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

[Brian Campbell <[e-mail]>]

I spent some time this holiday season futzing around with a few different
browsers to see what kind of UI, if any, they present to the user when
seeing different variations of the server requesting a client certificate
during the handshake.

In a non-exhaustive and unscientific look at the browsers I had easily at
my disposal (FF, Chrome, and Safari on Mac OS), it seems they all behave
basically the same. If the browser is configured with, or has access to,
one or more client certificates that match the criteria of the
CertificateRequest message from the server (basically if issued by one of
the CAs in the certificate_authorities of the CertificateRequest), a
certificate selection UI prompt will be presented to the user. Otherwise, a
certificate selection UI prompt is not presented all. When the
CertificateRequest message has an empty certificate_authorities list
(likely the case with a optional_no_ca type config), the browsers look for
client certificates with any issuer rather than narrowing it down.

The observed behavior of the browsers surveyed seems logical and rather
reasonable (and better than the last time I futzed with it). Importantly it
means that for the situation described in the email that started this
thread (a javascript client making a fetch/XHR request to an MTLS token
endpoint), users using browsers that are not configured with, or have
access to, any client keys/certs will not see any UI prompt at all. I
suspect that not having client certs set up is the situation for the vast
majority of users and their browsers. And for those that do have client
certs set up, I think they are more likely to be the kind of user that is
able to deal with the UI prompt okay.

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.



On Fri, Dec 28, 2018 at 10:50 AM Neil Madden <[e-mail]>
wrote:

> On the assumption that this is likely to be a requirement from customers,
> I’d be in favour of a new server metadata field. My favourite bikeshed
> colour would be:
>
> tls_client_auth_token_endpoint
>
> On another metadata-related note, given that the additional security of
> certificate-bound access tokens vanishes if the resource server doesn’t
> understand and enforce the certificate-binding associated with the access
> token, is there a general need for a client to be able to discover if any
> given RS does actually support this? Otherwise the whole scheme “fails
> open” in that the access token silently degrades to a normal bearer token.
> Or do we consider it unlikely that an RS is going to accept a TLS client
> certificate without supporting this?
>
> — Neil
>
> > On 17 Dec 2018, at 20:26, Brian Campbell <bcampbell=
> [e-mail]> wrote:
> >
> > While there's been some disagreement about the specific wording etc.,
> there does seem to be general consensus coming out of this WG to, in one
> form or another, recommend against the use of the implicit grant in favor
> of authorization code. In order to follow that recommendation, in-browser
> JavaScript clients will need to use XHR/fetch (and likely CORS) to make
> requests directly to the token endpoint.
> >
> > Meanwhile there is the MTLS document utilizes TLS client certificates at
> the token endpoint for client authentication and/or certificate bound
> access tokens. The security BCP draft even recommends sender/key
> constrained access tokens and MTLS is close to the only viable way to do
> that at this time.
> >
> > Unfortunately, however, these two things don't play very nice together.
> When a browser makes a TLS connection where a client cert is requested by
> the server in the handshake, even when client certificates are optional and
> even when it's fetch/XHR, most/many/all browsers will throw up some kind of
> certificate selection interface to the user.  Which is typically a very
> very bad user experience. From a practical standpoint, this means that a
> single deployment cannot really support the MTLS draft and have in-browser
> JavaScript clients using authorization code at the same time.
> >
> > In order to address the conflict here, I'd propose that the MTLS draft
> introduce a new optional AS metadata parameter that is an MTLS enabled
> token endpoint alias. Clients that are doing MTLS client authentication
> and/or certificate bound access tokens would/should/must use the
> alternative token endpoint when present in the AS's metadata. While all
> other clients continue to use the standard token endpoint as they always
> have. This would allow for an AS to deploy an alternative token endpoint
> alias on a distinct host or port where it will request client certs in the
> TLS handshake for OAuth clients that use it while keeping the regular token
> endpoint as it normally is for other clients, especially in-browser
> JavaScript clients.
> >
> > Thoughts, objections, agreements, etc., on this proposal?
> >
> > PS Bikeshedding on a name for the metadata parameter is also welcome.
> Some ideas to start:
> > token_endpoint_mtls_alias
> > token_endpoint_mtls
> > mtls_token_endpoint_alias
> > mtls_token_endpoint
> > alt_token_endpoint_mtls
> > mtls_token_endpoint_alt
> >
> a_token_endpoint_that_a_client_wanting_to_do_mtls_stuff_a_la_RFC_[TBD]_should_use
> > equally_poor_idea_here
> >
> >
> >
> >
> >
> >
> >
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