Re: [OAUTH-WG] draft-ietf-oauth-access-token-jwt-07

Denis <denis.ietf@free.fr> Tue, 08 September 2020 16:09 UTC

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From: Denis <denis.ietf@free.fr>
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Subject: Re: [OAUTH-WG] draft-ietf-oauth-access-token-jwt-07
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Hi Hannes,

Two comments between the lines.

> Hi Victorio, Hi all,
>
> I am doing my shepherd write-up for 
> draft-ietf-oauth-access-token-jwt-07. Reading through the draft I have 
> a few minor suggestions:
>
> Section 2:
>
> I would delete this sentence "JWT access tokens are regular JWTs 
> complying with the requirements described in this section."
>
> Reason: You pretty much make the same statement on the previous page 
> (see terminology section).
>
> Section 2.1
>
> s/asymmetric algorithms/asymmetric cryptography
>
> (same replacement in Section 4)
>
> s/   This specification registers the "application/at+jwt" media type,
>
>    which can be used to indicate that the content is an access 
> token./This specification registers the "application/at+jwt" media type,
>
>    which can be used to indicate that the content is a JWT access token.
>
> Use capitalized "Section" when a section number is indicated, such as 
> in Section 2.2.
>
> Section 2.2
>
> s/""aud"/"aud"
>
> 2.2.1
>
> s/   auth_time  OPTIONAL - as defined in section 2 of 
> [OpenID.Core]./   auth_time  OPTIONAL - as defined in Section 2 of 
> [OpenID.Core].
>
> s/   acr, amr  OPTIONAL - as defined in section 2 of [OpenID.Core]./   
> acr, amr  OPTIONAL - as defined in Section 2 of [OpenID.Core].
>
> s/Please see/See
>
> s/For example:/For example,
>
> Section 4
>
> You write:
>
> "Authorization servers SHOULD implement OAuth 2.0 Authorization Server 
> Metadata [RFC8414] ... "
>
> Are you sure you mean "implement" and not "use"? The paragraph gives 
> me the impression that you talk about "ASs using RFC 8414"
>
> s/Please see section Section 5 for further guidance on security 
> implications./Please see Section 5 for further guidance on security 
> implications.
>
> This sentence sounds strange to me:
>
> "
>
>    When invoked as described in OAuth 2.0 Bearer Token Usage [RFC6750],
>
>    resource servers receiving a JWT access token MUST validate it in the
>
>    following manner.
>
> "
>
> How about:
>
> "
>
>    Resource servers receiving a JWT access token MUST validate it in the
>
>    following manner.
>
> "
>
> Question: If you refer to RFC 6750 and then list the steps are you 
> just repeating the steps from RFC 6750 or are you augmenting them?
>
> You write:
>
> "
>
> If the JWT access token includes authorization claims as described in
>
>    the authorization claims section, the resource server SHOULD use them
>
>    in combination with any other contextual information available to
>
>    determine whether the current call should be authorized or rejected.
>
> "
>
> Include a reference to the authorization claims section
>
> s/ For more
>
>    details on cross-JWT confusion please refer to 2.8 of [RFC8725]./ 
> For more
>
>    details on cross-JWT confusion please refer to Section 2.8 of 
> [RFC8725].
>
> You write:
>
> "
>
>    Authorization servers should not rely on the use of different keys
>
>    for signing OpenID Connect ID Tokens and JWT tokens as a method to
>
>    safeguard against the consequences of leaking specific keys.
>
> "
>
> The phrase "leaking keys" is probably not the best term to describe 
> what follows afterwards in the text.
>
> You write:
>
> "
>
> The client MUST NOT inspect the content of
>
>    the access token
>
> "
>
> This RFC 2119 language is not really enforceable in terms of 
> interoperability. Maybe you could rephrase a bit. Something like the 
> following would work:
>
> "
>
>    Authorization server and the resource server
>
>    might decide to change token format at any time (for example by
>
>    switching from this profile to opaque tokens). Hence, any logic in the
>
>    client relying on the ability to read the access token content would
>
>    break without recourse. The OAuth 2.0 framework assumes that access 
> tokens
>
>    are treated opaque by clients.
>
>    Administrators of authorization servers should also take into 
> account that
>
>    the content of an access token is visible to the client. Whenever 
> client
>
>    access to the access token content presents privacy issues for a
>
>    given scenario, the authorization server should take explicit steps
>
>    to prevent it.
>
> "
>
>
/In the general case, /the OAuth 2.0 framework assumes that access 
tokens are treated as opaque by clients.
However, with this coming RFC, we are not in the general case: since the 
client gets back an access token conformant to _this_ RFC, then it knows
exactly its detailed structure. The argument about "changing the token 
format at any time" does not apply. In this case, the client is quite sure
that it would be able to understand most of its content (at least all 
the standard claims). The above text proposal would need to be reconsidered.

