Re: [OAUTH-WG] WGLC Review of PAR

[Dave Tonge <[e-mail]>]

Looks really good to me, thanks Brian.

On Wed, 2 Sep 2020 at 21:42, Brian Campbell <bcampbell=
[e-mail]> wrote:

> Thanks Torsten, Taka, and Justin,
>
> I took the revised text from Justin and tweaked it with some typo cleanup
> and minor adjustments to make what is hopefully a final proposal below. I
> had a similar feeling about the last paragraph not really fitting but don't
> have a better location to suggest so am just leaving it.
>
> 2.4. Management of Client Redirect URIs
>
> While OAuth 2.0 [RFC6749] allows clients to use unregistered redirect_uri
> values in certain circumstances, or for the authorization server to apply
> its own matching semantics to the redirect_uri value presented by the
> client at the authorization endpoint, the OAuth Security BCP
> [I-D.ietf-oauth-security-topics] as well as OAuth 2.1 [I-D.ietf-oauth-v2-1]
> require an authorization server exactly match the redirect_uri parameter
> against the set of redirect URIs previously established for a particular
> client. This is a means for early detection of client impersonation
> attempts and prevents token leakage and open redirection. As a downside,
> this can make client management more cumbersome since the redirect URI is
> typically the most volatile part of a client policy.
>
> The exact matching requirement MAY be relaxed by the authorization server
> for a confidential client using pushed authorization requests since the
> authorization server authenticates the client before the authorization
> process starts and thus ensures it is interacting with the legitimate
> client. The authorization server MAY allow such clients to specify
> redirect_uri values that were not previously registered with the
> authorization server. This will give the client more flexibility (e.g. to
> mint distinct redirect URI values per authorization server at runtime) and
> can simplify client management. It is at the discretion of the
> authorization server to apply restrictions on supplied redirect_uri values,
> e.g. the authorization server MAY require a certain URI prefix or allow
> only a query parameter to vary at runtime.
>
> The ability to set up transaction specific redirect URIs is also useful in
> situations where client ids and corresponding credentials and policies are
> managed by a trusted 3rd party, e.g. via client certificates containing
> client permissions. Such an externally managed client could interact with
> an authorization server trusting the respective 3rd party without the need
> for an additional registration step.
>
> On Wed, Sep 2, 2020 at 8:09 AM Justin Richer <[e-mail]> wrote:
>
>> The real conflict here is with the BCP and 2.1, both of which adopt the
>> stricter matching semantics for redirect URIs than 6749 does on its own.
>> This section would be needed to clarify how they relate to each other. That
>> said, I think adding some of Taka’s observations to Torsten’s text wouldn’t
>> hurt:
>>
>> 2.4. Management of redirect_uri
>>
>> While OAuth 2.0 [RFC6749] allows clients to use unregistered redirect_uri
>> values in certain circumstances, or for the AS to apply its own matching
>> semantics to the redirect_uri value presented by the client at the
>> authorization endpoint, the OAuth Security BCP
>> [I-D.ietf-oauth-security-topics] as well as OAuth 2.1 [I-D.ietf-oauth-v2-1]
>> require an AS to excactly match the redirect_uri parameter against the set
>> of redirect URIs previously established for a particular client. This is a
>> means to early detect attempts to impersonate a client and prevent token
>> leakage and open redirection. As a downside, it makes client management
>> more complex since the redirect URI is typically the most volatile part of
>> a client policy.
>>
>> This requirement MAY be relaxed by the AS if a confidential client uses
>> pushed authorization requests since the AS authenticates the client before
>> the authorization process starts and that way ensures it interacts with the
>> legit client. The AS MAY allow such clients to specify redirect_uri values
>> not previously registered with the AS. This will give the client more
>> flexibility (e.g. to mint AS-specific redirect URIs on the fly) and makes
>> client management much easier. It is at the discretion of the AS to apply
>> restriction on redirect_uri values, e.g. the AS MAY require a certain URI
>> prefix or allow only a query parameter to vary at runtime.
