Hello Vittorio,
Your reply was amazingly fast. Responses are between the lines.
Thanks Denis for the thorough commentary.
/> The title of this spec./
Fixed, thanks!
/> The client MUST NOT inspect the content of the access token/
This is really a sticky point. I really want to acknowledge your PoV
on this, but at the same time I found this to be one of the biggest
sources of issues
in the use of JWT for access tokens hence I feel we really need to
give solid guidance here. Let me expand further on the reasoning
behind it,
and perhaps we can get to language that satisfies both PoVs.
To me the key point is that clients should not write /code/ that
inspects access tokens. Taking a dependency on the ability to do so is
ignoring fundamental information
about the architecture and relationships between OAuth roles, and
suggests an ability of the client to understand the semantic of the
content that cannot be assumed
in the general case. I expanded on the details in my former reply to
you on this topic, I would recommend referring to it.
Clients violating this simple principle has been one of the most
common sources of production issues I had to deal with in the past few
years, and one of the hardest
to remediate given that clients are hard to update and sometimes the
things they relied on were irremediably lost. This is why I am
inclined to put in here strong language.
That said: I have nothing against client developers examining a
network trace and drawing conclusions based on the content of what
they see. That doesn’t create any hard
dependencies and has no implications in respect to changes in the
solution behavior. However I am not sure how to phrase that in the
specification, given that referring
to the client inevitably refers to its code. I am open to suggestions.
First of all, a "*MUST NOT* "should not be placed in a privacy
considerations section, but in the main body of the document.
The goal of a privacy considerations section is to provide explanations,
but not to introduce additional requirements.
Then after, since this section is about privacy considerations, it is
important to refer to an ISO standard that has been published 9 years ago.
It is ISO 29100 (Information technology — Security techniques — Privacy
framework). Most of the ISO standards need to be bought.
However, there are a few exceptions and this standard can be freely
downloaded from the ISO web site, if you accept some conditions,
using this URL:
https://standards.iso.org/ittf/PubliclyAvailableStandards/index.html
ISO 29100, even if it is outdated on some aspects (it only deals with
two entities), identifies ten principles (See Table 3).
1. Consent and choice
2. Purpose legitimacy and specification
3. Collection limitation
4. Data minimization
5. Use, retention and disclosure limitation
6. Accuracy and quality
7. Openness, transparency and notice
8. Individual participation and access
9. Accountability
10. Information security
11. Privacy compliance
If the client is unable to inspect the token, then openness and
transparency do not exist any more.
Since there is no formal protocol in OAuth 2.0 for "consent and
choice", the only way for a client for denying its consent is to inspect
the token
and to stop its transmission if the content of the token is not
considered to be adequate for it.
Since you are open to suggestions, would you please reconsider the text
I proposed: it simply states that if the client wants to inspect the token,
it will not necessarily be in a position to do it. It is simply a
warning, but this warning does not prevent the client to attempt to
inspect the token .
I will go one step further: if the client wants to inspect the token and
if the format of the token is unknown to the client, then the client
will simply
stop its further transmission. For some clients, preserving their
privacy may be more important than performing an access to a RS.
> 3)…
I have a pretty hard time following the chain of reasoning in this
section. Let me attempt to tackle it to the best of my understanding.
I think the key might be
/> a client should be able to choose whether it wishes the sub claim
to contain [..]
/
I don’t think that should be a choice left to the client. In business
systems, my experience is that the type of identifiers to be used
(when the IdP gives any choice at all)
is established at resource provisioning time. I am not aware of
mechanisms thru which a client signals the nature of the identifier to
be used, nor that would be fully
feasible (the resource knows what it needs to perform its function).
Consent and choice is the first of the ten privacy principles. Since
OAuth has not been constructed taking into considerations this privacy
principle,
it is quite "normal" that you don't observe protocols able to do it at
the present time.
Furthermore:
/> which has nothing to do with uniqueness since the value changes for
every generated token.
/
Again, this is something that was touched on in my former reply to
your message. As long as an identifier identifies one resource only,
it satisfies uniqueness. It doesn’t have to be a singleton.
It is a matter of interpretation. I stick to the definition given in RFC
7519 (section 4.1.2):
*T**he subject value MUST either be scoped to be locally unique in the
context of the issuer or be globally unique.*
However, the "context of the issuer" is left undefined.
