Re: [OAUTH-WG] MTLS and in-browser clients using the token endpoint

[Brian Campbell <[e-mail]>]

I guess I should have also said or been more straightforward in saying that
I don't particularly want to try and discuss/define the use of a 307 in the
document.

On Tue, Jan 15, 2019 at 6:59 AM Filip Skokan <[e-mail]> wrote:

> I don't know that the use of 307 would need to be discussed in the
>> document itself.
>
>
> If the clients are supposed to be ready for this, yeah. For instance, my
> client software by default doesn't follow redirects, in order for it to be
> ready for mtls client authentication i'd have to know 307 is a possibility
> and whitelist 307 as a valid code to be followed.
>
> S pozdravem,
> *Filip Skokan*
>
>
> On Tue, Jan 15, 2019 at 2:54 PM Brian Campbell <[e-mail]>
> wrote:
>
>> I don't know that the use of 307 would need to be discussed in the
>> document itself.
>>
>> On Tue, Jan 15, 2019 at 2:30 AM Filip Skokan <[e-mail]> wrote:
>>
>>> I'm in favour of both 307 and metadata.
>>>
>>>    - case 307 - I don't recall ever encountering an http client
>>>    software that wouldn't have an option for following redirects, same for a
>>>    server side frameworks not having the option to do a 307 response with a
>>>    location header.
>>>    - case 307 - Relying purely on a new metadata doesn't help in the
>>>    scenario David put forth earlier about clients not being aware of using
>>>    mtls, a device policy of sorts.
>>>    - case metadata - no second request if the client knows there's an
>>>    mtls endpoint it should use.
>>>
>>> Maybe we should specify both as optional for an AS to deploy and a
>>> client to be ready for?
>>>
>>> S pozdravem,
>>> *Filip Skokan*
>>>
>>>
>>> On Tue, Jan 15, 2019 at 10:05 AM Dave Tonge <[e-mail]>
>>> wrote:
>>>
>>>> I'm in favour of the `mtls_endpoints` metadata parameter - although it
>>>> should be optional.
>>>> While a 307 redirect seems kind of elegant I worry, like you,  that not
>>>> all clients would handle it appropriately.
>>>> There would probably need to be an error defined for clients who
>>>> attempt to use `tls_client_auth` at the regular endpoint.
>>>>
>>>> Dave
>>>>
>>>> On Mon, 14 Jan 2019 at 22:29, Brian Campbell <bcampbell=
>>>> [e-mail]> wrote:
>>>>
>>>>> Trying to summarize things somewhat here and focus in hopefully
>>>>> towards some decision. There's basically an idea on the table to add an AS
>>>>> metadata parameter to the draft-ietf-oauth-mtls doc that would be a JSON
>>>>> object which contains endpoints that a client doing MTLS would use rather
>>>>> than the regular endpoints. A straw-man example might look like this (with
>>>>> mtls_endpoints being that new parameter).
>>>>>
>>>>> {
>>>>>   "issuer":"https://server.example.com",
>>>>>   "authorization_endpoint":"https://server.example.com/authz",
>>>>>   "token_endpoint":"https://server.example.com/token",
>>>>>   "token_endpoint_auth_methods_supported":[
>>>>> "client_secret_basic","tls_client_auth", "none"],
>>>>>   "userinfo_endpoint":"https://server..example.com/userinfo
>>>>> <https://server.example.com/userinfo>",
>>>>>   "revocation_endpoint":"https://server.example.com/revo",
>>>>>   "jwks_uri":"https://server.example.com/jwks.json",
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> *  "mtls_endpoints":{
>>>>> "token_endpoint":"https://mtls.example.com/token
>>>>> <https://mtls.example.com/token>",    "userinfo_endpoint":"https://mtls
>>>>> <https://server.example.com/token>.example.com/userinfo
>>>>> <http://example.com/userinfo>",    "revocation_endpoint":"https://mtls
>>>>> <https://server.example.com/token>..example.com/revo
>>>>> <http://example.com/revo>"  }*
>>>>> }
>>>>>
>>>>> The idea behind this is that "regular" clients (those not doing MTLS)
>>>>> will use the regular endpoints. And only the host/port of the endpoints
>>>>> listed in mtls_endpoints will be set up to request TLS client certificates
>>>>> during handshake.. Thus any potential impact of the CertificateRequest
>>>>> message being sent in the TLS handshake can be avoided for all the other
>>>>> regular clients that are not going to do MTLS - including and most
>>>>> importantly in-browser javascript clients where there can be less than
>>>>> desirable UI presented to the end-user.
>>>>>
>>>>> The arguments in favor of that seem to be basically that it allows for
>>>>> AS deployments to support MTLS while still allowing for a "not broken" UX
>>>>> for end-users of clients (in-browser javascript clients) that aren't doing
>>>>> MTLS. And that it's not much in terms of adding to the spec and complexity
>>>>> of implementations.
>>>>>
>>>>> The arguments against it seem to be 1) the bad UX isn't really that
>>>>> bad and/or will only happen to a subset of users 2) there are other things
>>>>> that can be done, such as 307ing or renegotiation/post-handshake client
>>>>> auth, to avoid the bad UX.
>>>>>
>>>>> Speaking for myself, I'm kinda torn on it.
>>>>>
>>>>> I will say that, in addition to the folks that have pointed out that
>>>>> renegotiation just isn't possible in some cases, my experience trying to do
