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Re: ADsafe, Take 3

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  • Douglas Crockford
    ... In a .js file, it is harmless. In an .html file, it produces an error.
    Message 1 of 17 , Oct 3, 2007
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      --- 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.
    • Adam Barth
      ... When embedded in HTML, it calls alert (at least in IE7 and Firefox 2): http://crypto.stanford.edu/~abarth/jslint/parse.html Adam
      Message 2 of 17 , Oct 3, 2007
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        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
      • Adam Barth
        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
        Message 3 of 17 , Oct 3, 2007
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          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
          >
        • 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 4 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; //&#x0a; ]];
              //&#x0a; 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 5 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 6 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 7 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 8 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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