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. https://www.ietf.org/archive/id/draft-ietf-gnap-core-protocol-05.html#name-redirect-to-an-arbitrary-ur <https://www.ietf.org/archive/id/draft-ietf-gnap-core-protocol-05.html#name-redirect-to-an-arbitrary-ur> 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 <saschapreibi...@gmail.com> > 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): > https://datatracker.ietf.org/doc/html/draft-ietf-oauth-par > <https://datatracker.ietf.org/doc/html/draft-ietf-oauth-par>. I have also > made a video about this flow, maybe it matches what you are seeing on your > web server: https://www.youtube.com/watch?v=fE11HJRCL-k > <https://www.youtube.com/watch?v=fE11HJRCL-k> > > - 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 <amich...@amichais.net > <mailto:amich...@amichais.net>> 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@ietf.org <mailto:OAuth@ietf.org> > https://www.ietf.org/mailman/listinfo/oauth > <https://www.ietf.org/mailman/listinfo/oauth> > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth
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