Subject: [OAUTH-WG] Review of draft-ietf-oauth-native-apps-05

From: Samuel Erdtman <[email-masked]>
Date: Tue, 1 Nov 2016 17:41:36 +0100

Hi,

Thanks for the great work of putting this together. I have a few comments
on the current draft. See below

Best Regards
Samuel Erdtman

5.  Using Inter-app URI Communication for OAuth
The end of this section is a bit confusing with first a MUST statement and
then a RECOMMENDED statement
“Native apps MUST use an external user-agent to perform OAuth”
and
“This best practice focuses on the browser as the RECOMMENDED external
   user-agent for native apps.”

7.1.1.  Custom URI Scheme Namespace Considerations
“For example, an app that controls the domain name "app.example.com"
   can use "com.example.app:/" as their custom scheme.”
drop the slash in the custom schema.

7.2.  App-claimed HTTPS URI Redirection
“When the browser encounters a claimed URL, instead of the
   page being loaded in the browser, the native app is launched instead
   with the URL supplied as a launch parameter.”
drop one “instead” changing it to:
“When the browser encounters a claimed URL, instead of the
   page being loaded in the browser, the native app is launched
   with the URL supplied as a launch parameter.”

7.2.  App-claimed HTTPS URI Redirection
If this is the recommended way to do it when possible maybe it should be
first?

8.1.  Embedded User-Agents
“Embedded user-agents are an alternative method for authorization
   native apps.”
change to
“Embedded user-agents are an alternative method to authorize
   native apps.”
or
“Embedded user-agents are an alternative method for authorization
   of native apps.”

8.  Security Considerations
I see normative language here (MUST, RECOMMENDED, SHOULD, etc.), and it
felt a bit odd to me to have that under Security Considerations. Not sure
if there are guidelines around that, but to me it would make sense to keep
the normative parts out of Security Considerations. And it says
“Considerations”, to me that sounds mor like things to think about then
this is how it works.

8.2.  Protecting the Authorization Code

“A limitation of using custom URI schemes for redirect URIs is that
   multiple apps can typically register the same scheme, which makes it
   indeterminate as to which app will receive the Authorization Code.
   PKCE [RFC7636] details how this limitation can be used to execute a
   code interception attack (see Figure 1).  Loopback IP based redirect
   URIs may be susceptible to interception by other apps listening on
   the same loopback interface.”

I think it would be preferable to separate custom URI and Loopback IP based
redirect.

8.2.  Protecting the Authorization Code
“Loopback IP based redirect
   URIs may be susceptible to interception by other apps listening on
   the same loopback interface.”
Are you referring to an application listening to loopback traffic or an
application killing the original application and start listening on the
same port. The second alternative would be relatively intrusive and notable.

Appendix A.  Server Support Checklist
1.  Support custom URI-scheme redirect URIs.
I could not see that this was required in section Section 7.1.

Appendix A.  Server Support Checklist
5.  Support PKCE.
in Section 8.2 it says MUST
“Authorization servers MUST support PKCE [RFC7636]
   for public native app clients.”

---

From: William Denniss <[email-masked]>
Date: Tue, 1 Nov 2016 11:01:57 -0700

Hi Samuel,

Thanks for your review! I've replied inline:

>    user-agent for native apps.”
The browser is one external user-agent. Using an external agent is a MUST
to comply with this BCP, and the browser is the RECOMMENDED user agent.

> drop the slash in the custom schema.

Done.

>    with the URL supplied as a launch parameter.”
Good catch, thanks.

> If this is the recommended way to do it when possible maybe it should be

It's ideal in a security sense, but less broadly supported currently than
custom URI schemes. The order is roughly based on popularity.

> “Embedded user-agents are an alternative method for authorization

Fixed in 06.

> “Considerations”, to me that sounds mor like things to think about then

I actually thought it was common, but I might be wrong. I'll wait and see
if others weigh in on this too.

> I think it would be preferable to separate custom URI and Loopback IP
> based redirect.
>

Can add a paragraph break.

> same port. The second alternative would be relatively intrusive and notable.

The former. The assumption is that other desktop apps may be able to
observe all local HTTP traffic on the loopback interface.

> I could not see that this was required in section Section 7.1.

It's there in section 7:
"To fully support this best practice, authorization servers MUST support
        the following three redirect URI options. Native apps MAY use
        whichever redirect option suits their needs best, taking into
account
        platform specific implementation details."

>    for public native app clients.”

"Recommended" is used in normal sentence case here, but I can see how that
might be confusing, perhaps I should delete that word. Will think about
this section.

Incidentally, the idea for Appendix A was a non-normative checklist of
items for authorization servers to collect the core requirements of the
BCP, and give developers a quick way to evaluate authorization server
compliance.

06 changes staged:
https://github.com/WilliamDenniss/oauth-native-apps/pull/3

---

From: Samuel Erdtman <[email-masked]>
Date: Wed, 2 Nov 2016 07:15:03 +0100

see inline

Hannes could you have a look at the comment on Security Considerations.

On Tue, Nov 1, 2016 at 7:01 PM, William Denniss <[email-masked]> wrote:

>> The end of this section is a bit confusing with first a MUST statement
>> and then a RECOMMENDED statement

My comment is not that something is formally wrong, just that I had to read
it twice before I got it. I´m fine with keeping as is, just wanted to
hilight the potential confusion.

> custom URI schemes. The order is roughly based on popularity.

makes sense

> if others weigh in on this too.

Sounds like a plan.

> Can add a paragraph break.

Thanks

> observe all local HTTP traffic on the loopback interface.

Ok

>         platform specific implementation details."

My bad, I missed it.

> BCP, and give developers a quick way to evaluate authorization server

I like the idea of the checklist.

>
> 06 changes staged: https://github.com/WilliamDenniss/oauth-native-
> apps/pull/3
>
>