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RE: [xml-dev] json v. xml

Maybe one of you folks with more experience in the security aspects of the 
JSON/XML business could clarify something for me.  I've heard it alleged 
that among the other attractions of JSON is that typical browser security 
policies allow one to do cross-site retrieval of JavaScript in 
circumstances where XML retrieval would be disallowed.  Two questions:

1. Is this true?
2. If so, am I the only one who thinks this is bizarre?  Whatever the 
history of these policies, we have a situation in which information 
transmitted in the form of an executable Turing-complete programming 
language (JavaScript) are allowed, but in which information in the form of 
a declarative markup language are blocked as a security risk?

Now, I'm not against JSON.  The other good reasons for its use have been 
mentioned in this thread.  I also understand that anyone with any serious 
interest in security will not blindly eval whatever they get back as 
purported JSON, that the JSON subset of JavaScript is indeed declarative 
and not even close to Turing-complete.  I even agree that one can transmit 
Turing-complete code as XML (XSL comes to mind, or you can put C Code in 
into CDATA I suppose.)  The point is that almost no sensible default 
processing of XML raises the same sorts of security issues that would 
normally be associated with executing arbitrary JavaScript, and we all 
know that one of the ways to try and trick a JSON client is to send it 
non-JSON JavaScript..

So, insofar as my sketchy understanding of the situation is correct, and 
blithely ignoring the many compatibility issue that prevent sensible 
changes to already-deployed systems, wouldn't it make sense to ensure that 
the security limits on cross-site downloading of script fragments such as 
JSON are at least as tight as those on XML?  Then, insofar as cross site 
access to JSON survives such changes, we could go back to letting users 
choose whichever format makes them happier in a given situation.


Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142

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