Re: [OAUTH-WG] a token review of draft-ietf-oauth-access-token-jwt-01/-02

Vittorio Bertocci <Vittorio@auth0.com> Wed, 24 July 2019 21:46 UTC

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From: Vittorio Bertocci <Vittorio@auth0.com>
Date: Wed, 24 Jul 2019 14:45:56 -0700
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To: Brian Campbell <bcampbell=40pingidentity.com@dmarc.ietf.org>
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Subject: Re: [OAUTH-WG] a token review of draft-ietf-oauth-access-token-jwt-01/-02
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Thank you Brian for the thorough and insightful review!

Comments:

> On authenticated encryption.
I did chat with Neil about his draft, but as you mention I didn't reference
it given that it hasn't bee picked up (yet?).
On referencing JWE RFC7516 and more JWA RFC7518, I am reluctant. My
rationale is that this profile attempts to capture current practices or
practices that are close enough to the capabilities of existing systems to
have a reasonable chance of being adopted. None of the products/services
surveyed used authenticated encryption, and the requirement to acquire a
symmetric key out of band throws  a big wrench in the otherwise clear-cut
process of preparing your RS to handle JWT ATs. I will bring the matter up
as open issue on Friday, and if the consensus will be to include symmetric
authenticated encryption I'll do that; but my personal preference would be
to preserve the simplicity of a concrete,
nothing-of-substance-left-as-exercise-to-the-reader solution that can be
achieved when the key material can be advertised via metadata.
Typo fixed.


>Connect doesn't say anything about checking the "typ" header of id tokens
[..] this document probably shouldn't overstate what typing the JWT can
accomplish.  [..]  'nonce' in ID Tokens delivered via the front channel
[..] to be interpreted as id_tokens.

This is a very fair point, the current language is misleading. I rephrased
to position the header typing as a hint.
About the nonce mitigation you mention. Do you think this means that I
should explicitly forbid the presence of a "nonce" claim in JWT ATs?


>I think you want to say here that the scope claim in the token has to
correlate to the scope which was approved. Not necessarily what what
requested. The authorization request might ask for scope of a+b+c, for
example, while the user only approves b. Or any other variation on things
where what was asked for isn't what was gotten..

Here I am trying not to get into the details of what's inside the scope
claim, more requesting that if "scope" was in the request, the issued token
should express a delegation and a symptom of that should be the presence of
the scope claim. Paradoxically that scope claim might be empty if the user
only consented to scopes that have effect on the presence of other claims
in the token (say a functional equivalent of profile). Yes, it does sound
odd, but that's a side effect of the prohibition of sending id_tokens to
API that creates the requirement to create functional equivalents.


> https://tools.ietf.org/html/draft-ietf-oauth-resource-indicators has an
"invalid_target" error code that is intended for this kind of thing. So
that should probably be allowed. If not suggested. Probably not required
though.

Wonderful, that is a better fit- love it. I added it as possible
alternative to "invalid_request".


>I personally think the SHOULD is too strong here because it puts the onus
on the resource server to know about (via some config option or something)
and enforce on every transaction a setup/policy thing that the AS is
responsible for which isn't about the integrity of the authorization data
in the token. A MAY or even removing the list item would be preferred.

I borrowed this language straight from the OpenID Connect validation steps
for idtokens. Given that JWT ATs can carry identity info, the requirement
seemed appropriate... also, I think it is reasonable to expect the resource
server to know about its own registration- just like it must know about the
expected aud value and what key should be used to decrypt messages, it
should know whether encryption was requested or not. In fact, the fact that
such option was selected at registration time is likely the reflection of a
policy of the RS itself, something the RS itself would want to ensure has
been respected. WDYT?


>multi value audiences

I see the issue. I definitely don't want to redefine the semantic of aud,
but I also would like to be as crisp as possible on the audience-scope
correlation and prevent scopes from being misinterpreted as applying to the
wrong resource. The aud validation will likely happen in some middleware in
front of the API, but authorization checks might happen in the body of the
API itself when the actual access is being attempted, and having an OR list
in the aud claim might lead to false positives. RSes correctly written
should not suffer from this issue (the authorization logic should receive
only the value from the aud collection that is an actual match) but I have
seen enough sloppy implementations to be skeptical about this.
As a result, I would be inclined to  take out the sentence "or if it
contains additional audiences that are not known aliases of the resource
indicator of the current resource server", effectively restricting to
single value aud and eliminating this issue.


