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
From: John Bradley [mailto:[email protected]]
Sent: Thursday, June 27, 2013 8:49 AM
To: Tim Bray
Cc: Manger, James H; Mike Jones; Jim Schaad; [email protected];
[email protected]
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
<[email protected]<mailto:[email protected]>> 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
<[email protected]<mailto:[email protected]>> 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:[email protected]<mailto:[email protected]>]
Sent: Thursday, 27 June 2013 4:16 PM
To: Manger, James H
Cc: Mike Jones; Jim Schaad; [email protected]<mailto:[email protected]>;
[email protected]<mailto:[email protected]>
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
<[email protected]<mailto:[email protected]>> 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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