Re: [OAUTH-WG] Benjamin Kaduk's No Objection on draft-ietf-oauth-resource-indicators-05: (with COMMENT)

[Benjamin Kaduk <[e-mail]>]

On Wed, Sep 04, 2019 at 06:17:32PM -0600, Brian Campbell wrote:
> On Wed, Sep 4, 2019 at 5:55 PM Benjamin Kaduk <[e-mail]> wrote:
> 
> > On Wed, Sep 04, 2019 at 05:19:27PM -0600, Brian Campbell wrote:
> > > Thanks Ben, for the review and non-objectional ballot.
> > >
> > > On Wed, Sep 4, 2019 at 3:13 PM Benjamin Kaduk via Datatracker <
> > > [e-mail]> wrote:
> > >
> > > > Benjamin Kaduk has entered the following ballot position for
> > > > draft-ietf-oauth-resource-indicators-05: No Objection
> > > >
> > > > Section 3
> > > >
> > > >    An access token that is audience restricted to a protected resource
> > > >    that obtains that token legitimately cannot be used to access
> > > >    resources on behalf of the resource owner at other protected
> > > >    resources.  The "resource" parameter enables a client to indicate
> > the
> > > >
> > > > nit: This sentence has a pretty strange construction.  I think the
> > > > intent is to say that that a token, legitimately presented to a
> > > > resource, cannot then be taken by that resource server and
> > > > illegitimately present it somewhere else for access to other resources.
> > > > But with the current wording we seem to be missing part of the part
> > > > where some entity obtains the token with intent for illegitimate
> > access.
> > > >
> > >
> > > Yes, despite the pretty strange construction, you have the correct
> > intent.
> > > I'll work on improving that sentence (borrowing heavily from your words
> > > there).
> > >
> > >
> > >
> > > >    Some servers may host user content or be multi-tenant.  In order to
> > > >    avoid attacks that might confuse a client into sending an access
> > > >    token to a resource that is user controlled or is owned by a
> > > >    different tenant, it is important to use a specific resource URI
> > > >    including a path component.  This will cause any access token issued
> > > >    for accessing the user controlled resource to have an invalid
> > > >    audience if replayed against the legitimate resource API.
> > > >
> > > > I'm not entirely sure what this is trying to say.  What is the
> > > > "legitimate resource API"?  Why would a token be issued for accessing a
> > > > user-controlled resource if that's something we're trying to avoid
> > > > having confused clients access?
> > > >
> > >
> > > Um... so on rereading that I might say that it's also "pretty strange".
> > >
> > > What it was trying to somehow say is similar to the previous nit about
> > > audience-restricted not being usable at the resource for whom the weren't
> > > intended. But saying so in the context of a multi-tenant environment.
> > > Basically it's trying to say that, in a multi-tenant environment, the
> > > resource URI and subsequent token audience need to have something that
> > > identifies the tenant so as to prevent the token from being used by one
> > > tenant to illegitimately access resources at a different tenant. I'll
> > work
> > > on trying to improve that text to better explain all that.
> >
> > Ah, yes, that's a very good point to make.  I'm happy to look at some draft
> > text if you want.
> >
> 
> Thanks, here's what I've got now for this and the previous item in sec 3.
> Suggestions welcome.
> 
> 3.  Security Considerations
> 
>    An audience-restricted access token, legitimately presented to a
>    resource, cannot then be taken by that resource to present it
>    elsewhere for illegitimate access to other resources.  The "resource"

I think "by that resource and presented elsewhere" probbaly has a more
parallel flow.

>    parameter enables a client to indicate the protected resources where
>    the requested access token will be used, which in turn enables the
>    authorization server to apply the appropriate audience restrictions
>    to the token.
> 
>    Some servers may host user content or be multi-tenant.  In order to
>    avoid attacks where one tenant uses an access token to illegitimately
>    access resources owned by a different tenant, it is important to use
>    a specific resource URI including any portion of the URI that
>    identifies the tenant, such as a path component.  This will allow
>    access tokens to be audience-restricted in a way that identifies the
>    tenant and prevent their use, due to an invalid audience, at
>    resources owned by a different tenant.

But other than the above nit, this all looks really good; thank you!

-Ben