Re: [OAUTH-WG] Last Call: <draft-ietf-oauth-jwt-introspection-response-09.txt> (JWT Response for OAuth Token Introspection) to Proposed Standard

Benjamin Kaduk <kaduk@mit.edu> Mon, 31 August 2020 22:48 UTC

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Date: Mon, 31 Aug 2020 15:47:53 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Denis <denis.ietf@free.fr>
Cc: Torsten Lodderstedt <torsten=40lodderstedt.net@dmarc.ietf.org>, "last-call@ietf.org" <last-call@ietf.org>, oauth <oauth@ietf.org>, Mike Jones <Michael.Jones=40microsoft.com@dmarc.ietf.org>
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Subject: Re: [OAUTH-WG] Last Call: <draft-ietf-oauth-jwt-introspection-response-09.txt> (JWT Response for OAuth Token Introspection) to Proposed Standard
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Hi all,

On Mon, Aug 31, 2020 at 09:58:11AM +0200, Denis wrote:
> The last text that has been proposed on the list about this thread is 
> the following:
> 
> Implementers should be aware that a token introspection request lets the 
> AS know when the client is accessing the RS,
>        which can also indicate when the user is using the client. If 
> this implication is not acceptable, implementers can use other means
>        to carry access token data, e.g. directly transferring the data 
> needed by the RS within the access token.
> 
> The concerns of the implementers have nothing to do with the concerns of 
> the Users. Such a text proposal has nothing to do with a "User consent".
> 
> *Towards an RFC Errata to RFC 7662*
> 
> Mike Jones wrote:
> 
> I agree with Dick’s observation about the privacy implications of using 
> an Introspection Endpoint. That’s why it’s preferable to not use one at all
>        and instead directly have the Resource understand the Access 
> Token. One way of doing this is the JWT Access Token spec. There are 
> plenty of others.
> 
> I fully agree.
> 
> RFC 7662 should have incorporated a more detailed content such as:
> 
>       In OAuth 2.0 [RFC6749], the contents of tokens are opaque to 
> clients. However, the contents of tokens is not intended to be opaque to 
> RSs.
>       Token introspection is an OPTIONAL feature of an AS described in 
> OAuth Introspection [RFC 7662] intended for clients that are unable
>       to support structured access tokens including their validation. 
> The use of this call allows an AS to track where and when its clients 
> have indeed
>       presented an issued access token. As soon as the RS knows the 
> format of the access token, e.g. using structured token formats such as
>       JWT [RFC7519], and is able to validate its security features, the 
> call described in OAuth Introspection [RFC 7662] should be avoided, 
> otherwise
>       the AS will know exactly when the introspection call has been made 
> and thus be able to make sure which client has attempted perform an access
>       to that RS and at which instant of time. As soon as this call is 
> supported by an AS, the client or the user have no way to prevent the RS 
> to use it.
> 
> It might be useful to add it, e.g. using an RFC Errata.

I do not believe this would be an appropriate usage of an Errata Report --
it changes the meaning of the RFC away from what the WG intended at the
time of publication.

Use of tokens that are just opaque DB handles (along with some form of
introspection) is desirable when a prominent threat is leakage of token
contents from the browser.  We have had numerous discussions over the years
of various ways in which information can leak from the browser, including
history APIs, malicious javascript, and more.  While these threats are not
always applicable in all deployment models, they are still present, just as
the threats that you propose we defend against are not always of concern in
all deployment models.  AFAICT, given the technologies currently available,
there is not one universal solution that will address all concerns, and
deployments will have to make a trade-off.  I think we need to acknowledge
that there are different deployment models and that (for example) giving
the user visibility into the token contents is not always desired, given
the other risks that the current mechanisms for providing that visibility
open up.

-Ben

P.S. your usage of the phrase "the User and his client" (below) suggests
that you are still picturing the client as being local to the user, as is
the case for, e.g., a TLS client or an IMAP client.  This is not the
original model for an OAuth, where the client can just as well be a
headless server in a cloud somewhere.

