Re: [OAUTH-WG] client certs and TLS Terminating Reverse Proxies (was Re: I-D Action: draft-ietf-oauth-jwt-introspection-response-08.txt)

Justin Richer <jricher@mit.edu> Tue, 29 October 2019 19:13 UTC

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From: Justin Richer <jricher@mit.edu>
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Date: Tue, 29 Oct 2019 15:13:38 -0400
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Cc: Brian Campbell <bcampbell=40pingidentity.com@dmarc.ietf.org>, oauth <oauth@ietf.org>
To: Neil Madden <neil.madden@forgerock.com>
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Subject: Re: [OAUTH-WG] client certs and TLS Terminating Reverse Proxies (was Re: I-D Action: draft-ietf-oauth-jwt-introspection-response-08.txt)
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I would argue that making this standard would actually increase the likelihood of developers getting this right, as now instead of following some copy-pasted recipe for NGINX or Apache that they found on the web, they could turn on a standard setting that would take care of both stripping out incoming headers and injecting the appropriate values. And all of that can be covered in the security considerations with a bunch of normative text on top to make sure inbound headers are stripped. What you’re describing below is clever, but ultimately it’s just a small bit of obscurity more than anything real. 

The way things are today, you’ve got to not only pick a header and figure out its format, but also do the injection protection step yourself. Since all of these are disconnected, there are a lot more places that it could fall over. Even a typo where you throw out incoming “CLIENT_CERT” but inject “CLIENT_CERTS” or something like that would be disastrous. 

All in all, I am in favor of this being defined in one standard way, in addition to secure communication between proxies and backends being standardized — but this latter bit really seems like a separate problem.

 — Justin

> On Oct 28, 2019, at 12:32 PM, Neil Madden <neil.madden@forgerock.com>; wrote:
> 
> While there are some benefits to standardizing headers for this kind of communication, there are some significant downsides - particularly when using headers to communicate critical security information like certs. It is *very* easy to misconfigure a reverse proxy to not strip Forwarded (or whatever) headers from incoming requests, allowing a client to simply supply a certificate as a header without authenticating the TLS connection with the corresponding private key. One good practice to prevent this is to pick a random and unguessable header name (configurable per installation) to be used for communicating the certificate, rather than using something fixed and standard. That way even if you misconfigure the proxy an attacker still has to try and guess the correct header name.
> 
> I suppose the same thing could be accomplished by having an extension for including a shared secret (or HMAC tag) in the header to authenticate it.
> 
> -- Neil
> 
>> On 28 Oct 2019, at 15:32, Brian Campbell <bcampbell=40pingidentity.com@dmarc.ietf.org <mailto:bcampbell=40pingidentity.com@dmarc.ietf.org>> wrote:
>> 
>> I don't think there's anything beyond defining something to carry the client certificate information (including the format and encoding). And it could well be a new RFC7239 parameter. Or it might just be a new HTTP header on its own.  
>> 
>> On Mon, Oct 28, 2019 at 9:05 AM Rifaat Shekh-Yusef <rifaat.ietf@gmail.com <mailto:rifaat.ietf@gmail.com>> wrote:
>> Thanks Brian,
>> 
>> I guess my question is: given RFC7239 and the fact that it is straightforward to secure the channel between the terminating reverse proxy and the backend service in a cluster, is there anything, from a standard perspective, that we need to do beyond defining a new parameter to carry the client certificate information?
>> You seem suggest that the answer is yes. If so, can you please elaborate on why is that?
>> 
>> Regards,
>>  Rifaat
>> 
>> 
>> 
>> On Mon, Oct 28, 2019 at 8:42 AM Brian Campbell <bcampbell@pingidentity.com <mailto:bcampbell@pingidentity.com>> wrote:
>> 
>> 
>> On Sat, Oct 26, 2019 at 3:55 PM Rifaat Shekh-Yusef <rifaat.ietf@gmail.com <mailto:rifaat.ietf@gmail.com>> wrote:
>> 
>> On Fri, Oct 25, 2019 at 3:47 PM Brian Campbell <bcampbell@pingidentity.com <mailto:bcampbell@pingidentity.com>> wrote:
>> 
>> I did look at RFC7239 when doing that and it could have been made to work but felt the fit wasn't quite right and would have been more cumbersome to use than not.  
>> 
>> 
>> Can you elaborate on this?
>> These days, with the zero trust model in mind, there are orchestration tools, e.g. Istio, that easily allows you to establish an MTLS channel between the reverse proxy/load balancer/API GW and the backend servers.
>> Why is that not sufficient? 
>> Which part is cumbersome?
>> 
>> What I meant was only that in the course of writing https://tools.ietf.org/html/draft-ietf-tokbind-ttrp-09 <https://tools.ietf.org/html/draft-ietf-tokbind-ttrp-09>;, which aims to define HTTP header fields that enable a TLS terminating reverse proxy to convey information to a backend server about the validated Token Binding Message received from a client, it seemed more straightforward and sufficient for the use-case to use new HTTP headers to carry the information rather than to use new fields in the Forwarded header framework from RFC7239.    
>>  
>> 
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