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

Aaron Parecki <aaron@parecki.com> Wed, 06 May 2020 20:43 UTC

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From: Aaron Parecki <aaron@parecki.com>
Date: Wed, 6 May 2020 13:42:56 -0700
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To: Mike Jones <Michael.Jones@microsoft.com>
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Subject: Re: [OAUTH-WG] OAuth 2.1 - require PKCE?
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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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