Hiding (by encrypting it) the content of the access token to the client 
is odd when an access token contains claims about a human-user :
these claims are personal data and the human-user is usually allowed to 
have access to his own personal data.

Encryption is nice in theory but complicated in practice, since a key 
management system must put in place. Whenever possible, it should be 
avoided.

BTW, some questions raised during the WGLC have not been answered: How 
can a client request an access token compliant to this profile ?
Which parameter(s) allow it to ask an access token compliant to this 
profile ? How can the AS know that it got a call for the issuance of an 
access token
compliant to this profile ?

Another comment follows.

> You wrote:
>
> "
>
>    In scenarios in which JWT access tokens are accessible to the end
>
>    user, it should be evaluated whether the information can be accessed
>
>    without privacy violations (for example, if an end user would simply
>
>    access his or her own personal information) or if steps must be taken
>
>    to enforce confidentiality.  Possible measures include: encrypting
>
>    the access token, encrypting the sensitive claims, omitting the
>
>    sensitive claims or not using this profile, falling back on opaque
>
>    access tokens.
>
> "
>
> The first sentence is a repetition of the previous paragraph. I would 
> suggest to delete
>
> the first sentence in this paragraph and to move the second sentence 
> to the previous paragraph.
>
> You wrote:
>
> "
>
>    This profile mandates the presence of the "sub" claim in every JWT
>
>    access token, making it possible for resource servers to rely on that
>
>    information for performing tasks such as correlating incoming
>
>    requests with data stored locally for the authenticated principal.
>
>    Although the ability to correlate requests might be required by
>
>    design in many scenarios, there are scenarios where the authorization
>
>    server might want to prevent correlation to preserve the desired
>
>    level of privacy.  Authorization servers should choose how to assign
>
>    "sub" values according to the level of privacy required by each
>
>    situation.  For instance: if a solution requires preventing tracking
>
>    principal activities across multiple resource servers, the
>
>    authorization server should ensure that JWT access tokens meant for
>
>    different resource servers have distinct "sub" values tht cannot be
>
>    correlated in the event of resource servers collusion.  Similarly: if
>
>    a solution requires preventing a resource server from correlating the
>
>    principal's activity within the resource itself, the authorization
>
>    server should assign different "sub" values for every JWT access
>
>    token issued.  In turn, the client should obtain a new JWT access
>
>    token for every call to the resource server, to ensure that the
>
>    resource server receives different "sub" and "jti" values at every
>
>    call, thus preventing correlation between distinct requests.
>
> "
>
> The above paragraph suggests that there are different levels of 
> privacy. What you are
>
> talking about in the text is unlinkability and identification. Ways to 
> deal with such
>
> privacy threats are described in Section 6 of RFC 6973.
>
> Hence, I would suggest to slightly rephrase the paragraph to something 
> like:
>
> "
>
>    This profile mandates the presence of the "sub" claim in every JWT
>
>    access token, making it possible for resource servers to rely on that
>
>    information for correlating incoming
>
>    requests with data stored locally for the authenticated principal.
>
>    Although the ability to correlate requests might be required by
>
>    design in many scenarios, there are scenarios where the authorization
>
>    server might want to prevent correlation. The "sub" claim should be
>
>    populated by the authorization servers according to a privacy impact
>
>    assessment. For instance, if a solution requires preventing tracking
>
>    principal activities across multiple resource servers, the
>
>    authorization server should ensure that JWT access tokens meant for
>
>    different resource servers have distinct "sub" values that cannot be
>
>    correlated in the event of resource servers collusion.
>
While the idea is really nice, the use of the "sub" claim in this 
context is not compatible with the definition of the "sub" claim
as defined in RFC 7519:

