Re: [OAUTH-WG] Can a client send the Authorization Request?

"A. Rothman" <> Tue, 25 May 2021 20:21 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 9188B3A03F4 for <>; Tue, 25 May 2021 13:21:27 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: 1.393
X-Spam-Level: *
X-Spam-Status: No, score=1.393 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, FORGED_MUA_MOZILLA=2.309, HTML_MESSAGE=0.001, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_BLOCKED=0.001, RDNS_DYNAMIC=0.982, SPF_PASS=-0.001, URIBL_BLOCKED=0.001] autolearn=no autolearn_force=no
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id tJyG-J0hNHzV for <>; Tue, 25 May 2021 13:21:23 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id E9D093A03EF for <>; Tue, 25 May 2021 13:21:22 -0700 (PDT)
Message-ID: <1223731896.51621974079788.JavaMail.root@shefa>
MIME-Version: 1.0
X-UserIsAuth: true
Received: from ([]) by (JAMES SMTP Server 2.3.1) with SMTP ID 880; Tue, 25 May 2021 23:21:19 +0300 (IDT)
To: Justin Richer <>, Sascha Preibisch <>
Cc: IETF oauth WG <>
References: <952456782.01621954787444.JavaMail.root@shefa> <> <>
From: "A. Rothman" <>
Date: Tue, 25 May 2021 23:21:18 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1
In-Reply-To: <>
Content-Type: multipart/alternative; boundary="------------5B200D570595A564AC9E67FD"
Content-Language: en-GB
Archived-At: <>
Subject: Re: [OAUTH-WG] Can a client send the Authorization Request?
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: OAUTH WG <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Tue, 25 May 2021 20:21:28 -0000


Thanks for the clear answer. The distinction you make is indeed exactly 
the point I was asking about.

So I got the first answer, which is that it is not compliant.

Now the follow-up question is whether it is known to be any more or less 
secure than the normal flow, or simply unknown until further analysis is 
done, or dependent on their specific implementation in some way - 
although afaik they use an off-the-shelf standard OAUTH2 server, just 
expect it to be accessed from the client instead of from the user agent, 
due to their added mTLS requirement on the entire server (including the 
Authorization endpoint).

I'm still not sure what the motive is behind the mTLS requirement, 
though it's possible it just sounded like a good idea at the time to 
make it 'more secure', without realizing there are consequences (like 
being non-compliant with OAUTH2 and/or opening new potential attack 
vectors, if that's also the case - still trying to figure that one out). 
Is there any flaw in OAUTH2 that would require such mTLS on this 
endpoint? Is it worth the risks involved in deviating from the normal flow?



On 5/25/21 10:54 PM, Justin Richer wrote:
> One point, the client doesn’t POST to the authorization endpoint, the 
> resource owner’s browser is supposed to POST to the authorization 
> endpoint — it’s an important distinction. And in the wild, this is 
> really rare to see in use.
> As written, this is not compliant with OAuth2. I agree that this 
> sounds a lot like PAR, except for the fact that the URL getting sent 
> back sounds like it’s used directly as the redirect. Where PAR sends 
> back a URI to be tacked onto the authorization endpoint as a 
> parameter, this is sending back the full URL to send the browser to. 
> In this way, it sounds more like GNAP’s “redirect” interaction start 
> method, which follows that pattern.
> <>
> GNAP uses this pattern for both greater security and greater 
> flexibility in this step — In my opinion it’s basically what PAR would 
> have been if we hadn’t started with the parameterized authorization 
> endpoint.
>  — Justin
>> On May 25, 2021, at 11:28 AM, Sascha Preibisch 
>> < <>> wrote:
>> Hello Amichai!
>> There could be several reasons why you see that behaviour in your web 
>> browser. For example:
>> - This RFC suggests sending a request to the authorization server, 
>> get a session specific URL back which can be forwarded to the 
>> authorization server via the browser. This is OAuth PAR (Pushed 
>> Authorization Request): 
>> <>. I have 
>> also made a video about this flow, maybe it matches what you are 
>> seeing on your web server: 
>> <>
>> - In addition RFC 6749 also allows a client to POST to the 
>> authorization endpoint
>> I hope this helps,
>> Sascha
>> On Tue, 25 May 2021 at 08:00, A. Rothman < 
>> <>> wrote:
>>     Hi,
>>     In RFC 6749 section 4.1, the Authorization Code Grant flow starts
>>     with:
>>     (A)  The client initiates the flow by directing the resource owner's
>>              user-agent to the authorization endpoint. The client
>>     includes
>>              its client identifier, requested scope, local state, and a
>>              redirection URI to which the authorization server will
>>     send the
>>              user-agent back once access is granted (or denied).
>>     (B)  The authorization server authenticates the resource owner (via
>>              the user-agent) and establishes whether the resource owner
>>              grants or denies the client's access request.
>>      From this, and most explanation I've seen, I understand that the
>>     client
>>     (e.g. my web server) is supposed to prepare the Authorization
>>     Request
>>     URL but instead of sending it to the Authorization Server, it
>>     redirects
>>     the user agent which is the one actually making the HTTP request. It
>>     then goes back and forth with the Authorization Server (with HTML
>>     and
>>     posting forms and whatnot), and eventually receives the
>>     Authorization
>>     Response which redirects the user agent back to the client's
>>     callback
>>     URL with the included code parameter. So as far as the Authorization
>>     Request/Response flow goes, there is no direct communications
>>     between
>>     the client and Authorization Server up to this point (before the
>>     token
>>     exchange).
>>     1. Basically correct so far?
>>     Now, I've encountered a provider that works slightly differently
>>     (but
>>     still with the Authorization Code Grant scheme): the client (my web
>>     server) is supposed to send the Authorization Request directly to
>>     the
>>     Authorization Server, then receive some opaque URL, and redirect the
>>     user agent to there to continue the process. I suppose this URL is
>>     equivalent to one from the middle of the 'back and forth' in the
>>     previous scenario. The rest of the flow continues the same. So
>>     basically, the initial redirect response and HTTP request are
>>     reversed -
>>     instead of first redirect and then request (from user agent),
>>     there is
>>     first the request (from client)  and then redirect.
>>     So the questions are:
>>     2. Is this compliant with the RFC?
>>     3. Is it any less secure? (even if not strictly compliant with
>>     the RFC's
>>     flow, it may still be secure...)
>>     4. If it is less secure, what are the possible vulnerabilities or
>>     attacks made possible here that are mitigated in the original flow?
>>     5. They claim the change is made because they insist on using
>>     MTLS on
>>     all Authentication Server endpoints, including the Authorization
>>     Endpoint. Does this make sense? Does it add security, or is the
>>     OAUTH2
>>     flow just as secure without MTLS on the Authorization Endpoint?
>>     Thanks,
>>     Amichai
>>     _______________________________________________
>>     OAuth mailing list
>> <>
>>     <>
>> _______________________________________________
>> OAuth mailing list
>> <>