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

Sascha Preibisch <> Wed, 26 May 2021 05:39 UTC

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From: Sascha Preibisch <>
Date: Tue, 25 May 2021 22:38:50 -0700
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To: "A. Rothman" <>
Cc: IETF oauth WG <>
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Subject: Re: [OAUTH-WG] Can a client send the Authorization Request?
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Yes, I understand. I just wanted to clarify how I came up with ssying POST
is an option.

On Tue., May 25, 2021, 22:17 A. Rothman, <> wrote:

> Oh, I see what you mean... however as Justin clarified, the discussion
> here is not about GET vs POST, but rather about user agent vs client making
> the request. The former distinction doesn't really matter in this case,
> whereas in the latter distinction the client option seems to be breaking
> the spec (only the user agent should send it).
> Amichai
> On 5/26/21 4:15 AM, Sascha Preibisch wrote:
> Amichai,
> I know POST won't be seen often, but the /authorize endpoint may still
> accept POST as described here:
> I hope this helps,
> Sascha
> On Tue., May 25, 2021, 13:00 A. Rothman, <> wrote:
>> Hi Sacha,
>> Thanks for the links and video!
>> However I don't think this is what they're doing. There's no par
>> endpoint, no JSON response (just a redirect with a Location header, that
>> instead of following, the client is supposed to pass through to the user
>> agent), etc. It seems more like a regular OAUTH2 flow, just with the
>> initial request coming out of the client instead of the user agent, without
>> any of the specifics of the par mentioned in the video.
>> btw, where does RFC 6749 say the authorization request can be sent by the
>> client? In the quote I made below from 4.1, as well as e.g. 4.2.1, it seems
>> pretty explicit that it's the user agent that makes the actual HTTP request
>> (Authorization Request with all its params etc), after being redirected to
>> it from the client, no? I don't see much wiggle room there to allow for the
>> client to be sending it itself...
>> Amichai
>> On 5/25/21 6:28 PM, 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