I wouldn't be against lowering it to MAY but only if we stipulate a SHOULD on an expected lifetime of an authorization code. I think sending the message that these should be one time use except in exceptional circumstances.
Warren Parad Founder, CTO Secure your user data with IAM authorization as a service. Implement Authress <https://authress.io/>. On Fri, Oct 15, 2021 at 10:17 AM Pieter Kasselman <pieter.kasselman= 40microsoft....@dmarc.ietf.org> wrote: > Any weakening of the requirement should include a clear outline of the > risks to help implementors make informed decisions. > > > > *From:* OAuth <oauth-boun...@ietf.org> *On Behalf Of *Ash Narayanan > *Sent:* Friday 15 October 2021 01:51 > *To:* Aaron Parecki <aa...@parecki.com> > *Cc:* IETF oauth WG <oauth@ietf.org> > *Subject:* Re: [OAUTH-WG] [EXTERNAL] Re: Authorization code reuse and > OAuth 2.1 > > > > You don't often get email from ashvinnaraya...@gmail.com. Learn why this > is important <http://aka.ms/LearnAboutSenderIdentification> > > Yes, as I said before, authorization servers are free to enforce one-time > use of the authorization code even if there isn't a requirement to. The > proposal is just to remove the *requirement* of authorization servers > enforcing it. > > > > I agree, and therefore I think what it really ought to be is "MAY". > > > > Annabelle said: > > There are legitimate use cases for a client to replay an authorization > code. Connection failures happen. Servers fall over before completing > requests. Users hit browser refresh buttons. Permitting replay of > authorization codes (assuming valid PKCE, client creds, etc.) allows > clients to handle these failure modes simply and gracefully via retries. > > > > Couldn't agree more. Having experienced these exact use-cases, I can > honestly say that denying users a smooth experience just to be compliant > with the spec, which offers no additional security if PKCE is also being > used, makes no sense. > > It is also more effort (from a repository layer perspective) to implement > one-time use than do PKCE verification. > > > > What is the practical reason for allowing "plain" PKCE in OAuth 2.1? Are > there really use cases out there where SHA-256 is a deal breaker? > > > > I'd be interested in these use-cases as well (I can't think of any). > > > > On Thu, Oct 14, 2021 at 8:36 AM Aaron Parecki <aa...@parecki.com> wrote: > > Yes, as I said before, authorization servers are free to enforce one-time > use of the authorization code even if there isn't a requirement to. The > proposal is just to remove the *requirement* of authorization servers > enforcing it. > > > > I am okay with Mike's suggestion of changing the language to "SHOULD" to > continue to point out the possibility of enforcing one-time authorization > codes if desired. > > > > > > > > On Wed, Oct 13, 2021 at 2:15 PM Pieter Kasselman < > pieter.kassel...@microsoft.com> wrote: > > Log files can exist in lots of place (clients, servers, data lakes). The > question is whether it is a valid assumption that an attacker cannot obtain > an Authorization Code and a Code Verifier and present it a second time > round. Limiting the validity period is one layer of defence, PKCE is > another layer, one time use enforcement is another. Assuming breach and > designing from a defence in depth perspective is a good practice, so why > not give implementors options (and guidance) to add additional layers of > defence to match their risk profiles? > > > > > > *From:* OAuth <oauth-boun...@ietf.org> *On Behalf Of *Sascha Preibisch > *Sent:* Wednesday 13 October 2021 22:06 > *To:* Aaron Parecki <aa...@parecki.com> > *Cc:* IETF oauth WG <oauth@ietf.org> > *Subject:* Re: [OAUTH-WG] [EXTERNAL] Re: Authorization code reuse and > OAuth 2.1 > > > > Ok, if the goal is to avoid unnecessary requirements I am suggesting to > point out why MUST was changed to SHOULD. Otherwise developers will start > to mix and match OAuth 2.0 and OAuth 2.1 requirements as they see them fit > their needs. > > In regards to encrypted values in PKCE, Aaron, I can also not confirm that > as the general implementation. > > > > On Wed, 13 Oct 2021 at 13:56, Aaron Parecki <aa...@parecki.com> wrote: > > The PKCE spec actually says "Typically, the "code_challenge" and > "code_challenge_method" values are stored in encrypted form in the "code" > itself" which I feel like might be a stretch to say that's typical, but > this scenario was clearly thought of ahead of time. Doing that would enable > an AS to avoid storing server-side state. > > > > On Wed, Oct 13, 2021 at 1:50 PM Sascha Preibisch < > saschapreibi...@gmail.com> wrote: > > If the challenge is based on distributed authorization server > configurations, how would they handle PKCE? I imagine that managing the > state for PKCE is not less challenging than managing authorization codes on > the server side, preventing reuse of them. > > With that in mind I am not sure if I follow the given argument. I would > prefer to keep MUST as it is today. > > > > > > On Wed, 13 Oct 2021 at 13:37, Aaron Parecki <aa...@parecki.com> wrote: > > HTTPS, because if that's broken then the rest of OAuth falls apart too. > > > > On Wed, Oct 13, 2021 at 1:36 PM Warren Parad <wpa...@rhosys.ch> wrote: > > I feel like I'm missing something, what stops just plain old network > sniffing and replying the whole encrypted payload to the AS and getting > back a valid token? > > > [image: Image removed by sender.] > > *Warren Parad* > > Founder, CTO > > Secure your user data with IAM authorization as a service. Implement > Authress > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fauthress.io%2F&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871378875%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Q%2BVgxFWvU1T07%2FNrIriU3GsN5qRZjlnTWHyOYoaepbU%3D&reserved=0> > . > > > > > > On Wed, Oct 13, 2021 at 10:33 PM Aaron Parecki <aa...