Re: [OAUTH-WG] WGLC on draft-ietf-oauth-mtls-07

[Brian Campbell <[e-mail]>]

Strongly agree with Justin that any kind of TLS header forwarding standards
like that are well beyond the scope of this spec.


On Fri, Mar 30, 2018 at 10:02 PM, Justin Richer <[e-mail]> wrote:

> I don’t believe this is the spec to define TLS header forwarding standards
> in.
>
>  — Justin
>
>
> On Mar 30, 2018, at 2:03 PM, Vivek Biswas <[e-mail]> wrote:
>
> There are additional challenges which we have faced.
>
> A.      Most of the Mutual SSL communication as mentioned below
> terminates at the LBR and the LBR needs to have client certificates to
> trust the client. But lot of times the connection from LBR to Authorization
> server may be non-SSL.
>
> The CN, SHA-256 thumprint and serial number of the Client Cert are sent as
> header to the AuthzServer/Backend Server. However, if the connection from
> LBR to AuthzServer/Backend Server is unencrypted it is prone to MIM
> attacks. Hence, it’s a MUST requirement to have one-way SSL from LBR to
> AuthzServer/Backend Server, so that the headers passed are not compromised.
>
> This is a MOST common scenario in a real world. And we don’t want everyone
> come up with their own names for the header. There should be some kind of
> standardization around the header names.
>
> Regards
> Vivek Biswas, CISSP
>
> *From:* John Bradley [mailto:[e-mail] <[e-mail]>]
> *Sent:* Thursday, March 29, 2018 11:57 AM
> *To:* Neil Madden
> *Cc:* oauth
> *Subject:* Re: [OAUTH-WG] WGLC on draft-ietf-oauth-mtls-07
>
> Yes that is quite a common deployment scenario.   I think that is the way
> most of the Open Banking implementations have deployed it currently.
>
> The intent is to support that.   One problem is that how the certificate
> is transmitted to the application tends to be load balancer/reverse proxy
> specific as no real standard exists.
>
> If you think that needs to be clarified text is welcome.
>
> John B.
>
>
>
>
> On Mar 29, 2018, at 2:54 PM, Neil Madden <[e-mail]>
> wrote:
>
> Thanks, and understood.
>
> The privacy concerns are mostly around correlating activity of *clients*,
> which may or may not reveal activity patterns of users using those clients.
> I don’t know how much of a concern that is in reality, but thought it
> should be mentioned.
>
> A colleague also made the following comment about the draft:
>
> “It is still quite common to terminate TLS in a load balancer or proxy,
> and to deploy authorization servers in a secure network zone behind an
> intermediate in a DMZ. In these cases, TLS would not be established between
> the client and authorization server as per §2, but information about the
> TLS handshake may be made available by other means (typically adding to a
> downstream header) allowing lookup and verification of the client
> certificate as otherwise described. Given the prevalence of this approach
> it would be good to know whether such a deployment would be compliant or
> not.”
>
> Kind regards,
> Neil
> --
>
>
> On Thursday, Mar 29, 2018 at 4:47 pm, John Bradley <[e-mail]>
> wrote:
> Thanks for the feedback. We will review your comments and reply.
>
> One data point is that this will not be the only POP spec. The spec using
> token binding vs mtls has better privacy properties. It is UK Open banking
> that has pressed us to come up with a standard to help with
> interoperability.
>
> This spec has been simplified in some ways to facilitate the majority of
> likely deployments.
>
> I understand that in future certificates may have better than SHA256
> hashes.
>
> Regards
> John B.
>
>
>
> On Mar 29, 2018, at 12:18 PM, Neil Madden <[e-mail]>
> wrote:
>
> Hi,
>
> I have reviewed this draft and have a number of comments, below. ForgeRock
> have not yet implemented this draft, but there is interest in implementing
> it at some point. (Disclaimer: We have no firm commitments on this at the
> moment, I do not speak for ForgeRock, etc).
