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Re: [json] Re: JSON and the Unicode Standard

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  • Tatu Saloranta
    ... Yes. ... Ok. But in this case, would JSON specification itself help a lot? I understand that this is problematic, in that different platforms can choose
    Message 1 of 35 , Feb 26, 2011
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      On Fri, Feb 25, 2011 at 8:01 PM, johne_ganz <john.engelhart@...> wrote:
      > --- In json@yahoogroups.com, Tatu Saloranta <tsaloranta@...> wrote:
      > I have not seen a JSON implementation / parser that does such normalization.
      > On the other hand, I very strongly suspect that whether or not such normalization is taking place is not up to the writer of that parser.  In


      > my particular case (JSONKit, for Objective-C), I pass the parsed JSON String to the NSString class to instantiate an object.
      > I have ZERO control over what and how NSString interprets or manipulates the parsed JSON String that finally becomes the instantiated object that ostensibly the same as the original JSON String used to create it.  It could be that NSString decides that the instantiated object is
      > always converted to its precomposed form.  Objective-C is flexible enough where someone might decide to swizzle in some logic at run time that forces all strings to be precomposed before being handed off to the main NSString instantiation method.

      Ok. But in this case, would JSON specification itself help a lot? I
      understand that this is problematic, in that different platforms can
      choose different default (and possible opaque dealing).

      > I don't have a particular opinion on the matter one way or the other other than to highlight the point that in many practical, real-world situations, whether or not such things take place may not be under the control of the JSON parser.
      > I also suspect that it's one of those things that most people haven't really given a whole lot of consideration to- they just had the parsed string over to "the Unicode string handling code", and that's that. Most people may not realize that such string handling code may subtly alter the original Unicode text as a result (ala precomposing the string).

      Right. And if specification says nothing, it can uncover real
      complexities and ambiguities.

      >> to tackle such complexity). While it would seem wrong to punt the
      >> issue, there is the practical question of whether full solution would
      >> matter.
      > I can guarantee you that the practical question of whether a full solution would matter will be answered the first time someone exploits it in a security vulnerable way that results in a major security fiasco.

      I would be interested in how you would see this leading to security
      issues, outside of problems specific String handling on platforms has.
      Or are you equally concerned in general about parser implementation
      quality (which is understandable), above and beyond question of what
      JSON specification says? At least to me it would seem more likely that
      issues would be outside of realm of core specification itself.

      > Then it will be with 20/20 hindsight, and the question will be "Why didn't anyone address (this behavior) that allowed two keys that were not bit for bit identical, but became identical after converting them to their precomposed form, and the security checks allowed the
      > decomposed form through because it assumed that everything was in precomposed form?"

      I can see how this can be problematic from side of applications that
      make assumptions on uniqueness. And also that it is important that
      parsers will clearly define how they handle things -- not all parsers
      necessarily even check for uniqueness for same byte patterns, much
      less for normalization (and I think this is even allowed by the spec,
      i.e. uniqueness checks are not mandated).

      So in a way, it would be useful to have bit more concrete examples of
      known practical issues. Links below may give some insight -- but it
      would seem that they are typically platform specific. Which makes it
      even harder to find shared solutions, or to recommend best practices.

      > Unfortunately, the use of Unicode coupled with the fact that most JSON implementations are dependent on external code for their Unicode support means that this is an extremely non-trivial issue.  I can't think of a simple solution to the problem at the moment, other than it exists.
      > You really ought to read:
      > http://www.unicode.org/faq/security.html
      > http://www.unicode.org/reports/tr36/#Canonical_Represenation
      > Microsoft Security Bulletin (MS00-078): Patch Available for 'Web Server Folder Traversal' Vulnerability (http://www.microsoft.com/technet/security/bulletin/MS00-078.mspx, http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2000-0884)
      > Creating Arbitrary Shellcode In Unicode Expanded Strings (http://www.net-security.org/article.php?id=144)
      > There's a long history of "Those little Unicode details aren't really important" causing huge security problems later on.

      Thank you. While I had heard about issues with request to
      non-canonical UTF-8 code sequences (which were discussed to have such
      issues), I admit I had not heard much about issue regarding

      -+ Tatu +-
    • johne_ganz
      ... There is another relevant section (ECMA-262, 8.4 The String Type, pg 28) When a String contains actual textual data, each element is considered to be a
      Message 35 of 35 , Mar 3, 2011
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        --- In json@yahoogroups.com, Dave Gamble <davegamble@...> wrote:
        > To save people looking it up:
        > ECMA-262, section 7.6:
        > Two IdentifierName that are canonically equivalent according to the
        > Unicode standard are not equal unless they are represented by the
        > exact same sequence of code units (in other words, conforming
        > ECMAScript implementations are only required to do bitwise comparison
        > on IdentifierName values). The intent is that the incoming source text
        > has been converted to normalised form C before it reaches the
        > compiler.
        > ECMAScript implementations may recognize identifier characters defined
        > in later editions of the Unicode Standard. If portability is a
        > concern, programmers should only employ identifier characters defined
        > in Unicode 3.0.

        There is another relevant section (ECMA-262, 8.4 The String Type, pg 28)

        When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

        NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

        > I think it's fairly clear that a JSON parser has ABSOLUTELY NO
        > BUSINESS poking around with actual data strings; Douglas has been very
        > clear that you are to pass them bit-identical to the recipient. On the
        > other hand, there's an argument for some kind of sanitation when it
        > comes to object member names.
        > I'm really tempted by the idea of a JSON-secure spec, which clamps
        > down on these details.

        I disagree with your first statement. The ECMA-262 standard, at least in my opinion, tries to side step a lot of these issues. It makes a fairly clear distinction between "what happens inside the ECMA-262 environment (which it obviously has near total control over)" and "what happens outside the ECMA-262 environment".

        IMHO, the ECMA-262 standard advocates that "stuff that happens outside the ECMA-262 environment should be treated as if it is NFC".

        Since the sine qua non of JSON is the interchange of information between different environments and implementations, it must address any issues that can and will cause difficulties. Like it or not, the fact that it's Unicode means these things can and will happen, and it's simply not practical to expect or insist that every implementation treat JSON Strings as "just a simple array of Unicode Code Points".

        > Arguing the Unicode details is decidedly NOT compatible with the
        > "spirit" of JSON, which Douglas has been very clear about; a
        > lightweight, simple, modern data representation.

        I completely agree that these details are NOT compatible with the "spirit" of JSON.

        But.... so what? Unicode is not simple. I'm not the one who made it that way, but the way that RFC 4627 is written, you must deal with it. There are ways RFC 4627 could have been written such that the JSON to be parsed is considered a stream of 8 bit bytes, and therefore stripped of its Unicode semantics (if any). However, it very clearly and plainly says "JSON text SHALL be encoded in Unicode.", which pretty much kills the idea that you can just treat it as raw bytes.

        There's a saying about formalized standards: The standard is right. Even it's mistakes.

        As an aside, there is a RFC for "Unicode Format for Network Interchange", RFC 5198 (http://tools.ietf.org/html/rfc5198). It is 18 pages long. RFC 4627 is just 9 pages.

        Actually, I would encourage people to read RFC 5198. I'm not sure I agree with all of it, but it goes over a lot of the issues I think are very relevant to this conversation. It's great background info if you're not familiar with the details.
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