Re: [OAUTH-WG] Secdir last call review of draft-ietf-oauth-jwsreq-30

Mike Jones <Michael.Jones@microsoft.com> Fri, 26 February 2021 20:54 UTC

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From: Mike Jones <Michael.Jones@microsoft.com>
To: "watsonbladd@gmail.com" <watsonbladd@gmail.com>, nat <nat@nat.consulting>, "rdd@cert.org" <rdd@cert.org>
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Thread-Topic: Secdir last call review of draft-ietf-oauth-jwsreq-30
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Subject: Re: [OAUTH-WG] Secdir last call review of draft-ietf-oauth-jwsreq-30
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Thanks again for your review, Watson.  My replies to your comments below are prefixed by "Mike>".



-----Original Message-----
From: Watson Ladd <watsonbladd@gmail.com>
Sent: Tuesday, December 15, 2020 9:01 PM
To: Nat Sakimura <nat@nat.consulting>
Cc: secdir <secdir@ietf.org>rg>; IETF oauth WG <oauth@ietf.org>rg>; last-call@ietf.org; draft-ietf-oauth-jwsreq.all@ietf.org
Subject: [EXTERNAL] Re: Secdir last call review of draft-ietf-oauth-jwsreq-30



On Sat, Oct 31, 2020 at 6:13 AM Nat Sakimura <nat@nat.consulting<mailto:nat@nat.consulting>> wrote:

>

> Hi Watson,

>

> Thanks very much for the review. I thought I have sent my response

> earlier, which I actually did not. It was sitting in my draft box. I

> apologize for it.



My apologies for missing it in my inbox for a number of months.

>

> My responses inline:

>

> On Sat, Sep 26, 2020 at 9:46 AM Watson Ladd via Datatracker

> <noreply@ietf.org<mailto:noreply@ietf.org>> wrote:

> >

> > Reviewer: Watson Ladd

> > Review result: Serious Issues

> >

> > I generated this review of this document as part of the security

> > directorate's ongoing effort to review all IETF documents being

> > processed by the IESG.  These comments were written with the intent

> > of improving security requirements and considerations in IETF

> > drafts.  Comments not addressed in last call may be included in AD

> > reviews during the IESG review.  Document editors and WG chairs should treat these comments just like any other last call comments.

> >

> > Two minor issues: On page 4, "This offers an additional degree of

> > privacy protection." should be reworded. I don't think it makes

> > sense in context, where authenticity was discussed.

>

>

> In the course of the edit, explanation about two distinct privacy

> benefits was collated in one sentence and has become very difficult to

> parse.

>

> What it is trying to express as privacy benefits are the following.

>

> 1) The authorization request content is sent to the AS in the

> backchannel so it will not be exposed through the browser to the eyes

> of an active or passive outsider observing what is going on in the

> browser.  In the RFC6749 framework case, the authorization request

> goes through the browser redirect and it could leak to the adversary

> via WPAD/PAC Attack, referrer, browser history, etc. Also, if the

> browser was infected by an adversary controlled malware, the content

> can be sniffed by the adversary. In the case of JAR, it does not

> happen. This is one privacy benefit it is trying to explain.

>

> 2) The location that the authorization request is getting pushed to

> does not have to be the AS. A trusted third party that examines the

> content for the conformance to the collection minimization principle

> may act as the party that accepts the authorization request and issues

> the request_uri. AS can then just evaluate the domain part of

> request_uri to evaluate that the authorization request is conformant

> to this principle. This is another privacy benefit from the point of

> view of the individual user.



I'm fine with any fix to the sentence that makes sense. Don't think we need to insert the above but I very much appreciate the explanation.



Mike> Given that its meaning is fairly inscrutable, and that the two benefits described by Nat above are partially related to paragraphs other than the one with the privacy statement, I propose that we simply delete the sentence "This offers an additional degree of privacy protection." from this paragraph.



> > It took me a while to understand what the by reference method is:

> > maybe the intro should say via URL instead of by reference.

>

>

> request_uri can be URL or a handle such as URN. That is why the "by

> reference" word is being used, per the suggestion of the WG.



I'm fine with that, just noting my confusion.



Mike> Understood.  I looked at ways to possibly move the "by reference" text that follows in a few paragraphs earlier to possibly make this clearer, but it would mess up the logical flow of the Introduction.  Unless you have a better suggestion, I propose that we leave this as-is.



> > And now for the thorny issues with this draft. Signatures and

> > encryption are different. And encrypting a signed blob doesn't mean the signer encrypted it.

> > Then there are a plethora of methods specified in the draft  to

> > authenticate the blob, which will give different results in

> > maliciously constructed examples. The security considerations

> > section should discuss what the encrypted vs signed choices give in

> > the way of security, and it doesn't. This makes me worry.

>

> We don’t expect the encryption to ensure authenticity, that’s what the

> signatures are used for.



