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

Aaron Parecki <aaron@parecki.com> Wed, 06 May 2020 19:24 UTC

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From: Aaron Parecki <aaron@parecki.com>
Date: Wed, 6 May 2020 12:24:07 -0700
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To: Mike Jones <Michael.Jones@microsoft.com>
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Subject: Re: [OAUTH-WG] [EXTERNAL] Re: OAuth 2.1 - require PKCE?
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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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