Re: [OAUTH-WG] OAuth 2.1 - require PKCE?

Dominick Baier <dbaier@leastprivilege.com> Mon, 11 May 2020 07:20 UTC

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To: Neil Madden <neil.madden@forgerock.com>, Dick Hardt <dick.hardt@gmail.com>, Mike Jones <michael.jones=40microsoft.com@dmarc.ietf.org>
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Subject: Re: [OAUTH-WG] OAuth 2.1 - require PKCE?
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In IdentityServer, the PKCE requirement is per client.

We started with a default of false - and now switched to true.

FWIW
———
Dominick Baier

On 10. May 2020 at 22:22:35, Mike Jones (
michael.jones=40microsoft.com@dmarc.ietf.org) wrote:

Exactly!



I believe that this also the same point that Brian Campbell was making
earlier about interoperability implications.



                                                       -- Mike



*From:* Neil Madden <neil.madden@forgerock.com>
*Sent:* Sunday, May 10, 2020 1:06 PM
*To:* Dick Hardt <dick.hardt@gmail.com>
*Cc:* Mike Jones <Michael.Jones@microsoft.com>om>; oauth@ietf.org; Torsten
Lodderstedt <torsten@lodderstedt.net>
*Subject:* Re: [OAUTH-WG] Re: OAuth 2.1 - require PKCE?



But if an AS upgrades to OAuth 2.1 then it MUST reject authorization
requests that don’t include a code_challenge (section 4.1.2.1), so this
will only be possible when all clients support PKCE.



This makes it impossible for a 2.1-compliant AS to also support non-PKCE
2.0 clients (i.e., the vast majority of them).



I think we can have a 2.1 spec that says clients and servers MUST support
PKCE without this hard-fail clause. Otherwise I can’t see how we’d ever
ship with 2.1-compliance enabled out-of-the-box.



— Neil



On 10 May 2020, at 20:38, Dick Hardt <dick.hardt@gmail.com> wrote:



Hi Mike, I would consider upgrading to OAuth 2.1 to be voluntary, just as
the other extensions. Similarly, OAuth 1.0 deployments upgrading to OAuth
2.0 was voluntary.



Would you clarify why you think upgrading to OAuth 2.1 would be mandatory?





On Sun, May 10, 2020 at 12:02 PM Mike Jones <Michael.Jones=
40microsoft.com@dmarc.ietf.org> wrote:

I agree with actively maintaining and improving the OAuth 2.0 specs by
adding enhancements that are voluntary to use.  I’ve worked on many such
improvements, including Dynamic Client Registration, Authorization
Metadata, the Device Flow, Token Exchange, DPoP, and support PAR and RAR,
etc.  The issue that’s the subject is the current discussion is whether to
make use of another enhancement, PKCE, mandatory in cases where it’s
actually not needed, rather than making its use voluntary like the other
enhancements, which I certainly support.



                                                       -- Mike



*From:* Torsten Lodderstedt <torsten=40lodderstedt.net@dmarc.ietf.org>
*Sent:* Sunday, May 10, 2020 3:15 AM
*To:* Mike Jones <Michael.Jones@microsoft.com>
*Cc:* Daniel Fett <fett@danielfett.de>de>; oauth@ietf.org
*Subject:* [EXTERNAL] Re: [OAUTH-WG] OAuth 2.1 - require PKCE?



Hi Mike,



Mike Jones <Michael.Jones=40microsoft.com@dmarc.ietf.org> schrieb am Fr. 8.
Mai 2020 um 18:55:

OAuth 2.1 was supposed to not introduce breaking changes.

I cannot remember the WG met that decision. Can you please refer to the
respective thread?



Requiring exact redirect URI matching is already a breaking change. Do you
oppose against this as well?





If you want to do that, please do it in TxAuth instead.



Interesting statement. Does it mean you want to conserve OAuth 2.0 and
force any enhancements/improvements to go into TXAuth? This would cause
huge migration efforts for existing deployments wanting to benefit from
those enhancements.