Finally, the scope is optional (for good reasons: 1^st party and non
delegation scenarios don’t require it) hence it cannot be relied upon
for properties that should hold in every scenario.
I would not say that the scope request parameter is a nice way to choose
which attributes should be placed in a JWT. However, considering the
limitations placed
in this document, it is the ONLY way to do. In my email sent before the
virtual meeting, I wrote: "Allowing a client to only specify a scope and
a resource is very restrictive".
Clause 2.2.2 (second paragraph) states:
***This profile does not introduce any mechanism for a client to**
******directly request the presence of specific claims in JWT access**
******tokens*, as the authorization server can determine what additional
claims are required by a particular resource server by taking in
consideration the client_id of the client, the scope and the resource
parameters included in the request.
Currently, the client has no control of the claims that will be placed
in a JWT and further more the current wording of privacy consideration
section
prevents the client to have any inspection of the attributes that have
been placed in a JWT. Privacy principles 1 and 7 from ISO 29100 are not
fulfilled.
There are more implications. If, in the future, a mechanism is defined
to allow the client to choose the type of the attributes that should be
placed in a JWT,
then the "sub" claim is insufficient. Taking into consideration my
previous email, four types of attributes types would need to be specified:
The client should be able to choose whether it wishes a claim that contains:
* a global unique identifier for all ASs ("globally unique"),
* a unique identifier for each AS ("locally unique in the context
of the issuer"),
* a different pseudonym for each RS, or
* a different pseudonym for each authorization token request.
Clause 2.2.2 states:
* the authorization server can determine what additional**
******claims are required by a particular resource server* by taking in
consideration the client_id of the client, the scope and the resource
parameters included in the request.
I don't believe this is true in general. Considering its privacy
preferences, only the client may know what is appropriate: not the AS,
neither the RS.
In summary: per the preceding thread on this topic, the consensus was
that varying the sub content was a satisfactory way of protecting
against correlation.
"/if all you have is a hammer, everything looks like a nail/".
When the only parameter that can be used is the "sub" claim, then the
only solution is to place everything into it.
I don’t a gree that clients should have a mechanism to request
different sub flavors, as that decision should be done out of band by
the AS and RS; and the scope isn’t always available anyway.
While it would be nice to have such a mechanism, we are not going to
define such a mechanism during a second WGLC.
The privacy considerations section should only inform the readers about
the limitations of the protocol in terms of privacy.
I have proposed text to address the point. If you would prefer a
different wording to address this point, please make another proposal.
/> targeting of access tokens/
Let me think about that a bit longer.
I acknowledge that the decision of including an audience has the
effect of letting the AS track when the client accesses a particular
resource,
but at the same time that’s completely mainstream and very much by
design in a very large number of cases. As such, I find the language
you are suggesting to be potentially confusing, as it positions this
as an exception vs a privacy protecting mainstream that is in fact not
common,
and ascribes to the client more latitude than I believe is legitimate
to expect or grant.
I’ll try to come up with concise language that clarifies to the reader
that the current mechanism does allow AS tracking.
Please do so.
Denis
*From: *OAuth <oauth-boun...@ietf.org> on behalf of Denis
<denis.i...@free.fr>
*Date: *Wednesday, April 29, 2020 at 09:12
*To: *"oauth@ietf.org" <oauth@ietf.org>
*Subject: *Re: [OAUTH-WG] Second WGLC on "JSON Web Token (JWT) Profile
for OAuth 2.0 Access Tokens"
You will find four comments numbered 1) to 4).
*1) *The title of this spec. is:
JSON Web Token (JWT) Profile for OAuth *2.0* Access Tokens
So, this spec. is supposed to be targeted to OAuth *2.0. * However,
the header at the top of the page omits to mention it.
Currently, it is :
Internet-Draft OAuth Access Token JWT Profile April 2020
It should rather be:
Internet-Draft OAuth *2.0* Access Token JWT Profile April 2020
*2)* The following text is within section 6.
The client MUST NOT inspect the content of
the access token: the 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.
Nonetheless, authorization servers should
not assume that clients will comply with the above.
It is of a primary importance that clients MAY be able to inspect
tokens before transmitting them.
The "MUST NOT" is not acceptable.
The above text should be replaced with:
Reading the access token content may be useful for the user to verify
that
the access token content matches with its expectations. However,
the authorization server and the resource server might decide to
change the
token format at any time. Thus, the client should not expect to
always be
in a position to read the access token content.
