Re: [OAUTH-WG] rfc6749 question about the optional use of the client_id in the request body

[Tom Van Oppens <[e-mail]>]

@Brian +1
 I agree that the section is confusing and that errata should be published 
about it, but before we go there it might be interesting why someone took 
the effort to describe the situation with an empty client secret, because 
including these suggestions will break the ability for an sever to dectect 
a double authentication when the client secret is an empty string

- As for the client secret not being an empty, it might be that it's 
updateable by the client.Maybe specifying that a client secret can't be an 
empty string might be the most elegant solution. 
- Secondly I couldn't find the the "If a client_id parameter is present in 
conjunction with some client authentication mechanism, then both must 
refer to the same client. " section nor can i find anywhere in the spec 
what the 

I do agree that many to be specs and implementations use the client_id in 
the body, and there are valid arguments to be made for that. But without 
errata in the OAuth2 spec they are violating the OAuth2 spec, And seeing 
the nature of it i think adding errata to the OAuth2 spec is the way to go

Do you have another suggestion to interpret the problematic section ?

@Nat
Nowhere in the spec i can find that they must match or even that the AS is 
supposed to do anything with it
When you read the specs at this point the problem is that if a client_id 
is in the body it is the same as a client_id and a secret (but a blank 
secret) and that is a double authentication when the Auth header is 
present

@all

Shoudln't we define or maybe in the OIDC spec add some information so that 
the AS needs to do something with that clien_id in the body, saying it 
must match the client_id coming in somewhere else ?
Or at least have the AS do something with it .


From:   Nat Sakimura <[e-mail]>
To:     Brian Campbell <[e-mail]>
Cc:     Tom Van Oppens <[e-mail]>, oauth <[e-mail]>
Date:   26/01/2018 01:16
Subject:        Re: [OAUTH-WG] rfc6749 question about the optional use of 
the client_id in the request body



+1 to Brian. 

I would like to point out that listing the participant in the protocol 
message and the authentication of the message sender is an entirely 
different thing. I see no problems in duplicating client_id in body and 
header. Of course, they have to match and must fail the authentication if 
they do not, but this should not be a problem. In fact, it may even be 
desirable for the message body to have self-contained references to the 
participants in the authentication protocol as shown in [1]. In such a 
case, it will necessarily duplicate in case of the basic authentication. 

[1] Basin, D., Cremers, C., Meier, S.: Provably Repairing the ISO/IEC 9798 
Standard for Entity Authentication. Journal of Computer Security - 
Security and Trust Principles archive Volume 21 Issue 6, 817-846 (2013) 

Best, 

---
Nat Sakimura

On Thu, Jan 25, 2018 at 11:28 PM Brian Campbell <
[e-mail]> wrote:
Hi Tom,

Indeed RFC 6749 is not well written with respect to this situation and 
unfortunately leaves some room for varied interpretations. However, in my 
own not entirely uninformed view having worked on this stuff for awhile 
now, it is erroneous to interpret the presence of the client_id parameter 
in the request body as client_secret_post authentication when there is no 
corresponding client_secret parameter. As you alluded to, there are other 
types of client authentication that explicitly allow (JWT, SAML, and their 
base spec) or require (MTLS) the client_id parameter and the OIDC core 
spec even has an example of the client_id parameter in the body when doing 
JWT client auth. If client_id with no client_secret in the request body 
actually implies client_secret_post, then those RFCs (one soon to be RFC) 
and OIDF standards are all contradicting OAuth 2.0 /RFC 6749. Those 
supplementary standards as well as widespread implementations/deployments 
in practice should, I believe, be considered more authoritative than one 
particular implementation's problematic (in terms of interoperability) 
interpretation of a not particularly well written area of the OAuth spec.

The problematic text from 
https://tools.ietf.org/html/rfc6749#section-2.3.1 says that "the client 
MAY omit the [client_secret] parameter if the client secret is an empty 
string" so it would only really be reasonable for an AS to reject a 
request as having two client authentication methods in the case that it 
issued a client the empty string as a client secret (not a public client 
but a client with an empty string as its actual secret), which should 
never happen in practice, and that client sent both a basic authorization 
header without a password and a client_id without a client_secret in the 
body. That's one way to read it anyway. And regardless that text in Sec 
2.3.1 is problematic and should probably be updated with an errata on RFC 
6749 to get rid of the text about empty string password and just state 
that the client_secret parameter is required when doing client_secret_post 
authentication. Unfortunately the errata often get overlooked but it'd 
still be good to have that fixed somewhere and a published RFC can't be 
changed so errata is the only real option to document the actual intent of 
the original specification.

The presence of the client_secret parameter should be the only thing that 
implies client_secret_post authentication. 

If a client_id parameter is present in conjunction with some client 
authentication mechanism, then both must refer to the same client. 

   


On Wed, Jan 24, 2018 at 3:19 AM, Tom Van Oppens <[e-mail]
> wrote:
Dear Oauth Mailing List 

After some discussion i had i wanted to ask you for some guidance. 

For the following request 

request_uri 
https://example.com/token
request_method 
POST 
request_headers 
{"Accept":"application/json","Authorization":"Basic 
bWFnaWNpZDpwb3RhdG9zZWNyZXQ=","Content-Type":"application/x-www-form-urlencoded","Content-Length":"91"} 

request_body 
grant_type=client_credentials&scope=accounts&client_id=magicid 

We had some discussions whether or not this request is a valid request, to 
be more exact wether the clientid can be in the body. 
Section 2.3.1 states 
A client MAY use the "client_id" request parameter to identify itself when 
sending requests to the token endpoint. 

But at the same time in the case of a client password (2.3.1) 
The clientid and secret are carried in the basic auth header as a form of 
authentication as a preferred method , 
But the standard states that if you choose to use the body as a form of 
authentication that if you can ommit the clientsecret the clientsecret is 
an empty string, therefore passing only the client_id is the same as 
passing the client_id and an empty string clientsecret . 

So the current request would be according to the spec interpreted as 
follows 
Authentication 1) basic auth cleintid:secret 
Authentication 2) body auth  clientd and blank secret 

You can choose to use the client_id in the body with public clients or in 
the confidential client (the Lloyds situation) if you choose to add the 
clientsecret there as well and are not using the basic auth header (this 
is due to spec section 2.3 which states 
The client MUST NOT use more than one authentication method in each 
request. 

In short there is no way in the spec that allows for the oauth provider to 
distinguish between your intention of sending in the client_id again for 
identification and a malformed request with double authentication. 


So my stance is (for now) that you cannot send a clientid when you find 
yourself in the clientid with a corresponding password situation. 
Is that a correct statement ? 
and if it is not how would that work ? 
and if it is, when can you send the clientid in the body but use something 
else for authentication  (something like mtls ?) ? 

Kind Regards 
Van Oppens Tom

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International Business Machines of Belgium sprl / bvba
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