Re: [OAUTH-WG] [EXTERNAL] Re: Authorization code reuse and OAuth 2.1

[Warren Parad <[e-mail]>]

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=
[e-mail]> wrote:

> Any weakening of the requirement should include a clear outline of the
> risks to help implementors make informed decisions.
>
>
>
> *From:* OAuth <[e-mail]> *On Behalf Of *Ash Narayanan
> *Sent:* Friday 15 October 2021 01:51
> *To:* Aaron Parecki <[e-mail]>
> *Cc:* IETF oauth WG <[e-mail]>
> *Subject:* Re: [OAUTH-WG] [EXTERNAL] Re: Authorization code reuse and
> OAuth 2.1
>
>
>
> You don't often get email from [e-mail]. 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 <[e-mail]> 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 <
> [e-mail]> 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 <[e-mail]> *On Behalf Of *Sascha Preibisch
> *Sent:* Wednesday 13 October 2021 22:06
> *To:* Aaron Parecki <[e-mail]>
> *Cc:* IETF oauth WG <[e-mail]>
> *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 <[e-mail]> 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 <
> [e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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 <[e-mail]> 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 <
> [e-mail]> 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 <[e-mail]> *On Behalf Of *Aaron Parecki
> *Sent:* Wednesday 13 October 2021 18:40
> *To:* Warren Parad <wparad=[e-mail]>
> *Cc:* Mike Jones <Michael.Jones=[e-mail]>;
> [e-mail]
> *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=
> [e-mail]> 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=
> [e-mail]> 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
>
>
>
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