I think existing deployments are better served by actively maintaining and
evolving the 2.x line. For example, PAR and RAR are attempts to improve
OAuth 2.x and make it usable for new use cases. That’s better protection of
existing investments than sending them of to TXAuth.

Kind regards,

Torsten.





                                                       -- Mike



*From:* OAuth <oauth-bounces@ietf.org> *On Behalf Of *Daniel Fett
*Sent:* Thursday, May 7, 2020 11:50 PM
*To:* oauth@ietf.org
*Subject:* Re: [OAUTH-WG] OAuth 2.1 - require PKCE?



+1 to all what Aaron said. Thanks for pointing this out!



We need to address this in the security BCP and this will be a normative
change that affects OpenID Connect Core (just as our current recommendation
on the usage of nonce).



We would then have:



- use PKCE, except if you use OIDC with a nonce, then you don't need PKCE,
except if you are a public client, then you still need PKCE.

- use state, except if you use PKCE, then you don't need state.



I think there are very good reasons to simplify this down to



- use PKCE

- you may or may not use state



First and foremost, not many people will understand why there are cases
when the BCP/OAuth 2.1 mandate PKCE and some where they don't. However,
understanding *why* you have to do something is key to compliance. The
short version "PKCE protects the code; there is a specific case where it is
not needed, but its better to use it all the time" is easy to understand.
We will not see many implementations following the long version above
correctly.



Second, we dramatically reduce technical complexity by reducing cases that
need to be handled. We reduce correctness and compliance testing complexity
in the same way. We reduce the cost of security analysis, which scales
really badly to more cases.



And finally, using nonce to protect against code injection is less robust
than PKCE. AS have a better track record than clients when it comes to
correctly implementing security mechanisms.



Yes, this will make a number of implementations non-spec-compliant, but I
do not think that this is a huge problem. Software needs to adapt all the
time and a software that has not been changed in a while is probably not
one you would want to use anyway. We are setting a new goal for
implementations to meet and eventually, maintained implementations will get
there.



-Daniel





Am 08.05.20 um 01:38 schrieb Aaron Parecki:

Backing up a step or two, there's another point here that I think has been
missed in these discussions.



PKCE solves two problems: stolen authorization codes for public clients,
and authorization code injection for all clients. We've only been talking
about authorization code injection on the list so far. The quoted section
of the security BCP (4.5.3) which says clients can do PKCE or use the
nonce, is only talking about preventing authorization code injection.



The nonce parameter solves authorization code injection if the client
requests an ID token. Public clients using the nonce parameter are still
susceptible to stolen authorization codes so they still need to do PKCE as
well.



The only case where OpenID Connect clients don't benefit from PKCE is if
they are also confidential clients. Public client OIDC clients still need
to do PKCE even if they check the nonce.



OpenID Connect servers working with confidential clients still benefit from
PKCE because they can then enforce the authorization code injection
protection server-side rather than cross their fingers that clients
implemented the nonce check properly.



I really don't think it's worth the amount of explanation this will take in
the future to write an exception into OAuth 2.1 or the Security BCP for
only some types of OpenID Connect clients when all clients would benefit
from PKCE anyway.



Aaron







On Wed, May 6, 2020 at 10:48 AM Dick Hardt <dick.hardt@gmail.com> wrote:

Hello!



We would like to have PKCE be a MUST in OAuth 2.1 code flows. This is best
practice for OAuth 2.0. It is not common in OpenID Connect servers as the
nonce solves some of the issues that PKCE protects against. We think that
most OpenID Connect implementations also support OAuth 2.0, and hence have
support for PKCE if following best practices.



The advantages or requiring PKCE are:



- a simpler programming model across all OAuth applications and profiles as
they all use PKCE



- reduced attack surface when using  S256 as a fingerprint of the verifier
is sent through the browser instead of the clear text value



- enforcement by AS not client - makes it easier to handle for client
developers and AS can ensure the check is conducted



What are disadvantages besides the potential impact to OpenID Connect
deployments? How significant is that impact?



Dick, Aaron, and Torsten



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