Re: [jose] #27: member names MUST be unique needs additional text

[Richard Barnes <[e-mail]>]

On Thu, Jun 27, 2013 at 12:42 PM, Mike Jones <[e-mail]>wrote:

>  I’ve come to believe that James’ proposed language does a good job of
> making security-appropriate restrictions while allowing the use of existing
> parsers.  It was:****
>
> ** **
>
> A creator of a JOSE message MUST NOT put duplicate names in any JSON
> object in a JOSE header.  To prevent attacks where a JOSE message is
> interpreted as different valid messages by different recipients, each
> recipient MUST either reject messages with duplicate names or accept only
> the last name.****
>
> ** **
>
> I might change “or accept only the last name” to “or use a parser that
> always uses the lexically last name for any duplicate member names, per
> [ECMAScript]” for clarity, but the intent is the same.****
>
> ** **
>
> Would that work for people?****
>
> ** **
>
>                                                             -- Mike
>


That's close.  I don't think we can allow the "lexically last" because
elements in a JSON dictionary can be re-ordered.  Suppose there a helpful
intermediary that sorts a JSON dictionary in descending alphabetical order
of values.
When it gets: { "payload": "bad stuff",  "payload": "good stuff", ... }
It emits: { "payload": "good stuff",  "payload": "bad stuff", ... }

The only way to avoid the ambiguity is to reject objects with duplicate
keys.


I realize that this renders most JSON parsers incompatible.  Implementors
will need to add a secondary check.  I feel your pain.  But hopefully this
pain will be temporary; the ambiguity will be fixed by the JSON WG and
ECMA, and parsers will be updated.  Even in the short run, it doesn't seem
like it would be that hard to mod existing parsers to have a "strict" flag.

"""
A creator of a JOSE message MUST NOT put duplicate names in any JSON object
in a JOSE header.  To prevent attacks where a JOSE message is interpreted
as different valid messages by different recipients, each recipient MUST
reject messages.  Implementations MUST NOT select the lexically last name,
as required by [ECMAScript], since this could lead them to select the
incorrect value if an intermediary re-orders the fields in a dictionary.
 If an implementation uses a JSON parser that does not reject objects with
duplicate fields, then it MUST verify that there are no dictionaries with
duplicate fields in the JSON object.
"""

