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

Brian Campbell <bcampbell@pingidentity.com> Wed, 06 May 2020 21:03 UTC

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From: Brian Campbell <bcampbell@pingidentity.com>
Date: Wed, 6 May 2020 15:02:57 -0600
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To: Aaron Parecki <aaron@parecki.com>
Cc: Mike Jones <Michael.Jones@microsoft.com>, "oauth@ietf.org" <oauth@ietf.org>
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Subject: Re: [OAUTH-WG] OAuth 2.1 - require PKCE?
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I think the point is that the Security BCP in
https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-2.1.1
requires that the authz request has either the PKCE "code_challenge" or the
OIDC "nonce". Whereas 2.1 in
https://tools.ietf.org/html/draft-parecki-oauth-v2-1-01#section-4.1.1.3
flat out requires PKCE "code_challenge" in the authz request. They are not
equivalent and have very different ramifications on interoperability etc..


On Wed, May 6, 2020 at 2:43 PM Aaron Parecki <aaron@parecki.com> wrote:

> Going back to this point about server vs client requirements, since both
> the Security BCP and OAuth 2.1 currently say that ASs MUST support PKCE,
> isn't that already imposing additional requirements on OpenID Connect
> providers that don't currently exist in OpenID Connect alone?
>
> OPs that want to be compliant with the Security BCP will need to add PKCE
> support if they don't already have it (many of them already support it so
> for many of them this will not be any change), so it seems like a very
> small leap to also require clients implement PKCE as well.
>
> On Wed, May 6, 2020 at 12:31 PM Mike Jones <Michael.Jones@microsoft.com>
> wrote:
>
>> I realize what it says about servers.  My point is that OAuth 2.1’s
>> requirements on *clients* should match those in the security BCP and not
>> try to go beyond them.
>>
>>
>>
>>                                                        -- Mike
>>
>>
>>
>> *From:* Aaron Parecki <aaron@parecki.com>
>> *Sent:* Wednesday, May 6, 2020 12:24 PM
>> *To:* Mike Jones <Michael.Jones@microsoft.com>
>> *Cc:* Dick Hardt <dick.hardt@gmail.com>om>; oauth@ietf.org
>> *Subject:* Re: [OAUTH-WG] OAuth 2.1 - require PKCE?
>>
>>
>>
>> Yes, the BCP says *clients* may use either PKCE or nonce to prevent
>> authorization code injection. Shortly after that quoted segment is the
>> below:
>>
>>
>>
>> > Authorization servers MUST support PKCE [RFC7636].
>>
>>
>>
>> On Wed, May 6, 2020 at 12:22 PM Mike Jones <Michael.Jones@microsoft.com>
>> wrote:
>>
>> Aaron, the section you cited at
>> https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-2.1.1
>> makes it clear that clients can support EITHER PKCE or the OpenID Connect
>> nonce.   The text is:
>>
>>
>>
>>    Clients MUST prevent injection (replay) of authorization codes into
>>
>>    the authorization response by attackers.  The use of PKCE [RFC7636
>> <https://tools.ietf.org/html/rfc7636>]
>>
>>    is RECOMMENDED to this end.  The OpenID Connect "nonce" parameter and
>>
>>    ID Token Claim [OpenID
>> <https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#ref-OpenID>]
>> MAY be used as well.  The PKCE challenge or
>>
>>    OpenID Connect "nonce" MUST be transaction-specific and securely
>>
>>    bound to the client and the user agent in which the transaction was
>>
>>    started.
>>
>>
>>
>> We should not attempt to change that in OAuth 2.1, as doing so would
>> needlessly break already working and secure clients.
>>
>>
>>
>>                                                        -- Mike
>>
>>
>>
>> *From:* Aaron Parecki <aaron@parecki.com>
>> *Sent:* Wednesday, May 6, 2020 11:56 AM
>> *To:* Mike Jones <Michael.Jones@microsoft.com>
>> *Cc:* Dick Hardt <dick.hardt@gmail.com>om>; oauth@ietf.org
>> *Subject:* [EXTERNAL] Re: [OAUTH-WG] OAuth 2.1 - require PKCE?
>>
>>
>>
>> > In particular, authorization servers shouldn’t be required to support
>> PKCE when they already support the OpenID Connect nonce.
>>
>>
>>
>> The Security BCP already requires that ASs support PKCE:
>> https://tools.ietf.org/html/draft-ietf-oauth-security-topics-15#section-2.1.1 Are
>> you suggesting that the Security BCP change that requirement as well? If
>> so, that's a discussion that needs to be had ASAP. If not, then that's an
>> implicit statement that it's okay for OpenID Connect implementations to not
>> be best-practice OAuth implementations. And if that's the case, then I also
>> think it's acceptable that they are not complete OAuth 2.1 implementations
>> either.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> On Wed, May 6, 2020 at 11:21 AM Mike Jones <Michael.Jones=
>> 40microsoft.com@dmarc.ietf.org> wrote:
>>
>> The disadvantage of requiring PKCE for OpenID Connect implementations is
>> that you’re trying to add a normative requirement that’s not required of
>> OpenID Connect deployments today, which would bifurcate the ecosystem.
>> There are hundreds of implementations (including the 141 certified ones at
>> https://openid.net/certification/), none of which have ever been
>> required to support PKCE.  Therefore, most don’t.
>>
>>
>>
>> Per feedback already provided, I believe that OAuth 2.1 should align with
>> the guidance already in the draft Security BCP, requiring EITHER the use of
>> PKCE or the OpenID Connect nonce.  Trying to retroactively impose
>> unnecessary requirements on existing deployments is unlikely to succeed and
>> will significantly reduce the relevance of the OAuth 2.1 effort.
>>
>>
>>
>> In particular, authorization servers shouldn’t be required to support
>> PKCE when they already support the OpenID Connect nonce.  And clients
>> shouldn’t reject responses from servers that don’t support PKCE when they
>> do contain the OpenID Connect nonce.  Doing so would unnecessarily break
>> things and create confusion in the marketplace.
>>
>>
>>
>>                                                           -- Mike
>>
>>
>>
>> *From:* OAuth <oauth-bounces@ietf.org> *On Behalf Of *Dick Hardt
>> *Sent:* Wednesday, May 6, 2020 10:48 AM
>> *To:* oauth@ietf.org
>> *Subject:* [OAUTH-WG] OAuth 2.1 - require PKCE?
>>
>>
>>
>> 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
>>
>>
>>
>> ᐧ
>>
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