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

Justin Richer <> Tue, 25 May 2021 19:54 UTC

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From: Justin Richer <>
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Date: Tue, 25 May 2021 15:54:42 -0400
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Cc: "A. Rothman" <>, IETF oauth WG <>
To: Sascha Preibisch <>
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Subject: Re: [OAUTH-WG] Can a client send the Authorization Request?
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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
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