>>
>> I also feel like this paragraph belongs in a different section outside of
>> here. I’m not sure where, but it doesn’t quite seem to fit, to me. It’s not
>> the end of the world if it stays here though as it’s a decent view on the
>> “why".
>>
>>
>> The aibility to set up transaction specific redirect URIs is also useful
>> in situations where client ids and correspoding credentials and policies
>> are managed by a trusted 3rd party, e.g. via client certifiates containing
>> client permissions. Such an externally managed client could interact with
>> an AS trusting the respective 3rd party without the need for an additional
>> registration step.
>>
>>
>>  — Justin
>>
>> On Sep 1, 2020, at 11:05 PM, Takahiko Kawasaki <[e-mail]> wrote:
>>
>> Under existing specifications (RFC 6749, OIDC Core 1.0 and FAPI), a
>> client can choose an arbitrary redirect_uri without registering it only
>> when all the following conditions are satisfied.
>>
>> 1. The client type of the client is "confidential". (RFC 6749 Section
>> 3.1.2.2 requires that public clients register redirect URIs.)
>> 2. The flow is not "implicit". (RFC 6749 Section 3.1.2.2 requires that
>> confidential clients utilizing the implicit grant type register redirect
>> URIs.)
>> 3. The authorization request is not an OIDC request. (OIDC Core 1.0
>> Section 3.1.2.1 requires that redirect_uri match a pre-registered one.)
>> 4. The authorization request is not a FAPI request. (FAPI Part 1 Section
>> 5.2.2 Clause 8 requires that redirect URIs be pre-registered.)
>>
>> In short, under existing specifications, pure RFC-6749
>> authorization-code-flow requests from confidential clients can choose an
>> arbitrary redirect_uri without registering it. Once OIDC or FAPI is used,
>> existing specifications require pre-registration of redirect URIs. I'm not
>> sure but if PAR's "redirect_uri Management" is going to introduce rules
>> that conflict with existing specifications, it is better to list the
>> conflicts explicitly in the section.
>>
>> Best Regards,
>> Takahiko Kawasaki
>>
>>
>> On Wed, Sep 2, 2020 at 3:48 AM Torsten Lodderstedt <torsten=
>> [e-mail]> wrote:
>>
>>> Here is my proposal for the new section:
>>>
>>> 2.4. redirect_uri Management
>>>
>>> The OAuth Security BCP [I-D.ietf-oauth-security-topics] as well as OAuth
>>> 2.1 [I-D.ietf-oauth-v2-1] require an AS to excactly match the redirect_uri
>>> parameter against the set of redirect URIs previously established for a
>>> particular client. This is a means to early detect attempts to impersonate
>>> a client and prevent token leakage and open redirection. As a downside, it
>>> makes client management more complex since the redirect URI is typically
>>> the most volatile part of a client policy.
>>>
>>> This requirement MAY be relaxed by the AS, if a confidential client uses
>>> pushed authorization requests since the AS authenticates the client before
>>> the authorization process starts and that way ensures it interacts with the
>>> legit client. The AS MAY allow such clients to specify redirect_uri values
>>> not previously registered with the AS. This will give the client more
>>> flexibility (e.g. to mint AS-specific redirect URIs on the fly) and makes
>>> client management much easier. It is at the discretion of the AS to apply
>>> restriction on redirect_uri values, e.g. the AS MAY require a certain URI
>>> prefix or allow only a query parameter to vary at runtime.
>>>
>>> Note: The aibility to set up transaction specific redirect URIs is also
>>> useful in situations where client ids and correspoding credentials and
>>> policies are managed by a trusted 3rd party, e.g. via client certifiates
>>> containing client permissions. Such an externally managed client could
>>> interact with an AS trusting the respective 3rd party without the need for
>>> an additional registration step.
>>>
>>> > On 29. Aug 2020, at 17:22, Justin Richer <[e-mail]> wrote:
>>> >
>>> > I completely agree with the utility of the function in question here
>>> and it needs to be included. I’m in favor of creating a dedicated section
>>> for redirect_uri management, so that we can explain exactly how and why to
>>> relax the requirement from core OAuth. In addition, I think we want to
>>> discuss that the AS might have its own restrictions on which redirect URIs
>>> an authenticated client might be able to use. For example, registering a
>>> client with a Redirect URI prefix, or allowing only a query parameter to
>>> vary at runtime. All of these can be enforced in PAR because the client is
>>> presenting its authentication, as you point out, so the AS can determine
>>> which policies should apply.