This needs to be very clearly spelled out in the text. Lots of people will not understand this. The wording of section 10.2 is at best ambivalent, with multiple alternatives presented as acceptable.



Mike> After the sentence at 10.2 (b) about symmetric encryption, I propose that we add the following sentence to emphasize the point you're raising:  "Note however, that if public key encryption is used, no source authentication is enabled by the encryption, as any party can encrypt content to the public key."



<chop>

>

> I didn't quite get what is meant by "plethora of methods specified in

> the draft to authenticate the blob ... "

> There is a bit of text about authenticating the source (=client) but

> not much on the blob itself.

> The discussion around the signature and/or encryption is covered in

> RFC7519 (JWT), the format that the request object assumes.

> This is required reading when implementing this spec, so WG thought it

> is not worth repeating here.

> Attacks etc. on the signature and encryption are covered in RFC7515

> and RFC7516 respectively.



Well, the draft happens to include the following text:

   "The Authorization Server MUST validate the signature of the JSON Web

   Signature [RFC7515] signed Request Object.  The signature MUST be

   validated using the key for that "client_id" and the algorithm

   specified in the "alg" Header Parameter."



Shouldn't the key be associated with a single algorithm? How do we ensure that the common attack of telling the server to use hmac to verify the signature doesn't work here?



Mike> Good point.  This attack is described in Section 2.1 of RFC 8725 and mitigated by the practices in Sections 3.1 and 3.2 of the same.  I propose that we replace the sentence:

    "The signature MUST be validated using the key for that "client_id" and the algorithm specified in the "alg" Header Parameter."

with:

    "The signature MUST be validated using a key associated with the client and the algorithm specified in the "alg" Header Parameter.  If a "kid" Header Parameter is present, the key identified MUST be the key used, and MUST be a key associated with the client.  Algorithm verification MUST be performed, as specified in Sections 3.1 and 3.2 of RFC 8725."



> > Looking at the cited reference for attacks, I see the fix is to

> > include information about which IPD was used by the RP. But the

> > draft before us doesn't mandate that. It's not clear than how the

> > cited attack is prevented by the draft. Saying that the

> > communication through the user-agent is subject to

>

> The mention of mix-up attack was introduced after the Last call by one

> of the comment. It just added it in the sentence with a reference. I

> am ok to remove it.



That works for me.



Mike> I will remove the mix-up attack reference in the next draft.



> Having said that, the heart of mix-up attack is that the combination

> of the client believes that it is communicating with the

> attacker-controlled AS (AAS) while it in-fact is talking to Honest AS

> (HAS), AND HAS unable to find out that the client is thinking that it

> is talking to AAS not him.

>

> OAuth JAR seems to mitigate it in two ways:

>

> a) Use request_uri which is hosted by the AS. Then, if the client is

> thinking that it is talking to the AAS, then it will push it to AAS

> and when the user is redirected to HAS, HAS will find out that the

> request_uri is not created by herself and return an error, making the

> mix-up attack fail.

>

> b) Include `aud` in the request. Then, when the HAS will find that the

> request was minted to AAS and not her. So, it will result in an error,

> making the mix-up attack fail.



If the draft mandates doing this it addresses the attack and the sentence can stay.



Mike> The draft mandates use of "aud" but it does not mandate that the request_uri be hosted by the AS.  Therefore, I think we should remove the mix-up attack reference.



> So, I added mix-up attack to the sentence thinking the commenter's

> request to add it is fine, but I am fine with removing it.

>

> > manipulation, and this prevents it, ignores that the attacker in

> > that position sees a lot more. The user-agent as resource owner

> > modifying the requested resources is a very funny sort of attack:

> > can't they do what they want with the resources since they control the access?

>

> If the client is in the browser, yes.

> But in the mainstream case, the client is not in the browser but the

> web-server that the browser is communicating with and the resource

> access happens without being mediated by the browser.



My concern on this point is resolved.



Mike> Thanks



> > Key management is ignored. This is a very important issue,

> > especially

>

> A lot of ground is covered by RFC 7515, 7516, 7517, 7518, 7519, 7591,

> and 8414 so this document is not specifically restating them.

>

> >

> > considering the potential problems with the reuse of JWT. I'd like

> > to see a

>

> Are you talking about the reuse of the request object by an adversary

> trying to act as an honest client?

> Even if it happens, the malicious client does not have the proper

> client credential so it cannot redeem the code it obtained with the

> token. It is no different than RFC6749 code grant. Protocols that

> extend it, such as OpenID Connect, have introduced nonce to prevent

> the reuse and used JAR (it is called request object there) to further

> protect tampering and achieve client authentication even in the front

> channel.

>

> > recommendation that keys be separated by intended uses, rather than

> > limiting particular fields in an ad-hoc manner.

>

> Could you kindly elaborate on the "ad-hoc manner" part so that I can

> understand it more fully?



10.8, Cross-JWT Confusion discusses avoiding signing certain fields, rather than suggesting good key usage as a solution.