>I think it'd be worthwhile to explicitly disallow the use of the "none"
algorithm here.

Good point. I am all for doing everything we can to defuse that FUD. I
added language that explicitly disallow RS to accept JWTs with "alg":"none"


>Little 'o' in TOken -> Token
fixed


> A link directly to SCIM alone from the JSON Web Token Claims registry
might be rather confusing.

Added a reference as you suggested in all the entries. I am always confused
by forward references whenever the syntax requires the use of an RFC number
that isn't available, but the [[]] trick is a good one.
I didn't follow the "subcompact" considerations, I'll bug you offline for
clarifications. Thx in advance


>Ping<blank>identity

Fixed! Sorry for not having done in 01, I think you mentioned this already.

On Wed, Jul 24, 2019 at 12:59 PM Brian Campbell <bcampbell=
40pingidentity.com@dmarc.ietf.org>; wrote:

>
> 2.1.  Header
>
>>    NOTE: there were discussions about adding a reference to
>>    authenticated encyption methods as well, but there's no internet
>>    draft specifying interoperable public key methods at this time
>>
> Well, Neil did write this up a while back
> https://tools.ietf.org/html/draft-madden-jose-ecdh-1pu-01 which is
> certainly interesting and I wonder if it's something that this group (or
> some other group?) should look at working on. But it is only an individual
> draft (with all the authority that brings to bear
> <https://tools..ietf.org/html/draft-abr-twitter-reply-00>) and thus I do
> agree that it isn't appropriate or useful for this document to reference.
>
> However JWE RFC7516 and more JWA RFC7518 do have authenticated encryption
> with symmetric keys that, IMHO, can work very nicely in some cases for JWT
> access tokens by providing both integrity protection and confidentiality.
> And the size and performance benefits thereof can be sufficiently useful so
> as to justify the need to do an out-of-band exchange of the symmetric key.
>
> Also encyption should be encryption.
>
>
> 2.1.  Header
>
>>    The typ header parameter for a JWT access token MUST be at+jwt.  See
>>    the security considerations section for details on the importance of
>>    preventing JWT access tokens to be interpreted as id_tokens.
>
> 5.  Security Considerations
>
>>    The JWT access token data layout described here is very similar to
>>    the one of the id_token as defined by [OpenID.Core].  Without the
>>    explicit typing required in this profile, in line with the
>>    recommendations in [JWT..BestPractices] there would be the risk of
>>    attackers using JWT access tokens in lieu of id_tokens.
>
> Connect doesn't say anything about checking the "typ" header of id tokens
> so typing ATs with "at+jwt" doesn't actually do anything to prevent JWT
> access tokens being used as id tokens. It can prevent misuse in the other
> direction. But this document probably shouldn't overstate what typing the
> JWT can accomplish.
>
> BTW, there's been some discussion and agreement that the requirement
> around 'nonce' in ID Tokens delivered via the front channel (the only time
> connect is subject to that kind of token swapping) is sufficient to protect
> against JWT access tokens (and other types of JWTs) to be interpreted as
> id_tokens. So I think the risk is acceptable but just think the text in the
> draft shouldn't make claims which are untrue.
>
>
> 2.2.2.  Authorization Claims:
>
>>    If an authorization request includes a scope parameter, the
>>    corresponding issued JWT access token
>>
>   MUST in -01/SHOULD in -02 include a scope claim
>>
> as defined in
>>
> I think you want to say here that the scope claim in the token has to
> correlate to the scope which was approved. Not necessarily what what
> requested. The authorization request might ask for scope of a+b+c, for
> example, while the user only approves b. Or any other variation on things
> where what was asked for isn't what was gotten..
>
>
> 3.  Requesting a JWT Access Token
>
>>    If it receives a request for an access token containing more than one
>>    resource parameter, an authorization server issuing JWT access tokens
>>    MUST reject the request and fail with invalid_request as described in
>>    section 4.1.2.1 of [RFC6749].
>>
> https://tools.ietf.org/html/draft-ietf-oauth-resource-indicators has an
> "invalid_target" error code that is intended for this kind of thing. So
> that should probably be allowed. If not suggested. Probably not required
> though.
>
> 4.  Validating JWT Access Tokens
>
>>       If encryption was negotiated with the
>>        authorization server at registration time and the incoming JWT