The remaining of the text about this topic is fine.
*3) *The next topic is about the sub claim.
The text states:
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.
I have a set of questions:
1. How can authorization servers choose how to assign sub values
according to the level of privacy required "by each situation" ?
2. How can authorization servers know the level of privacy required
"by each situation" ?
3. How can the users be informed of the level of privacy required "by
each situation" ?
4. How can the users *consent* with the level of privacy required "by
each situation" ?
Currently, the request MUST include either a resource parameter or an
aud claim parameter, while it MAY include a scope parameter.
The syntax of the scope parameter is a list of space-delimited,
case-sensitive strings (RFC 6749). It is thus subject to private
agreements
between clients and Authorization Servers. Since the scope is being
returned, it is a primary importance that the returned scope matches
with its expectations before transmitting the token to a Resource Server.
In theory, a client should be able to choose whether it wishes the sub
claim to contain :
* a global unique identifier for all ASs ("globally unique"),
* a unique identifier for each AS ("locally unique in the context of
the issuer"),
* a different pseudonym for each RS, or
* a different pseudonym for each authorization token request.
The only variable parameter that it can use for this purpose in the
token request is the scope parameter.
RFC 7519 states is section 4.1.2:
The subject value MUST either be scoped to be locally unique in the
context of the issuer
or be globally unique.
It is quite hard to recognize that the sub claim is able to carry a
different pseudonym for each RS, i.e. for case (c), or
a different pseudonym for each authorization token request, i.e. for
case (d), which has nothing to do with uniqueness
since the value changes for every generated token.
This has implications about the following text:
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.
Since it addresses case (c).
and also about the following text:
4.b) 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.
Since it addresses case (d).
This means that the current text placed in the privacy considerations
section was a good attempt to address the case,
but that the text needs to be revised.
Proposed text replacement for all the previously quoted sentences:
According to RFC 7519 (4.1.2): The subject value MUST either be scoped
to be locally unique in the context of the issuer or be globally unique.
When the sub claim contains a globally unique identifier, this allows
to correlate principal activities across multiple resource servers,
while in addition,
this globally unique identifier may also allow to correlate the
principal activities on servers where no access has been performed by
the principals
to these servers but where the same globally unique identifiers are
being used by these servers.
When the sub claim contains a locally unique identifier in the context
of the issuer, this also allows the tracking of principal activities
across multiple resource servers.
The scope request parameter is the only way to influence on the
content of the sub claim parameter. Its meaning is subject to a
private agreement
between the client and the AS, which means that the use of the scope
parameter is the only way to choose between a locally unique identifier
in the context of the issuer or a globally unique identifier.
Since the scope parameter is being returned, it is a primary
importance that the returned scope matches with the expectations of
the client before transmitting
the token to a Resource Server.
However, there are other cases where the client would like to be able
to choose whether it wishes the sub claim to contain :
- a different pseudonym for each RS so that different resource
servers will be unable to correlate its activities, or
- a different pseudonym for each authorization token request, so
that the same resource server cannot correlate its activities
performed at different instant of time.
Considering the semantics of the sub claim, these two cases cannot be
currently supported.
*4) *The next topic is about the targeting of access tokens
Text had been proposed before the last conference call. Then, the
topic has been presented at the very end of the last conference call,
but no text has been included
in the next draft.
Here is a revised text be included in the privacy considerations section:
For security reasons, some clients may be willing to target their
access tokens but, for privacy reasons, may be unwilling to disclose
to Authorization Servers
an identification of the Resource Servers they are going to access, so
that Authorization Servers will be unable to know which resources
servers are being accessed.
The disclosure of the Resource Servers names allows the Authorization
Servers to list all the Resource Servers being access by all its users
and in addition to list pairs
of (Principal, Resource Servers) which allow to trace all the users
accesses to Resource Servers performed through a given Authorization
Server. When a token is targeted,
this profile does not contain provisions to address these two threats.
Denis
Hi all,
This is a second working group last call for "JSON Web Token (JWT)
Profile for OAuth 2.0 Access Tokens".
Here is the document:
https://tools.ietf.org/html/draft-ietf-oauth-access-token-jwt-06
Please send your comments to the OAuth mailing list by April 29, 2020.
Regards,
Rifaat & Hannes
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