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

[Phil Hunt <[e-mail]>]

Brian,

Thanks for clarifying. Given the browser uncertainty that happens with optional configs and browsers, I agree with your suggestion of using ‘mtls_endpoints’.

I’m expecting this will be a big deal as we migrate apps and clients. We have to make sure old clients keep working and don’t do unexpected things. Because of this, returning the CertificateRequest message may cause problems for legacy clients. It seems practical to just to tell the MTLS clients where they will definitely work given they will make changes to support MTLS certs anyway.

Thanks.

Phil

Oracle Corporation, Cloud Security and Identity Architect
@independentid
www.independentid.com <http://www.independentid.com/>[e-mail] <mailto:[e-mail]>

> On Feb 1, 2019, at 3:26 PM, Brian Campbell <[e-mail]> wrote:
> 
> Yes client certificate authen can be made optional. In the optional case the server still sends the CertificateRequest message during the handshake which efficiently asks the client to present a cert. The client send a cert or not at that point. In the optional case, the server will continue with the connection even when no client cert is presented. In the required case, the server will terminate the handshake when no client cert is presented. 
> 
> So yes, optional is possible. The potential problem is the (sometimes pretty bad and/or confusing) UI that browsers will present to the end-user anytime the CertificateRequest message is sent by the server in the handshake. And that message is sent in the optional case and the required case. Giving an AS a way to avoid the potentially bad UX for the end user is the impetus behind this whole discussion. 
> 
> On Fri, Feb 1, 2019 at 3:49 PM Phil Hunt <[e-mail] <mailto:[e-mail]>> wrote:
> Brian,
> 
> Let me turn this around. Is it possible for a single endpoint to have MTLS clients (mutual auth) and bearer clients (server auth TLS)?  
> 
> I had thought that client certificate authen can be made optional, but I must confess I’m confused as to how optionality works in TLS when it comes to client certificate authentication.
> 
> If we have to do multiple endpoints for the APIs and/or the token endpoints, we have to send clients to the correct endpoints to make TLS set up correctly.   If this is the case, I think Brian is right—we need a parameter.
> 
> Phil
> 
> Oracle Corporation, Cloud Security and Identity Architect
> @independentid
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> 
>> On Feb 1, 2019, at 1:43 PM, Brian Campbell <[e-mail] <mailto:[e-mail]>> wrote:
>> 
>> I guess I'm not clear what you are driving at. 
>> 
>> On Fri, Feb 1, 2019 at 2:32 PM Phil Hunt <[e-mail] <mailto:[e-mail]>> wrote:
>> That way works. But one of the modes on most tls terminators is client cert optional.
>> 
>> This works ok when you want dual mode to support bearer and mtls for apps (e.g. mobile) because the client will decide to use MTLS.  With browsers, they only use it if forced.
>> 
>> Phil
>> 
>> Oracle Corporation, Cloud Security and Identity Architect
>> @independentid
>> www.independentid.com <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.independentid.com&d=DwMFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=na5FVzBTWmanqWNy4DpctyXPpuYqPkAI1aLcLN4KZNA&m=zghYuxpPtYeMn2IhNNwG0lo64yhSMZ5ER8oPt3G95oE&s=1K1v6KsnRhJrNtN6LVDS7U_kzRD5xLCy59j-ij-MGNA&e=>[e-mail] <mailto:[e-mail]>
>> 
>>> On Feb 1, 2019, at 1:14 PM, Brian Campbell <[e-mail] <mailto:[e-mail]>> wrote:
>>> 
>>> The server has to ask during the handshake for a client to send a cert. 
>>> 
>>> On Fri, Feb 1, 2019 at 2:12 PM Phil Hunt <[e-mail] <mailto:[e-mail]>> wrote:
>>> If a client attempts to force mtls would typical tls terminators accept it enough to redirect?
>>> 
>>> My worry is how optionality works in browsers. It seems like they have to hit an mtls required endpoint before they reliably use a client cert. 
>>> 
>>> Phil
>>> 
>>> On Feb 1, 2019, at 12:56 PM, Brian Campbell <[e-mail] <mailto:[e-mail]>> wrote:
>>> 
>>>> It would be more a client having reached a non-MTLS endpoint and is 307'd to an MTLS enabled endpoint. 
>>>> 
>>>> On Fri, Feb 1, 2019 at 1:40 PM Phil Hunt <[e-mail] <mailto:[e-mail]>> wrote:
>>>> I was a bit confused by how the 307 would work.
>>>> 
>>>> To confirm, Is the client having reached an MTLS optional endpoint being redirected to an MTLS mandatory endpoint if the AT (or a cookie) is detected to have a “cnf” claim in order to make the browser invoke MTLS?
>>>> 
>>>> Phil
>>>> 
>>>> Oracle Corporation, Cloud Security and Identity Architect
>>>> @independentid
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>>>> 
>>>>> On Feb 1, 2019, at 11:56 AM, Brian Campbell <bcampbell=[e-mail] <mailto:bcampbell=[e-mail]>> wrote:
>>>>> 
>>>>> I'm finally getting around to working on the document updates (there's quite a few things that came out of AD review too). As far as the issue in this thread goes though, I'm leaning towards adding "mtls_endpoints" as a new metadata parameter. Maybe mention that a 307 might happen but it'd be more of a considerations type text. 
>>>>> 
>>>>> On Wed, Jan 16, 2019 at 5:52 AM Brian Campbell <[e-mail] <mailto:[e-mail]>> wrote:
>>>>> 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] <mailto:[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] <mailto:[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] <mailto:[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] <mailto:[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] <mailto:[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 <https://server.example.com/>",
>>>>>   "authorization_endpoint":"https://server.example.com/authz <https://server.example.com/authz>",
>>>>>   "token_endpoint":"https://server.example.com/token <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 <https://server.example.com/revo>",
>>>>>   "jwks_uri":"https://server.example.com/jwks.json <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] <mailto:[e-mail]>> wrote:
>>>>> 
>>>>> 
>>>>> > On Jan 11, 2019, at 3:32 AM, Neil Madden <[e-mail] <mailto:[e-mail]>> wrote:
>>>>> > 
>>>>> > On 9 Jan 2019, at 05:54, David Waite <[e-mail] <mailto:[e-mail]>> wrote:
>>>>> >> 
>>>>> >>> On Dec 28, 2018, at 3:55 PM, Brian Campbell <bcampbell=[e-mail] <mailto:[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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