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

Sascha Preibisch <> Tue, 25 May 2021 15:29 UTC

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From: Sascha Preibisch <>
Date: Tue, 25 May 2021 08:28:56 -0700
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To: "A. Rothman" <>
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Subject: Re: [OAUTH-WG] Can a client send the Authorization Request?
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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

I hope this helps,

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
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