>
> 1. https://tools.ietf.org/html/draft-ietf-oauth-mtls-07#section-3.1 defines
> a new confirmation method “x5t#S256”. However, there is already a
> confirmation method “jwk” that can contain a JSON Web Key, which itself can
> contain a “x5t#S526” claim with exactly the same syntax and semantics. The
> draft proposes:
>
> { “cnf”: { “x5t#S256”: “…” } }
>
> but you can already do:
>
> { “cnf”: { “jwk”: { … , “x5t#S256”: “…” } } }
>
> If the intent is just to save some space and avoid the mandatory fields of
> the existing JWK types, maybe this would be better addressed by defining a
> new JWK type which only has a thumbprint? e.g., { “kty”: “x5t”, “x5t#S256”:
> “…” }.
>
> 2. I find the naming “mutual TLS” and “mTLS” a bit of a misnomer: it’s
> really only the client authentication that we are interested here, and the
> fact that the server also authenticates with a certificate is not hugely
> relevant to this particular spec (although it is to the overall security of
> OAuth). Also, TLS defines non-certificate based authentication mechanisms
> (e.g. TLS-SRP extension for password authenticated key exchange, PSK for
> pre-shared key authentication) and even non-X.509 certificate types (
> https://www.iana.org/assignments/tls-extensiontype-
> values/tls-extensiontype-values.xhtml#tls-extensiontype-values-3). I’d
> prefer that the draft explicitly referred to “X.509 Client Certificate
> Authentication” rather than mutual TLS, and changed identifiers like
> ‘tls_client_auth’ (https://tools.ietf.org/html/draft-ietf-oauth-mtls-07#
> section-2.1.1) to something more explicit like ‘tls_x509_pki_client_auth’.
>
>
> This is especially confusing in section 3 on sender constrained access
> tokens, as there are two different servers involved: the AS and the
> protected resource server, but there is no “mutual” authentication between
> them, only between each of them and the client.
>
> 3. The draft links to the TLS 1.2 RFC, while the original OAuth 2.0 RFC
> only specifies TLS 1.0. Is the intention that TLS 1.2+ is required? The
> wording in Section 5.1 doesn’t seem clear if this could also be used with
> TLS 1.0 or 1.1, or whether it is only referring to future TLS versions.
>
> 4. It might be useful to have a discussion for implementors of whether TLS
> session resumption (and PSK in TLS 1.3) and/or renegotiation impact the use
> of client certificates, if at all?
>
> 5. Section 3 defines sender-constrained access tokens in terms of the
> confirmation key claims (e.g., RFC 7800 for JWT). However, the OAuth 2.0
> Pop Architecture draft defines sender constraint and key confirmation as
> different things (https://tools.ietf.org/html/draft-ietf-oauth-pop-
> architecture-08#section-6.2). The draft should decide which of those it
> is implementing and if sender constraint is intended, then reusing the
> confirmation key claims seems misleading. (I think this mTLS draft is doing
> key confirmation so should drop the language about sender constrained
> tokens).
>
> 6. The OAuth 2.0 PoP Architecture draft says (https://tools.ietf.org/html/
> draft-ietf-oauth-pop-architecture-08#section-5
> <https://tools..ietf.org/html/draft-ietf-oauth-pop-architecture-08#section-5>
> ):
>
> Strong, fresh session keys:
>
> Session keys MUST be strong and fresh.. Each session deserves an
> independent session key, i.e., one that is generated specifically
> for the intended use. In context of OAuth this means that keying
> material is created in such a way that can only be used by the
> combination of a client instance, protected resource, and
> authorization scope.
>
>
> However, the mTLS draft section 3 (https://tools.ietf.org/html/
> draft-ietf-oauth-mtls-07#section-3) says:
>
> The client makes protected resource requests as described in
> [RFC6750], however, those requests MUST be made over a mutually
> authenticated TLS connection using the same certificate that was used
> for mutual TLS at the token endpoint.