>>        access token is not encrypted, the resource server SHOULD reject
>>        it.
>>
> I personally think the SHOULD is too strong here because it puts the onus
> on the resource server to know about (via some config option or something)
> and enforce on every transaction a setup/policy thing that the AS is
> responsible for which isn't about the integrity of the authorization data
> in the token. A MAY or even removing the list item would be preferred.
>
> 4.  Validating JWT Access Tokens
>
>>      The JWT access token MUST be
>>        rejected if aud does not list the resource indicator of the
>>        current resource server as a valid audience, or if it contains
>>        additional audiences that are not known aliases of the resource
>>        indicator of the current resource server.
>>
> 5.  Security Considerations
>
>>    This profile explicitly forbids the use of multi value aud claim when
>>    the individual values refer to different resources, as that would
>>    introduce confusion about what scopes apply to which resource-
>>    possibly opening up avenues for elevation of delegated privileges
>>    attacks.  Alternative techniques to prevent scope confusion include
>>    "scope stuffing", imposing to every individual scope string to
>>    include a reference to the resource they are meant to be applied to,
>>    but its application is problematic (scope opacity violations, size
>>    inflation, more error conditions become possible when the combination
>>    of requested scopes and resource indicators is invalid) and the
>>    observed frequency of the scenario doesn't warrant complicating the
>>    more common cases.
>>
> I do think I see the simplification you're aiming at with this stuff but
> I'd like to offer the perspective of how this is likely more complicated
> for standards based JWT library implementations. The semantics of aud in
> RFC 7519 basically say that the recipient has to identify with at least one
> of the aud values. It's effectively a big OR. While the text in this draft
> changes that semantic to say that the recipient has to identify with at
> every one of the aud values. Effectively making it a big AND. Normal JWT
> libraries will need nonstandard one-off functionality or adaptations to get
> this right.
>
>
>  4.  Validating JWT Access Tokens
>
>>       The resource server MUST validate the signature of all incoming
>>        JWT access token according to [RFC7515] using the algorithm
>>        specified in the JWT alg Header Parameter.  The resource server
>>        MUST use the keys provided by the authorization server.
>>
> I think it'd be worthwhile to explicitly disallow the use of the "none"
> algorithm here. I think/suspect that it is implied already. But at least
> some of the JWT hate out there seems to stem from or focus on statements
> like this one and conclude that words like "MUST validate ... according to
> [the] algorithm specified in the JWT alg Header Parameter" means that to be
> spec compliant one has to accept "alg":"none" as valid. That's more of a
> logical leap than I think is reasonable but I'd like to avoid it anyway if
> possible. And it's not a bad thing to remind folks not to accept "none"
> here.
>
> 7.2.  Claims Registration
>
>>     claims in the JSON Web TOken (JWT) IANA
>>
> Little 'o' in TOken -> Token
>
>
> 7.2.1.  Registry Contents
> |   Specification Document(s): section 4.1.2 of [RFC7643]
> Ultimately this is what will show up in the "References" column in the
> registry https://www.iana.org/assignments/jwt/jwt.xhtml and thus I'd
> suggest that it also include something like "Section 2.2.2.1 of [[this
> specification]]" there too so someone who starts from the registry has the
> link to and context of this document. A link directly to SCIM alone from
> the JSON Web Token Claims registry might be rather confusing.
> For the folks that will have to review and act on these registrations at
> some point, it might be nice to give em a little better
> grouping/formatting. I'm thinking along the lines of what is done here
> https://tools.ietf.org/html/draft-ietf-oauth-token-exchange-18#section-7.4.1,
> which you can corice xml2rfc into doing with <?rfc subcompact="yes"?> and <?
> rfc subcompact="no"?> as in the src at
> https://github.com/oauth-token-exchange/spec/blob/master/draft-ietf-oauth-token-exchange.xml#L1191
>
> Appendix A.  Acknowledgements
>
>>     Brian Campbell (PingIdentity)
>>
> My corporate overloads will be happier if you put a space between Ping and
> Identity.
>
>
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