--Richard






> ****
>
> ** **
>
> *From:* John Bradley [mailto:[e-mail]]
> *Sent:* Thursday, June 27, 2013 8:49 AM
> *To:* Tim Bray
> *Cc:* Manger, James H; Mike Jones; Jim Schaad; [e-mail];
> [e-mail]
>
> *Subject:* Re: [jose] #27: member names MUST be unique needs additional
> text****
>
>  ** **
>
> The problem is not wanting to leave a hole for malicious signers to create
> JWT that may be misinterpreted by the receiver.  Accidental interop
> problems from bad signers are not a big concern for me and are covered by
> MUST NOT emit dups.****
>
> ** **
>
> Accepting malformed JSON as the envelope may lead to real security issues.
> ****
>
> ** **
>
> This may be more of an issue for the JSON encoding than the compact as
> there you need to be careful the body you process for the signature is the
> body that the application uses.****
>
> ** **
>
> What happens if you receive.****
>
> ** **
>
> {"protected":<integrity-protected shared header contents>",****
>
>       "unprotected":<non-integrity-protected shared header contents>",****
>
>       "payload":"<original payload contents>",****
>
>       "payload":"<bad payload contents inserted by attacker>"****
>
>       "signatures":[****
>
>        {"header":"<per-signature unprotected header 1 contents>",****
>
>         "signature":"<signature 1 contents>"},****
>
>        ...****
>
>        {"header":"<per-signature unprotected header N contents>",****
>
>         "signature":"<signature N contents>"}],****
>
>      }****
>
>  ** **
>
> If the JOSE library validates on the first version of "payload" and the
> application using a different JSON parser only sees the second "payload" we
> have a wrapping attack similar to xmldsig.****
>
> ** **
>
> I can't off the top of my head think of what you could do in the compact
> serialization that would lead to a successful attack given that the
> signature is over the base64url encoded segments, that prevents attackers
> from modifying the JSON while maintaining integrity.****
>
> ** **
>
> It is a real issue for the JSON serialization though.****
>
> ** **
>
> John B.****
>
> ** **
>
> On 2013-06-27, at 11:20 AM, Tim Bray <[e-mail]> wrote:****
>
>
>
> ****
>
> The approach just seems wrong.  If we require that conforming
> implementations MUST NOT emit dupes, then authors of receiving software can
> just pick the best-performing parser with the nicest API, because they
> *all* perform correctly when there are no dupes.
>
> Who’s going to own the responsibility for making the authoritative finding
> that of the many JSON parsers in the Java ecosystem, these ones, but not
> those ones, are usable in JOSE applications?  And suppose one that has been
> previously OK is quietly optimized on GitHub to run 30% faster and as a
> side-effect silently ignores dupes?  ****
>
> The benefit of using JSON is that there is widely deployed software to
> handle it.  It is a known problem that sloppy spec drafting has allowed
> various kinds of problems to occur when dupe keys are received. “Doctor, it
> hurts when I do this.” “So... don’t do that.”****
>
> Are we also going to specify the behavior of receiving software when the
> JSON has a misplaced quotation mark?  A missing trailing “}”?****
>
>  -T****
>
> ** **
>
> ** **
>
> On Wed, Jun 26, 2013 at 11:29 PM, Manger, James H <
> [e-mail]> wrote:****
>
> Most JSON libraries that silently accept [sob] dupe keys at least
> consistently use the last key. Allowing just that behaviour (or rejecting
> dupes), while forbidding acceptance of a non-last dupe (eg first key), is
> safe and allows most JSON libraries to be used. Since this accepts most
> libraries I think it is practical.****
>
>  ****
>
> Hence I suggest:****
>
>  ****
>
> >   A creator of a JOSE message MUST NOT put duplicate names in any JSON**
> **
>
> > object in a JOSE header.****
>
> >   To prevent attacks where a JOSE message is interpreted as different***
> *
>
> > valid messages by different recipients, each recipient MUST either****
>
> > reject messages with duplicate names or accept only the last name.****
>
>  ****
>
> This isn’t “truly predictable” as a message with dupes might be accepted
> or rejected. The crucial point is that if a message is accepted it is
> predictable.****
>
>  ****
>
> --****
>
> James Manger****
>
>  ****
>
> *From:* Tim Bray [mailto:[e-mail]]
> *Sent:* Thursday, 27 June 2013 4:16 PM
> *To:* Manger, James H
> *Cc:* Mike Jones; Jim Schaad; [e-mail];
> [e-mail]****
>
>
> *Subject:* Re: [jose] #27: member names MUST be unique needs additional
> text****
>
> ** **
>
>  ****
>
> Well, you have a practical problem in that most implementers will want to
> use a standard JSON library, which is good practice because it will be
> well-debugged, and most libraries [sob] silently take care of dupe keys and
> don’t have a way to tell the client software what happened. So if you want
> truly predictable behavior, you’re forcing the use of hand-constructed JSON
> parsers. And that sucks, because getting good performance in JSON parsing
> is surprisingly hard, with dramatic performance differences between
> implementations. So you’re forcing receiving software which wants to be
> conformant to use a hand-rolled parser which will probably have lousy
> performance and have other bugs which in fact may compromise security more
> than dupe-key tricks could.  -T****
>
>  ****
>
> On Wed, Jun 26, 2013 at 10:39 PM, Manger, James H <
> [e-mail]> wrote:****
>
> > I think it’s a nice clean minimal solution to say that producers MUST
> > NOT generate dupes, end of story.  I don’t think saying anything beyond
> > that adds value. -T****
>
> Clean and minimal that may be, but it ignores the security issue. We don't
> want a malicious producer (who is so malicious they ignore a MUST) to
> create JOSE messages that a JOSE-compliant security layer accepts as
> "benign interpretation #1" so it passes the message on to the
> JOSE-compliant backend app that acts on "nasty interpretation #2".
>
> --
> James Manger****
>
>  ****
>
> ** **
>
> ** **
>
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