Re: [secdir] Security review of draft-ietf-oauth-token-exchange-14

Brian Campbell <bcampbell@pingidentity.com> Mon, 13 August 2018 17:37 UTC

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From: Brian Campbell <bcampbell@pingidentity.com>
Date: Mon, 13 Aug 2018 11:36:49 -0600
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To: Benjamin Kaduk <kaduk@mit.edu>
Cc: hilarie@purplestreak.com, The IESG <iesg@ietf.org>, secdir@ietf.org, draft-ietf-oauth-token-exchange.all@ietf.org
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Subject: Re: [secdir] Security review of draft-ietf-oauth-token-exchange-14
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On Fri, Aug 10, 2018 at 3:26 PM Benjamin Kaduk <kaduk@mit.edu>; wrote:

> ...
> With the disclaimer that I haven't read token-exchange recently and am
> mostly going off my memory of the mailing list discussions, it seems like
> the key consideration here is that token exchange allows you to convert a
> token that may be opaque to you into one that has inspectable fields, and
> those fields can be privacy sensitive.  So a JWT response should only be
> given to a requestor that trusted to know such information; a lot of the
> time, that has large overlap with being trusted to have the token in the
> first place, but potentially there is some amount of non-overlap.
>
> The considerations from 7523 of course also apply, and sensitive
> information should not be sent over unencrypted channels, but isn't the
> above paragraph an aspect inherently new to token exchange?


I don't think it's unique to token exchange. RFC 7523, for example, defines
the use of a JWT as an authorization grant used to obtain an access token
from a authorization server. Unless encrypted as a JWE, the JWT in that
scenario has fields that are inspectable to the client. And an OAuth access
token is considered opaque to the client but whether or not information in
the token is actually accessible to a client is an implementation detail of
the authorization server. The latter is true for OAuth in general and not
just these more specialized exchanges.

The idea behind maybe incorporating some text similar to (but adapted to
the context) what's in the privacy considerations of RFC 7523
<https://tools.ietf.org/html/rfc7523#section-7> is that the considerations
are pretty much the same - use TLS in transit, minimize the amount of
information in the token, encrypt the token if disclosure of its
information to the client is a concern.

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