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

[Filip Skokan <[e-mail]>]

307 indeed seems doable, similar to a discovery namespace it requires the
client software to be prepared for this and follow the redirect in that
case, but in David’s case it doesn’t require the client to “know” it is
bound to a device wide policy. The client just assumes it has no form of
authentication and only sends the client_id.

I’ll try to do a quick PoC for 307 and share any findings i may have.

Renegotiation not only isn’t easy to do in some common setups (self managed
tls terminating LB or proxy), but in some is outright impossible - e.g.
managed services like AWS ALB or API Gateway that may be in use for regular
requests and with self-hosted endpoints running nxinx/apache for just mtls
requests on the side.

Best,
*Filip*


On Fri, Jan 11, 2019 at 11:32 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.
>
> > 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.
>
> — Neil
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