Loading ...
Sorry, an error occurred while loading the content.

Re: ADsafe, Take 3

Expand Messages
  • collin_jackson
    I am concerned about browser differences in the handling of null bytes (and other special characters). Example:
    Message 1 of 17 , Oct 3, 2007
    • 0 Attachment
      I am concerned about browser differences in the handling of null bytes
      (and other special characters).

      Example: http://crypto.stanford.edu/jsonrequest/nullbyte.html

      The Adsafe checks pass on Firefox, and the code is harmless. On IE,
      the checks fail, and the code is dangerous. If you only checked the
      script on Firefox but the visitor viewed it in IE, you'd have a
      problem.

      Another concern is that if the code is copy-pasted from Notepad to the
      browser, the null byte may be converted to a space, so JSLint won't
      detect it. Is there a better way to feed code to JSLint other than
      copy paste?

      --- In caplet@yahoogroups.com, "Douglas Crockford" <douglas@...> wrote:
      >
      > --- In caplet@yahoogroups.com, "collin_jackson" <collinj@> wrote:
      > >
      > > /*@cc_on alert("Conditional compilation considered harmful"); @*/
      >
      > Good one. I owe you a plate of shrimp.
      >
    • Adam Barth
      This seems to get through the filter: (function() { var str = alert( script tags affect parsing )/* ; })(); /**/ Adam
      Message 2 of 17 , Oct 3, 2007
      • 0 Attachment
        This seems to get through the filter:

        (function() {
        var str = "</script><script>alert('script tags affect parsing')/*";
        })();
        /**/

        Adam
      • David Hopwood
        Douglas Crockford wrote: [...] ... IMHO the rejections should not be silent; they should throw an exception. In any case, I prefer my suggestion to use
        Message 3 of 17 , Oct 3, 2007
        • 0 Attachment
          Douglas Crockford wrote:
          [...]
          > return {
          > get: function (object, name) {
          > var value;
          > if (reject(object, name)) {
          > return;
          > }
          > value = object[name];
          > if (typeof value === 'function') {
          > return;
          > }
          > return value;
          > },
          > set: function (object, name, value) {
          > if (reject(object, name) || typeof value === 'function') {
          > return;
          > }
          > object[name] = value;
          > return object;
          > }
          > };
          > }();

          IMHO the rejections should not be silent; they should throw an exception.

          In any case, I prefer my suggestion to use object.get(name) and
          object.set(name, value). That way, these methods can be added only to
          prototypes of objects that should be indexable, rather than all objects
          being indexable by default. Since implementations of .get and .set in user
          code would not be able to use the [] syntax, there is then no need for
          explicit checks. (The trusted ADsafe implementation can of course use
          [] where it needs to.)

          --
          David Hopwood <david.hopwood@...>
        • Douglas Crockford
          ... Good point. I am now scanning for the presence of any control character. ... JSLint runs in a number of configurations, including Rhino and WSH, which read
          Message 4 of 17 , Oct 3, 2007
          • 0 Attachment
            --- In caplet@yahoogroups.com, "collin_jackson" <collinj@...> wrote:
            >
            > I am concerned about browser differences in the handling of null bytes
            > (and other special characters).

            Good point. I am now scanning for the presence of any control character.

            > Another concern is that if the code is copy-pasted from Notepad to the
            > browser, the null byte may be converted to a space, so JSLint won't
            > detect it. Is there a better way to feed code to JSLint other than
            > copy paste?

            JSLint runs in a number of configurations, including Rhino and WSH,
            which read text directly from files.
          • Douglas Crockford
            ... In a .js file, it is harmless. In an .html file, it produces an error.
            Message 5 of 17 , Oct 3, 2007
            • 0 Attachment
              --- 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 6 of 17 , Oct 3, 2007
              • 0 Attachment
                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 7 of 17 , Oct 3, 2007
                • 0 Attachment
                  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 8 of 17 , Oct 4, 2007
                  • 0 Attachment
                    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 9 of 17 , Oct 4, 2007
                    • 0 Attachment
                      --- 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 10 of 17 , Oct 4, 2007
                      • 0 Attachment
                        --- 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 11 of 17 , Oct 4, 2007
                        • 0 Attachment
                          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 12 of 17 , Oct 6, 2007
                          • 0 Attachment
                            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
                          Your message has been successfully submitted and would be delivered to recipients shortly.