>
> These two statements are contradictory: the OAuth 2.0 PoP architecture
> effectively requires a fresh key-pair to be used for every access token
> request, whereas this draft proposes reusing the same long-lived client
> certificate for every single access token and every resource server.
>
> In the self-signed case (and even in the CA case, with a bit of work -
> e.g., https://www.vaultproject.io/docs/secrets/pki/index.html) it is
> perfectly possible for the client to generate a fresh key-pair for each
> access token and include the certificate on the token request (e.g., as per
>  https://tools.ietf.org/html/draft-ietf-oauth-pop-key-
> distribution-03#section-5.1 - in which case an appropriate “alg” value
> should probably be described). This should probably at least be an option.
>
>
> 7. The use of a single client certificate with every resource server (RS)
> should be called out in a Privacy Considerations section, as it allows
> correlation of activity.
>
> 8. This is maybe a more general point, but RFC 6750 defines the
> Authorization: Bearer scheme (https://tools.ietf.org/html/
> rfc6750#section-2) for a client to communicate it’s access token to the
> RS in a standard way. As sender-constrained access tokens are not strictly
> bearer tokens any more, should this draft also register a new scheme for
> that? Should there be a generic PoP scheme?
>
> 9. The Security Considerations should really make some mention of the long
> history of attacks against X.509 certificate chain validation, e.g. failure
> to check the “CA” bit in the basic constraints, errors in parsing DNs, etc.
> It should be strongly suggested to use an existing TLS library to perform
> these checks rather than implementing your own checks. This relates to
> Justin’s comments around DN parsing and normalisation.
>
> 10. The PKI client authentication method (https://tools.ietf.org/html/
> draft-ietf-oauth-mtls-07#section-2.1) makes no mention at all of
> certificate revocation and how to handle checking for that (CRLs, OCSP -
> with stapling?). Neither does the Security Considerations. If this is a
> detail to be agreed between then AS and the CA (or just left up to the AS
> TLS stack) then that should perhaps be made explicit. Again, there are
> privacy considerations with some of these mechanisms, as OCSP requests are
> typically sent in the clear (plain HTTP) and so allow an observer to see
> which clients are connecting to which AS.
>
> 11. The same comment applies to how the protected resource checks for
> revocation of the certificate presented during sender constrained access
> token usage. Should the RS make its own revocation checks based on the
> information in the certificate presented, or should it trust the
> certificate while the access token is still valid? If the latter case, is
> the AS responsible for revoking any access tokens whose certificate have
> been revoked (if so, should it be doing an OCSP call on every token
> introspection request, and should the RS be passing on the
> certificate/serial number on that request)? If the Client request uses OCSP
> Stapling (https://en.wikipedia.org/wiki/OCSP_stapling) how can the RS
> verify the signature on that if it does not have a separate trust
> relationship with the CA already?
>
> 12. The use of only SHA-256 fingerprints means that the security strength
> of the sender-constrained access tokens is limited by the collision
> resistance of SHA-256 - roughly “128-bit security" - without a new
> specification for a new thumbprint algorithm. An implication of this is
> that is is fairly pointless for the protected resource TLS stack to ever
> negotiate cipher suites/keys with a higher level of security. In more
> crystal ball territory, if a practical quantum computer becomes a
> possibility within the lifetime of this spec, then the expected collision
> resistance of SHA-256 would drop quadratically, allowing an attacker to
> find a colliding certificate in ~2^64 effort. If we are going to pick just
> one thumbprint hash algorithm, I would prefer we pick SHA-512.
>
> Cheers,
>
> Neil
>
>
>
> On 19 Mar 2018, at 22:34, Rifaat Shekh-Yusef <[e-mail]>
> wrote:
>
> All,
>
> As discussed during the meeting today, we are starting a WGLC on the MTLS
> document:
> https://tools.ietf.org/html/draft-ietf-oauth-mtls-07
>
> Please, review the document and provide feedback on any issues you see
> with the document.
>
> The WGLC will end in two weeks, on April 2, 2018.
>
> Regards,
> Rifaat and Hannes
>
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