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Re: ADsafe, Take 3

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  • Douglas Crockford
    ... you need ... alert Is it sufficient to disallow
    Message 1 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 2 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 3 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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