>>>>> something like that in the past was not particularly successful or
>>>>> encouraging. That could have been my fault, of course, but still seems a
>>>>> relevant data point. I also have my doubts about the actual difficulty of
>>>>> getting an AS to issue a 307 like response for requests based on the
>>>>> calling client and the likelihood that some/all OAuth client software would
>>>>> handle it appropriately.
>>>>>
>>>>>
>>>>> On Fri, Jan 11, 2019 at 12:32 PM David Waite <
>>>>> [e-mail]> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> > On Jan 11, 2019, at 3:32 AM, Neil Madden <[e-mail]>
>>>>>> wrote:
>>>>>> >
>>>>>> > On 9 Jan 2019, at 05:54, David Waite <[e-mail]>
>>>>>> wrote:
>>>>>> >>
>>>>>> >>> On Dec 28, 2018, at 3:55 PM, Brian Campbell <bcampbell=
>>>>>> [e-mail]> wrote:
>>>>>> >>>
>>>>>> >> <snip>
>>>>>> >>
>>>>>> >>> All of that is meant as an explanation of sorts to say that I
>>>>>> think that things are actually okay enough as is and that I'd like to
>>>>>> retract the proposal I'd previously made about the MTLS draft introducing a
>>>>>> new AS metadata parameter. It is admittedly interesting (ironic?) that Neil
>>>>>> sent a message in support of the proposal as I was writing this. It did
>>>>>> give me pause but ultimately didn't change my opinion that it's not worth
>>>>>> it to add this new AS metadata parameter.
>>>>>> >>
>>>>>> >> Note that the AS could make a decision based on the token endpoint
>>>>>> request - such as a policy associated with the “client_id”, or via a
>>>>>> parameter in the ilk of “client_assertion_type” indicating MTLS was desired
>>>>>> by this public client installation. The AS could then to TLS 1.2
>>>>>> renegotiation, 1.3 post-handshake client authentication, or even use 307
>>>>>> temporary redirects to another token endpoint to perform mutual
>>>>>> authentication.
>>>>>> >
>>>>>> > Renegotiation is an intriguing option, but it has some practical
>>>>>> difficulties. Our AS product runs in a Java servlet container, where it is
>>>>>> pretty much impossible to dynamically trigger renegotiation without
>>>>>> accessing private internal APIs of the container. I also don’t know how you
>>>>>> could coordinate this in the common scenario where TLS is terminated at a
>>>>>> load balancer/reverse proxy?
>>>>>> >
>>>>>> > A 307 redirect could work though as the server will know if the
>>>>>> client either uses mTLS for client authentication or has indicated that it
>>>>>> wants certificate-bound access tokens, so it can redirect to a
>>>>>> mTLS-specific endpoint in those cases.
>>>>>>
>>>>>> Agreed. There are trade-offs for both. As you say, I don’t know a way
>>>>>> to have say a custom error code or WWW-Authenticate challenge to trigger
>>>>>> renegotiation on the reverse proxy - usually this is just a static,
>>>>>> location-based directive.
>>>>>>
>>>>>> >
>>>>>> >> Both the separate metadata url and a “client_assertion_type”-like
>>>>>> indicator imply that the client has multiple forms of authentication and is
>>>>>> choosing to use MTLS. The URL in particular I’m reluctant to add support
>>>>>> for, because I see it more likely a client would use MTLS without knowing
>>>>>> it (via a device-level policy being applied to a public web or native app)
>>>>>> than the reverse, where a single client (represented by a single client_id)
>>>>>> is dynamically picking between forms of authentication.
>>>>>> >
>>>>>> > That’s an interesting observation. Can you elaborate on the sorts
>>>>>> of device policy you are talking about? I am aware of e.g. mobile device
>>>>>> management being used to push client certificates to iOS devices, but I
>>>>>> think these are only available in Safari.
>>>>>>
>>>>>> The primary use is to set policy to rely on device level management
>>>>>> in controlled environments like enterprises when available. So an AS may
>>>>>> try to detect a client certificate as an indicator of a managed device, use
>>>>>> that to assume a device with certain device-level authentication, single
>>>>>> user usage, remote wipe, etc. characteristics, and decide that it can
>>>>>> reduce user authentication requirements and/or expose additional scopes.
>>>>>>
>>>>>> On more thought, this is typically done as part of the user agent
>>>>>> hitting the authorization endpoint, as a separate native application may be
>>>>>> interacting with the token endpoint, and in some operating systems the
>>>>>> application’s network connections do not utilize (and may not have access
>>>>>> to) the system certificate store.
>>>>>>
>>>>>> In terms of user agents, I believe you can perform similar behavior
>>>>>> (managed systems using client certificates on user agents transparently) on
>>>>>> macOS, Windows, Chrome, and Android devices, Chrome (outside iOS) typically
>>>>>> inherits device level policy. Firefox on desktop I assume you can do that
>>>>>> in limited fashion as well.
>>>>>>
>>>>>> -DW
>>>>>
>>>>>
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>>>>
>>>>
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