>>> >
>>> > — Justin
>>> >
>>> >> On Aug 29, 2020, at 7:52 AM, Torsten Lodderstedt <
>>> [e-mail]> wrote:
>>> >>
>>> >>
>>> >>>
>>> >>>
>>> >>>   ¶6: Does the AS really have "the ability to authenticate and
>>> authorize clients”? I think what we mean here is "the ability to
>>> authenticate clients and validate client requests”, but I’m not positive of
>>> the intent.
>>> >>>
>>> >>> I think the intent is that the AS can check whether a client is
>>> authorized to make a particular authorization request (specific scopes,
>>> response type, etc.). But checking authorization to request authorization
>>> is confusing wording. I think your working is less confusing and still
>>> allows for the intent.
>>> >>>
>>> >>> I'll let Torsten interject if he feels differently as I think he
>>> originally wrote the text in question.
>>> >>
>>> >> that was the original intent. I think “validate" is fine.
>>> >>
>>> >>>
>>> >>>
>>> >>>
>>> >>>   ¶7: I’m not sure I buy this example. Even if the clientID is
>>> managed externally, the association with a set or pattern of allowed
>>> redirect URIs is still important, and the AS will need to know what that
>>> is. I think this example could lead an AS developer to (erroneously and
>>> dangerously) conclude that they don’t have to check any other values in a
>>> request, including scope and redirect URI. It’s important that DynReg
>>> doesn’t alleviate that issue, but removal of DynReg doesn’t really change
>>> things in that regard. Suggest removing example or reworking paragraph.
>>> >>>
>>> >>> I'm going to have to defer to Torsten on this because, to be honest,
>>> I'm not too sure about it myself. I tend to lean towards thinking the draft
>>> would be better off without it.
>>> >>>
>>> >>
>>> >> In the traditional authorization flow, the redirect_uri serves as way
>>> to make sure the AS is really talking to the legit client and the allowed
>>> redirect_uri values are determined by the legit client at registration time
>>> (might be manually).
>>> >>
>>> >> With PAR, we have a much stronger means to ensure the AS is talking
>>> to the legit client. That’s why I don’t see an issue with letting the
>>> client set a per transaction redirect_uri. This will give the client more
>>> flexibility (mint AS-specific redirect URIs on the fly) and makes client
>>> management much easier since redirect URIs are the most volatile part of a
>>> client policy.
>>> >>
>>> >> It also makes use of OAuth much easier in deployments where client
>>> identities are managed by external entities (even without any idea of
>>> OAuth). A prominent example is open banking in the EU (aka PSD2). The
>>> (technical) identity of any PSD2-licensed client is asserted by an eIDAS
>>> compliant CA in a special X.509 certificate. Those certificates contain the
>>> permissions (access to account information and/or payment initiation
>>> allowed) and the identity (member state specific). But they don’t contain
>>> OAuth policy values. Nevertheless, the regulation requires any financial
>>> institution in the EU to at runtime, without any registration, to accept
>>> and process calls from any licensed PSD2 clients.
>>> >>
>>> >> There are two ways to cope with it in OAuth context:
>>> >> a) use dynamic client registration with the X.509 cert as credential.
>>> Unfortunately, RFC 7591 does not support other client authentication means
>>> then an initial access token. Beside that, it would violate the text of the
>>> regulation.
>>> >> b) establish a redirect URL with every transaction. This is the
>>> recommended approach in at least one of the PSD2 specs.
>>> >>
>>> >> PAR is a clean way to solve that problem.
>>> >>
>>> >> I don’t want this text to cause confusing. On the other hand this
>>> potential of PAR is way too important to not mention it at all. What about
>>> moving it into a special section "redirect_uri management”?
>>> >>
>>> >>>
>>> >>
>>> >
>>>
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>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/oauth
>>>
>>
>>
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