> *Differences with RFC 7662*
> 
> RFC 7662 defines a protocol that allows authorized protected resources 
> to query the authorization server to determine
> the set of metadata for a given token that was presented to them by an 
> OAuth 2.0 client.
> 
> At a first glance, draft-ietf-oauth-jwt-introspection-response-09 seems 
> to be simply a JWT Response for OAuth Token Introspection
> instead of a JSON document representing the meta information surrounding 
> the token.
> 
> However, this is not the case since major differences can be identified.
> 
> RFC 7662 describes an OPTIONAL call able to return a JSON object with 
> the following top-level members: active (REQUIRED), scope (OPTIONAL),
> client_id (OPTIONAL), username (OPTIONAL), token_type (OPTIONAL), exp 
> (OPTIONAL), iat (OPTIONAL), nbf (OPTIONAL), sub (OPTIONAL),
> aud (OPTIONAL), iss (OPTIONAL), jti (OPTIONAL) and claims from the "JSON 
> Web Token Claims" registry (OPTIONAL).
> 
> draft-ietf-oauth-jwt-introspection-response-09 is able to return a JWT 
> as the introspection response. However, the request and the returned 
> information
> are not the same.
> 
> Section 4 (Requesting a JWT Response) only provides an example and does 
> not describe the mandatory and optional parameters from the request.
> Are they identical to those described in RFC 7662 ? No one is able to say.
> 
> draft-ietf-oauth-jwt-introspection-response-09 describes a response 
> structure which is different from RFC 7662 where a single top-level 
> member is required,
> i.e. active. The text from 
> draft-ietf-oauth-jwt-introspection-response-09 requires three (in fact 
> four) top-level members. The text states:
> 
> The JWT MUST include the following top-level claims:
> 
>   * issMUST be set to the issuer URL of the authorization server.
>   * audMUST identify the resource server receiving the token
>     introspection response.
>   * iatMUST be set to the time when the introspection response was
>     created by the authorization server.
> 
> Note that the text about "active" (i.e. the fourth top-level member) is 
> misplaced in the middle of the token_introspection claim and does not allow
> to easily understand that the top-level claim "active" MUST be set to 
> either true or false.
> 
> The text states:
> 
> The AS determines based on its RS-specific policy what claims about the 
> resource owner to return in the token introspection response.
> 
> Such a sentence (which does not exist in RFC 7662) is a door wide-opened 
> to return claims that are NOT included into the access token.
> This protocol should NOT be a protocol that allows authorized RSs to 
> query the AS to obtain metadata that was NOT included into an access token
> that was presented to them.
> 
> Torsten wrote:
> 
> Token introspection has several advantages over structured access 
> tokens, also when it comes to privacy. If one uses a structured access 
> token
>        in conjunction with different *services*, then this access token 
> needs to contain ALL data required to call ALL these services. This 
> effectively means
>        the services learn more data than required. One could try to 
> mitigate this by carrying encrypted compartments in the same token, each 
> of them
>        encrypted with for a certain service. That would be complex and 
> is not covered by current technical standards. Introspection, however, 
> allows the AS
>        issue a minimal access token (even without any id) and mint 
> specific response for the different RSs.
> 
> Instead of attempting to imagine complex mechanisms like "encrypted 
> compartments in the same token", it is much simpler to use different 
> access tokens.
> AFAIK, in addition, there is no notion of "services" in OAuth in RFC 
> 6749, but only a notion of "server". A "server" is not a "service".
> 
> resource server
> 
> The server hosting the protected resources, capable of accepting
> and responding to protected resource requests using access tokens.
> 
> RFC 6750 is using the wording "audience restriction" to restrict the use 
> of a token to "the intended relying party or set of relying parties"
> (i.e. to one or more RSs).
> 
> So if the access token is targeted to one or more RSs, it should only 
> contain what is necessary to accomplish the call on these RSs.
> This means the RS(s) learn(s) exactly the data that it required. If it 
> is not the case, several access tokens should be used.
> 
> The principle of "data minimization" implies that an access token should 
> only contain what is necessary to accomplish a call to a given RS.
> 
> Since this new call is meant to be OPTIONAL, there should be no 
> difference whether the RS decodes and validates the access token by itself
> or sub-contracts the same operations to the AS.
> 
> The explanations from Torsten imply that there can be a difference .... 
> which furthermore is left at the full discretion of the AS.
> 
> *About the secondary use*
> 
> I wrote:
> 
> This concern is identified in RFC 6973 as:
> 
> 5.2.3.Secondary Use
> 
> Secondary use is the use of collected information about an individual
> without the individual’s consent for a purpose different from that