Mike> I propose to add this new paragraph at the end of Section 10.8 to implement your suggestion:

    "Finally, another way to prevent cross-JWT confusion is to use a key management regime in which keys used to sign Request Objects are identifiably distinct from those used for other purposes.  Then, if an adversary attempts to repurpose the Request Object in another context, a key mismatch will occur, thwarting the attack."



> > Then we have section 11. What section 11 introduces is an entire new

> > dramatis personae, the Trust Framework Provider, with no prior

> > discussion of what it is or a reference to where it is defined and a

> > good number of statements about how it works that aren't really  clear what they mean from the document to me.

>

> Trust Framework Provider first appears in 5.2.1.

> At the time of writing the related text, it was a pretty well-known

> concept. In the United State, it was part of its National Strategy

> (NSTIC) and internationally, it was even taken up at WEF Davos

> meeting. It is quite surprising that such a mainstream concept faded

> into obscurity so quickly. The reason for introducing it was to a)

> justify request_uri as some WG members wanted it to be removed, b)

> justify that requst_uri to be served from a different domain. Now that

> people appreciate it, e.g., it can be seen from PAR, the justification

> for a) probably is no longer required. A full explanation for b) would

> probably be a much longer text but I doubt if it belongs to this

> document. I am fine with removing the reference to Trust framework

> etc. as long as the capability to push the authorization request to a

> place other than the client or the authorization server is not

> removed.



Let's remove the text then, but keep the capability.



Mike> I propose to remove uses of the term "Trust Framework Provider" and instead replace them by the more generic term "trusted third-party service".



> > My biggest concern is that these issues are signs that the problem

> > this draft is trying to solve and the mechanisms to solve it haven't

> > been analyzed as thoroughly as they should have been. Without that

> > sort of thorough analysis it's not certain that the mechanisms

> > actually solve the problem and it's not clear what the

> > recommendations to implementers have to be to preserve those properties.

>

> OAuth JAR, as the name "The OAuth 2.0 Authorization Framework: JWT

> Secured Authorization Request (JAR)" suggests, is a framework and not

> a house itself. One such example is FAPI [1] which was formally

> verified [2].



"It's possible to use this draft security" I don't think should be enough anymore. Rather it should be impossible to use insecurely.



Mike> The draft describes how to use the mechanism securely.  Yes, if portions of the draft (and those it depends upon) are not followed, insecure usage can result.  That's true of any security draft.  If there are additional specific requirements and/or recommendations needed, we'd be glad to add them.  If so, please identify them.  (Indeed, we're already doing so in response to your existing specific feedback.)



Mike> As for past JWT problems, the JWT BCP [RFC 8725] was written at the request of the IESG to identify and mitigate them - especially in light of JWTs being used for additional use cases, such as STIR secured telephony and securing first-responder services.  If you believe that additional generic JWT issues should be discussed and addressed, we could always revise this BCP.  But doing so is beyond the scope of this RFC.



> [1]

> https://bitbucket.org/openid/fapi/src/master/Financial_API_WD_002.md

> [2] https://arxiv.org/abs/1901.11520

>

> >

> > Obviously this draft has had a long and tortured history with

> > multiple reviews,  and what I'm suggesting needs to happen is a lot

> > of work. But it's essential in any security protocol to do this

> > analysis and be clear about what is, and what is not, protected by the protocol.

>

> OAuth JAR is nothing but just another binding to OAuth 2.0. Where

> RFC6749 binds it to form encoding, it provides two additional bindings:

>     1) binding to JWT, and

>     2) binding to the pushed authorization request that is referenced by a URI.

> It is this simple. As such, it would also inherit some of the

> shortcomings in RFC6749. However, it is not this document to address

> them. It should be done by other documents so that the result can be

> encoded using the mechanisms provided in this document.



This is not a simple matter. JWT has a long and twisted history with some pervasive errors in common libraries, and is a fairly large standard. OAuth 2.0 is also large. Ensuring that the mapping has the right properties is going to be a mess. If the encoding does not respect the semantics we have a serious security issue. If implementors assume the encoding provides properties it does not, we again have a security issue.



Mike> See my previous comments on past JWT implementation errors and the writing of the JWT BCP [RFC 8725] to address these.



> It is quite surprising that this fact is not getting appreciated and

> is taking such a long time to complete.

> Maybe I should delete all the explanation text and leave it with just

> the core text. Explanation and justification text for defining

> additional bindings probably are just distractions now as it is now

> appreciated and used all over the world unlike when the project was

> started.



Mike> I would propose that we make only necessary changes to the draft at this point.  Let's finish this long-overdue specification!



>

> >

> > Sincerely,

> > Watson Ladd

> >

>

> Thanks again for your detailed comments.

>

> Best wishes,

>

> --

> Nat Sakimura

> NAT.Consulting LLC



Mike> I now plan to create edits incorporating the proposed resolutions above for review.



                                                       Best wishes,

                                                       -- Mike



--

Astra mortemque praestare gradatim