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

[Aaron Parecki <[e-mail]>]

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 <[e-mail]> 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
>
> ᐧ
>