> for which the information was collected.Secondary use may violate
> people’s expectations or desires.The potential for secondary use
> can generate uncertainty as to how one’s information will be used in
> the future, potentially discouraging information exchange in the
> first place.Secondary use encompasses any use of data, including
> disclosure.
> 
> Torsten replied:
> 
>        This is no secondary use, it’s the primary use *the user 
> consented with*.
> 
> No "user consent" phase is ever mentioned in order to allow a RS to ask 
> and receive a jwt-introspection-response.
> 
> The text states:
> 
> The AS MUST ensure the release of any privacy-sensitive data is legally 
> based.
> 
> Such a statement does not make sense in practice. A different legal 
> system applies depending upon three factors:
> the location of the client, the location of the AS and the location of 
> the RS.
> 
> Unless these three entities are located within the same country (e.g. 
> Switzerland), the same state (e.g. Maryland) or
> the same union (e.g. the European Union), the AS will be unable to know 
> how to enforce such a statement.
> 
> The release of any privacy-sensitive data must be under the control of 
> the user who must consent to the release
> of that privacy-sensitive data. This makes it independent from any legal 
> system.
> 
> *About the token introspection endpoint
> *
> The token introspection endpoint is advertised in RFC 8414 in the 
> following way:
> 
> introspection_endpoint
> OPTIONAL.URL of the authorization server’s OAuth 2.0 introspection 
> endpoint [RFC7662].
> 
> This endpoint is intended to explicitly support RFC 7662, but not 
> anything else. This draft is intended to support a new functionality 
> that is different
> from RFC 7662. This means that the AS should not use the same token 
> introspection endpoint. If supported, this new functionality should be 
> supported
> using a new endpoint, e.g. an introspection_JWT_endpoint.
> 
> When this new functionality is advertised by an AS by the disclosure of 
> an access point, this does not necessarily mean that it is supported for 
> all the RSs
> with which that AS has a relationship.
> 
> The current situation is that the User and his client have no way to 
> know whether or not this new call will indeed be supported by the AS for 
> a given RS.
> Even the RS can't know it directly, since the only way to know it is to 
> use a "trial and error" mechanism.
> 
> *A proposal on how to solve the issue*
> 
> Hereafter is one way on how to solve the issue.
> 
> The User is the entity that is the best placed to give the User Consent. 
> It would be simpler if the user could ask to the AS to provide to his 
> client
> a JWT for a given RS that could be used to issue certificates usable for 
> creating qualified electronic signatures. A specific scope could be defined
> for such a purpose which would detail the claims to be incorporated into 
> the access token and say that these claims shall be verified according to
> sections 6.2.2 (Initial identity validation) of both ETSI EN 319 411-1 
> and ETSI EN 319 411-2.
> 
> The client supporting the user would then communicate that JWT to the RS 
> of its choice. The scope placed into the access token would testify
> that the claims have indeed been verified according to sections 6.2.2 of 
> both ETSI EN 319 411-1 and ETSI EN 319 411-2.
> 
> Such a functionality cannot be supported using 
> draft-ietf-oauth-jwt-introspection-response.
> 
> In case the RS would be unable to decode the access token and/or to 
> validate it, it might make attempt to make a call to the AS according to 
> RFC 7662,
> if this service is available (but the user has no way to indicate that 
> he consents for making such a call).
> 
> As a conclusion, since draft-ietf-oauth-jwt-introspection-response harms 
> the user's privacy and fully by-passes the User consent,
> this draft should not be progressed to the RFC level.
> 
> Denis
> 
> 
> > The "can" works better, agreed. Thanks!
> > ᐧ
> >
> > On Sat, Aug 29, 2020 at 8:25 AM Justin Richer <jricher@mit.edu 
> > <mailto:jricher@mit.edu>> wrote:
> >
> >     Thanks, Dick. I agree with removing the excess parenthetical, but
> >     I intentionally avoided using a lowercase “may” in the middle of
> >     the text  (in favor of “can”) to avoid normative-sounding
> >     non-normative language, so I’d recommend that change be kept:
> >
> >     Implementers should be aware that a token introspection request
> >     lets the AS know when the client
> >         is accessing the RS, which can also indicate when the user is
> >     using
> >         the client. If this implication is not acceptable,
> >     implementers can use other means to carry
> >         access token data, e.g. directly transferring the data needed
> >     by the RS within the access token.
> >
> >>     On Aug 27, 2020, at 12:15 PM, Dick Hardt <dick.hardt@gmail.com
> >>     <mailto:dick.hardt@gmail.com>> wrote:
> >>
> >>     Here is a crisper revision.
> >>
> >>     Implementers should be aware that a token introspection request
> >>     lets the AS know when the client
> >>         is accessing the RS, which may indicate when the user is using
