I gave a short presentation about OAuth 2.0 Token Exchange
<http://www.slideshare.net/briandavidcampbell/oauth-20-token-exchange-an-sts-for-the-rest-of-us>
at a recent identity conference, which seemed well received and a lot of
folks expressed their support for it and a desire to see support for it in
offerings out in the wild.
However, I did get some feedback from Vittorio Bertocci of Microsoft that
he felt that the language around issuing refresh tokens was overly
restrictive based on real world deployment experience they have with a
token exchange like interaction that they are currently offering. He
described it as follows:
"To summarize our main use case: we use [a token exchange like protocol]
for achieving user impersonation thru tiers, or delegation for confidential
clients which do not have any web UX.
Those cases do call for the ability of the middle tier to access the
resource also when the original credential is no longer valid (e.g. there
is no longer any user entertaining an active session with the middle tier).
Think of a hypothetical new feature of the Uber mobile app, which can
monitor your Exchange calendar and book a ride whenever a suitable slot
opens up within a certain time range. The Uber app would call an Uber-owned
middle tier, in form of Web API, and the Web API would itself be a client
of the Exchange API. Given that the Uber API doesn’t offer any browser
ready surface, as it is meant to be accessed only via oauth2 bearer token,
the [token exchange] is the only way through which it can obtain a token
for the Exchange API; and the scenario definitely call for offline access,
as the uber API should be able to monitor the Exchange calendar of the user
even if the user closed the app on his/her phone."
While the current text in the draft allows for this kind of thing, it
stigmatizes it somewhat with a "NOT RECOMMENDED" on sending an RT in the
response. So the proposal here is to make refresh tokens OPTIONAL and
change the accompanying text in sec 2.2.1 to the following:
refresh_token
OPTIONAL. A refresh token will typically not be issued when the
the exchange is of one temporary credential (the subject_token)
for a different temporary credential (the issued token) for use in
some other context. A refresh token can be issued in cases where
the client of the token exchange needs the ability to access a
resource even when the original credential is no longer valid
(e.g. user-not-present or offline scenarios where there is no
longer any user entertaining an active session with the client).
Profiles or deployments of this specification should clearly
document the conditions under which a client should expect a
refresh token in response to "urn:ietf:params:oauth:grant-
type:token-exchange" grant type requests.
I plan to make the aforementioned change and get a new draft published with
that and some other updates later this week before the Internet Draft
submission cut-off on Friday.
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