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

[Brian Campbell <[e-mail]>]

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",
  "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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