> >>         the client. If this implication is not acceptable,
> >>     implementers can use other means to carry
> >>         access token data, e.g. directly transferring the data needed
> >>     by the RS within the access token.
> >>     ᐧ
> >>
> >>     On Thu, Aug 27, 2020 at 7:19 AM Justin Richer <jricher@mit.edu
> >>     <mailto:jricher@mit.edu>> wrote:
> >>
> >>         I would clarify that this doesn’t necessarily say that the
> >>         user’s there, and remove the normative requirement (which
> >>         doesn’t have enforceable teeth in this context):
> >>
> >>         Implementers should be aware that a token introspection
> >>         request lets the AS know when the client
> >>             (and potentially the user) is accessing the RS, which
> >>         *can also indicate* when the user is using
> >>             the client. If this implication is not acceptable,
> >>         *implementers can use other means* to carry
> >>             access token data, e.g. directly transferring the data
> >>         needed by the RS within the access token.
> >>
> >>
> >>          — Justin
> >>
> >>>         On Aug 27, 2020, at 9:48 AM, Torsten Lodderstedt
> >>>         <torsten=40lodderstedt.net@dmarc.ietf.org
> >>>         <mailto:torsten=40lodderstedt.net@dmarc.ietf.org>> wrote:
> >>>
> >>>         Will the following text work for you?
> >>>
> >>>         Implementers should be aware that a token introspection
> >>>         request lets the AS know when the client
> >>>             (and potentially the user) is accessing the RS, which is
> >>>         also an indication of when the user is using
> >>>             the client. If this impliction is not accepatable,
> >>>         implementars MUST use other means to carry
> >>>             access token data, e.g. directly transferring the data
> >>>         needed by the RS within the access token.
> >>>
> >>>
> >>>>         On 26. Aug 2020, at 23:12, Mike Jones
> >>>>         <Michael.Jones=40microsoft.com@dmarc.ietf.org
> >>>>         <mailto:Michael.Jones=40microsoft.com@dmarc.ietf.org>> wrote:
> >>>>
> >>>>         I agree with Dick’s observation about the privacy
> >>>>         implications of using an Introspection Endpoint. That’s why
> >>>>         it’s preferable to not use one at all and instead directly
> >>>>         have the Resource understand the Access Token.  One way of
> >>>>         doing this is the JWT Access Token spec.  There are plenty
> >>>>         of others.
> >>>>
> >>>>         The downsides of using an Introspection Endpoint should be
> >>>>         described in the Privacy Considerations section.
> >>>>
> >>>>                                                               -- Mike
> >>>>
> >>>>         From: OAuth <oauth-bounces@ietf.org
> >>>>         <mailto:oauth-bounces@ietf.org>> On Behalf Of Dick Hardt
> >>>>         Sent: Wednesday, August 26, 2020 9:52 AM
> >>>>         To: Torsten Lodderstedt
> >>>>         <torsten=40lodderstedt.net@dmarc.ietf.org
> >>>>         <mailto:torsten=40lodderstedt.net@dmarc.ietf.org>>
> >>>>         Cc: last-call@ietf.org <mailto:last-call@ietf.org>; oauth
> >>>>         <oauth@ietf.org <mailto:oauth@ietf.org>>
> >>>>         Subject: Re: [OAUTH-WG] Last Call:
> >>>>         <draft-ietf-oauth-jwt-introspection-response-09.txt> (JWT
> >>>>         Response for OAuth Token Introspection) to Proposed Standard
> >>>>
> >>>>
> >>>>
> >>>>         On Wed, Aug 26, 2020 at 4:37 AM Torsten Lodderstedt
> >>>>         <torsten=40lodderstedt.net@dmarc.ietf.org
> >>>>         <mailto:torsten=40lodderstedt.net@dmarc.ietf.org>> wrote:
> >>>>         Hi Denis,
> >>>>
> >>>>>         On 25. Aug 2020, at 16:55, Denis <denis.ietf@free.fr
> >>>>>         <mailto:denis.ietf@free.fr>> wrote:
> >>>>
> >>>>>         The fact that the AS will know exactly when the
> >>>>>         introspection call has been made and thus be able to make
> >>>>>         sure which client
> >>>>>         has attempted perform an access to that RS and at which
> >>>>>         instant of time. The use of this call allows an AS to
> >>>>>         track where and when
> >>>>>         its clients have indeed presented an issued access token.
> >>>>
> >>>>         That is a fact. I don’t think it is an issue per se. Please
> >>>>         explain the privacy implications.
> >>>>
> >>>>         As I see it, the privacy implication is that the AS knows
> >>>>         when the client (and potentially the user) is accessing the
> >>>>         RS, which is also an indication of when the user is using
> >>>>         the client.
> >>>>
> >>>>         I think including this implication would be important to
> >>>>         have in a Privacy Considerations section.
> >>>>
> >>>>         /Dick
> >>>>         ᐧ
> >>>
> >>>         _______________________________________________
> >>>         OAuth mailing list
> >>>         OAuth@ietf.org <mailto:OAuth@ietf.org>
> >>>         https://www.ietf.org/mailman/listinfo/oauth
> >>
> >
> >
> > _______________________________________________
> > OAuth mailing list
> > OAuth@ietf.org
> > https://www.ietf.org/mailman/listinfo/oauth
> 
> 

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