      4.1.2.  "sub" (Subject) Claim

         The "sub" (subject) claim identifies the principal that is the
         subject of the JWT.  The claims in a JWT are normally statements
         about the subject. *The subject value MUST either be scoped to be**
**        locally unique in the context of the issuer or be globally 
unique.*
         The processing of this claim is generally application 
specific.  The
         "sub" value is a case-sensitive string containing a StringOrURI
         value.  Use of this claim is OPTIONAL.

There are two options and two options only:

    "locally unique in the context of the issuer" means that it is the
    same for all RSs.
    "globally unique" means that it is the same not only for all the RSs
    but also for servers that have nothing to do with OAuth (e.g. an
    email address).


>     Similarly, if
>
>    a solution requires preventing a resource server from correlating the
>
>    principal's activity within the resource itself, the authorization
>
>    server should assign different "sub" values for every JWT access
>
>    token issued.  In turn, the client should obtain a new JWT access
>
>    token for every call to the resource server, to ensure that the
>
>    resource server receives different "sub" and "jti" values at every
>
>    call, thus preventing correlation between distinct requests.
>
The proposed text describes two different cases where the sub claim is 
either unique for an AS/RS pair orunique for each access token.

These two cases are not included in the definition found in RFC 7519.

In the general case, an identifier can be:

 1. locally unique in the context of the issuer (i.e. the same for all RSs),
 2. globally unique (i.e. the same not only for all the RSs but also for
    servers that have nothing to do with OAuth),
 3. unique for an AS/RS pair, or
 4. unique for each access token.

I see different ways to solve this problem:

    1° Stick to the definition of RFC 7519 and (unfortunately) remove
    these possibilities.
    2° Define two new claims which would support the two cases where the
    sub claim would be either unique for an AS/RS pair orunique for one
    access token.
    3° Define four new claims which would support the four above cases.

Denis

> "
>
> Section 7.2
>
> s/   Section Section 2.2.3.1 of this specification refers to the
>
>    attributes "roles", "groups", "entitlements" defined in [RFC7643] to
>
>    express authorization information in JWT access tokens.
>
> /   Section 2.2.3.1 of this specification refers to the
>
>    attributes "roles", "groups", "entitlements" defined in [RFC7643] to
>
>    express authorization information in JWT access tokens.
>
> References
>
> RFC 7519 has to be a normative reference:
>
>    [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
>
>               (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
>
> <https://www.rfc-editor.org/info/rfc7519>.
>
> RFC 7644 is an unused reference:
>
>    [RFC7644]  Hunt, P., Ed., Grizzle, K., Ansari, M., Wahlstroem, E.,
>
>               and C. Mortimore, "System for Cross-domain Identity
>
> Management: Protocol", RFC 7644, DOI 10.17487/RFC7644,
>
> September 2015, <https://www.rfc-editor.org/info/rfc7644>.
>
> The same is true for RFC 3986:
>
>    [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
>
>               Resource Identifier (URI): Generic Syntax", STD 66,
>
>               RFC 3986, DOI 10.17487/RFC3986, January 2005,
>
> <https://www.rfc-editor.org/info/rfc3986>.
>
> Ciao
>
> Hannes
>
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