@parecki.com> wrote: > > Aside from the "plain" method, the PKCE code verifier never leaves the > client until it's sent along with the authorization code in the POST > request to the token endpoint. The only place it can leak at that point is > if the authorization server itself leaks it. If you have things leaking > from the authorization server log, you likely have much bigger problems > than authorization code replays. > > > > Keep in mind that even with the proposed change to drop the requirement of > authorization codes being one time use, authorization servers are free to > enforce this still if they want. Authorization code lifetimes are still > expected to be short lived as well. > > > > Aaron > > > > > > On Wed, Oct 13, 2021 at 1:25 PM Pieter Kasselman < > pieter.kassel...@microsoft.com> wrote: > > Aaron, I was curious what prevents an attacker from presenting an > Authorization Code and a PKCE Code Verifier for a second time if the one > time use requirement is removed. Is there another countermeasure in PKCE > that would prevent it? For example, an attacker may obtain the > Authorization Code and the Code Verifier from a log and replay it. > > > > Cheers > > > > Pieter > > > > *From:* OAuth <oauth-boun...@ietf.org> *On Behalf Of *Aaron Parecki > *Sent:* Wednesday 13 October 2021 18:40 > *To:* Warren Parad <wparad=40rhosys...@dmarc.ietf.org> > *Cc:* Mike Jones <Michael.Jones=40microsoft....@dmarc.ietf.org>; > oauth@ietf.org > *Subject:* [EXTERNAL] Re: [OAUTH-WG] Authorization code reuse and OAuth > 2.1 > > > > Warren, I didn't see you on the interim call, so you might be missing some > context. > > > > The issue that was discussed is that using PKCE already provides all the > security benefit that is gained by enforcing single-use authorization > codes. Therefore, requiring that they are single-use isn't necessary as it > doesn't provide any additional benefit. > > > > If anyone can think of a possible attack by allowing authorization codes > to be reused *even with a valid PKCE code verifier* then that would warrant > keeping this requirement. > > > > --- > > Aaron Parecki > > > > > > On Wed, Oct 13, 2021 at 10:27 AM Warren Parad <wparad= > 40rhosys...@dmarc.ietf.org> wrote: > > Isn't it better for it to be worded as we want it to be, with the > implication being that of course it might be difficult to do that, but that > AS devs will think long and hard about sometimes not denying the request? > Even with MUST, some AS will still allow reuse of auth codes. Isn't that > better than flat out saying: *sure, there's a valid reason* > > > > In other words, how do we think about RFCs? Do they exist to be followed > to the letter or not at all? Or do they exist to stipulate this is the way, > but acknowledge that not everyone will build a solution that holds them as > law. > > > > Let's look at *SHOULD* > > This word, or the adjective "RECOMMENDED", mean that there may exist valid > reasons in particular circumstances to ignore a particular item, but the > full implications must be understood and carefully weighed before choosing > a different course. > > > > I think *recommended* here is not sufficient nor are there valid reasons. > "It's too hard" isn't really a valid reason. Isn't it better in this case > for an AS to not be compliant with the RFC, than it is to relax this to > SHOULD and have lots of AS thinking reusing auth codes is a viable > solution, "because they are a special snowflake where SHOULD should apply". > > > > Are we setting the standard or instead attempting to sustain a number of > "AS that are in compliance with the RFC"? > > > > [image: Image removed by sender.] > > *Warren Parad* > > Founder, CTO > > Secure your user data with IAM authorization as a service. Implement > Authress > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fauthress.io%2F&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871388836%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=yJZ3n6X6%2BeVBK5KrHCNWREEX6NYMl3lqQxhW%2FjwguaQ%3D&reserved=0> > . > > > > > > On Wed, Oct 13, 2021 at 7:17 PM Mike Jones <Michael.Jones= > 40microsoft....@dmarc.ietf.org> wrote: > > During today’s call, it was asked whether we should drop the OAuth 2.0 > language that: > > The client MUST NOT use the authorization code > > more than once. If an authorization code is used more than > > once, the authorization server MUST deny the request and SHOULD > > revoke (when possible) all tokens previously issued based on > > that authorization code.” > > > > The rationale given was that enforcing one-time use is impractical in > distributed authorization server deployments. > > > > Thinking about this some more, at most, we should relax this to: > > The client MUST NOT use the authorization code > > more than once. If an authorization code is used more than > > once, the authorization server SHOULD deny the request and SHOULD > > revoke (when possible) all tokens previously issued based on > > that authorization code.” > > > > In short, it should remain illegal for the client to try to reuse the > authorization code. We can relax the MUST to SHOULD in the server > requirements in recognition of the difficulty of enforcing the MUST. > > > > Code reuse is part of some attack scenarios. We must not sanction it. > > > > -- Mike > > > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Fmailman%2Flistinfo%2Foauth&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871388836%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=ikRSMsY7goK%2FXMfhd43ClqNoQmhfAsq754MsS2uOFAg%3D&reserved=0> > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Fmailman%2Flistinfo%2Foauth&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871398790%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=OY9y15o%2FBx7pcLWwrzcdvKvda7rW5mYcMAJAIVD2EzM%3D&reserved=0> > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Fmailman%2Flistinfo%2Foauth&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871398790%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=OY9y15o%2FBx7pcLWwrzcdvKvda7rW5mYcMAJAIVD2EzM%3D&reserved=0> > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth > <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.ietf.org%2Fmailman%2Flistinfo%2Foauth&data=04%7C01%7Cpieter.kasselman%40microsoft.com%7C36e879a0ca5e4defdda108d98f75e61d%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637698558871398790%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=OY9y15o%2FBx7pcLWwrzcdvKvda7rW5mYcMAJAIVD2EzM%3D&reserved=0> > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth >
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