuel
    If you do want to allow ADsafe JS to be embedded in a script tag, you need to deal with ]] as well, since the following could be used to throw allert
    Message 1 of 17 , Oct 4, 2007
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      If you do want to allow ADsafe JS to be embedded in a script tag, you need to deal with ]]> as well, since the following could be used to throw allert

      (function () {
        var a = [[]]> 0; //
 ]];
        //
 alert('hi');
      })();

      Once you have to deal with CDATA sections and entity expansion in HTML, you're opening up a huge can of worms.

      mike




      On 03/10/2007, Adam Barth <hk9565@...> wrote:

      I think I misunderstood your comment below. I meant that, when
      embedded in an HTML file, the script does indeed produce an error in a
      browser, but the browser continues on and executes the alert.

      I guess this really boils down to a question of how to use JSLint. I
      was imagining a web developer using JSLint as follows:

      1) A user supplies the site with some JavaScript that is supposed to be ADsafe.
      2) The site operator runs the code through JSLint, rejecting the
      submission if JSLint so advises.
      3) If JSLint approves, the site embeds the script in an HTML document
      served from his or her domain.

      This script causes problems in this scenario because JSLint approves
      of the script, but it escapes from the sandbox when embedded in an
      HTML page.

      Perhaps it would be safer to ban </ from scripts altogether (not just
      in string literals as described in <http://www.jslint.com/lint.html>).

      Here are some more dangerous examples courtesy of Collin Jackson:

      <html><body><script>
      //</script><script>alert("XSS");
      </script></body></html>

      <html><body>
      <script>/*</script><script>alert("XSS");*/</script>
      </body></html>

      Adam

      On 10/3/07, Adam Barth <hk9565@...> wrote:
      > On 10/3/07, Douglas Crockford <douglas@...> wrote:
      > > --- In caplet@yahoogroups.com, "Adam Barth" <hk9565@...> wrote:
      > > > This seems to get through the filter:
      > > >
      > > > (function() {
      > > > var str = "</script><script>alert('script tags affect parsing')/*";
      > > > })();
      > > > /**/
      > >
      > > In a .js file, it is harmless. In an .html file, it produces an error.
      >
      > When embedded in HTML, it calls alert (at least in IE7 and Firefox 2):
      >
      > http://crypto.stanford.edu/~abarth/jslint/parse.html
      >
      > Adam
      >


    • Douglas Crockford
      ... be ADsafe. ... The usage I was anticipating was that ads would be delivered in individual .js files. Ultimately, I want to banish all in page script. But I
      Message 2 of 17 , Oct 4, 2007
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        --- In caplet@yahoogroups.com, "Adam Barth" <hk9565@...> wrote:
        >
        > I think I misunderstood your comment below. I meant that, when
        > embedded in an HTML file, the script does indeed produce an error in a
        > browser, but the browser continues on and executes the alert.
        >
        > I guess this really boils down to a question of how to use JSLint. I
        > was imagining a web developer using JSLint as follows:
        >
        > 1) A user supplies the site with some JavaScript that is supposed to
        be ADsafe.
        > 2) The site operator runs the code through JSLint, rejecting the
        > submission if JSLint so advises.
        > 3) If JSLint approves, the site embeds the script in an HTML document
        > served from his or her domain.
        >
        > This script causes problems in this scenario because JSLint approves
        > of the script, but it escapes from the sandbox when embedded in an
        > HTML page.
        >
        > Perhaps it would be safer to ban </ from scripts altogether (not just
        > in string literals as described in <http://www.jslint.com/lint.html>).

        The usage I was anticipating was that ads would be delivered in
        individual .js files. Ultimately, I want to banish all in page script.
        But I agree that ADsafe should be aware of in page script.
      • Douglas Crockford
        ... you need ... alert Is it sufficient to disallow
        Message 3 of 17 , Oct 4, 2007
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          --- In caplet@yahoogroups.com, "Mike Samuel" <mikesamuel@...> wrote:
          >
          > If you do want to allow ADsafe JS to be embedded in a script tag,
          you need
          > to deal with ]]> as well, since the following could be used to throw
          alert

          Is it sufficient to disallow <![ ?
        • Mike Samuel
          No because ]] can end a CDATA section introduced by the embedding XHTML page which would then allow the embedding script to play tricks with entities that
          Message 4 of 17 , Oct 4, 2007
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            No because ]]> can end a CDATA section introduced by the embedding XHTML page which would then allow the embedding script to play tricks with entities that aren't recognized by your lexer.  Consider  /* &#42;/ .constructor /&#42;*/ where 42 === ord('*')

            Given that XHTML allows arbitrary entity definitions in DOCTYPE elements, you can't modify your lexer to recognize all entities, so if you want to restrict ADsafe JS to embeddable JS, the only thing you can do is disallow anything that looks like an entity in a pre-lexer pass.

            There's a few ways to do this:
            - require that the <, >, >=, >>, <<, %, &, and && operators and their self-assignment versions be separated by whitespace from other tokens
            - require that characters in [<>&%] in string literals and regular expressions be hex/octal/unicode escaped

            But even if you do that, if you advertise the output as "safe for embedding in script" tags, someone will go and put it in an onclick handler, and you can't produce javascript that contains string literals that is safe regardless of which quotes are used for html attribute values.

            And finally, embedding opens you up to all kinds of charset attacks.  IE guesses character encoding for HTML pages regardless of whether they are served with a Content-type header, but not for javascript files that have a content-type header.  You could approve javascript for embedding only to find that it causes the page to be interpreted in a completely different character set.  I can't think of any way to exploit it off the top of my head, but it would make me leery of embedding third-party javascript directly in my pages.

            mike





            On 04/10/2007, Douglas Crockford <douglas@...> wrote:

            --- In caplet@yahoogroups.com, "Mike Samuel" <mikesamuel@...> wrote:
            >
            > If you do want to allow ADsafe JS to be embedded in a script tag,
            you need
            > to deal with ]]> as well, since the following could be used to throw
            alert

            Is it sufficient to disallow <![ ?


             



             


          • Adam Barth
            Another situation you may or may not have considered is the following: (function(){ throw hi! ; })(); This brings up the issue of what exactly is ADsafe
            Message 5 of 17 , Oct 6, 2007
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              Another situation you may or may not have considered is the following:

              (function(){
              throw "hi!";
              })();

              This brings up the issue of what exactly is ADsafe guaranteeing its
              users. One simple guarantee is that the JavaScript is unable to read
              or write any global variables and is unable to call any global
              function (except as mediated by the ADSAFE object). This fails here
              because this code is able to call the global "onerror" function.

              The simplest solution is to require ADsafe code to be written using
              the following idiom:

              ADSAFE.call(function(){
              ...
              });

              Here the supplied JavaScript does not actually invoke the anonymous
              function it declares but asks the ADSAFE object to call the function.
              The ADSAFE object can then call the function inside a try ... catch
              block and expose an interface, like ADSAFE.onerror, to embedding
              applications to handle exceptions from ADsafe JavaScript.

              This idiom might be useful for other ADsafe features as well as the
              ADSAFE object would have a pointer to the JavaScript function.

              Adam
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