I disagree that validating endpoints "at this step" (which refers to
right before making the token request) should be the default way of
handling this. The vast majority of OAuth client deployments connecting
to more than one AS will have a static configuration of the ASes
issuer/endpoint information anyhow and they just need to confirm that
the issuer value received in the authorization response is the same
issuer as that the request was sent to.
Hans.
On 1/11/16 1:14 PM, Mike Jones wrote:
Correct
*From:*George Fletcher [mailto:gffle...@aol.com]
*Sent:* Monday, January 11, 2016 12:13 PM
*To:* Mike Jones <michael.jo...@microsoft.com>; oauth@ietf.org
*Subject:* Re: [OAUTH-WG] OAuth 2.0 Mix-Up Mitigation
So just to make sure I understand...
This specification requires the response from the Authorization Server
to an successful /authorize call to pass back code=, state= and jwt= (?)
or individually iss= and aud= as URL parameters on the 302 to the
redirect_url. This way, before the client issues a call to the /token
endpoint (with the code), it can verify that the token endpoint is correct.
If the client doesn't validate the endpoints at this step, then it could
disclose it's secret to a malicious endpoint. Correct?
Thanks,
George
On 1/11/16 2:59 PM, Mike Jones wrote:
The alternatives for the code flow are to return them either in a
new JWT added to the reply containing them in the “iss” and “aud”
claims or to return them in new individual “client_id” and “iss”
authorization response parameters. Both alternatives are described
in the draft. I’m sure that we’ll now be having a good engineering
discussion on the tradeoffs between the alternatives.,
In a separate draft, John Bradley will shortly also be describing
the possibility of securing the “state” value using a “state_hash”
value that works in a way that’s similar to how “at_hash” and
“c_hash” secure the “access_token” and “code” values in Connect.
This would be an alternative means of binding the authorization
request and response to the session – accomplishing the same thing
that the Connect “nonce” does.
While I fully get that some OAuth implementations want to avoid
having to have crypto, it seems like at least support for
cryptographic hashing (SHA-256, etc.) may be necessary to mitigate
some of these attacks (at least for clients that use more than one
authorization server).
The other important engineering discussion I know we’re going to
have is whether, when an OAuth profile already returns the
information needed for the mitigation, whether we want to specify
that the client obtain it from the existing location, or whether to
also return it in a duplicate location. I’ll note that OpenID
Connect already returns the client ID and issuer for the flows that
return an ID Token in the authorization response, so this isn’t a
hypothetical question.
Finally, I know that we’ll need to discuss the impact of
cut-and-paste attacks when the issuer and client ID are returned as
individual authorization response parameters that are not
cryptographically bound to the rest of the response. The
cut-and-paste attack that returning the issuer and client_id values
as separate parameters enables, even when state_hash or nonce is
used, is for the attacker to capture the legitimate response
containing “iss” and “client_id” results and substitute different
values for these fields, then post that altered response to the
legitimate client. The state and/or nonce values are protected
against substitution but “iss” and “client_id” are not.
And yes, I absolutely agree that good examples are essential.
That’s high on my list for the -01 version.
Thanks a
bunch,
-- Mike
*From:*George Fletcher [mailto:gffle...@aol.com]
*Sent:* Monday, January 11, 2016 10:21 AM
*To:* Mike Jones <michael.jo...@microsoft.com>
<mailto:michael.jo...@microsoft.com>; oauth@ietf.org
<mailto:oauth@ietf.org>
*Subject:* Re: [OAUTH-WG] OAuth 2.0 Mix-Up Mitigation
Thanks Mike. One question after reading about the different attack
vectors and this spec...
How are the 'iss' and 'aud' values returned with the 'code' and
'state' parameters. It seems the client needs to verify the
endpoints before making the request to exchange the code for a
token. If the client is using the default OAuth2 client_id and
client_secret these values will be sent to the malicious endpoint if
the client can't verify the endpoints before hand.
Also, it would be nice to add some non-normative examples to the spec.
Thanks,
George
On 1/11/16 4:27 AM, Mike Jones wrote:
Yesterday Hannes Tschofenig announced an OAuth Security Advisory
on Authorization Server Mix-Up
<https://mailarchive.ietf.org/arch/msg/oauth/JIVxFBGsJBVtm7ljwJhPUm3Fr-w>.
This note announces the publication of the strawman OAuth 2.0
Mix-Up Mitigation draft he mentioned that mitigates the attacks
covered in the advisory. The abstract of the specification is:
This specification defines an extension to The OAuth 2.0
Authorization Framework that enables an authorization server to
provide a client using it with a consistent set of metadata
about itself. This information is returned in the authorization
response. It can be used by the client to prevent classes of
attacks in which the client might otherwise be tricked into
using inconsistent sets of metadata from multiple authorization
servers, including potentially using a token endpoint that does
not belong to the same authorization server as the authorization
endpoint used. Recent research publications refer to these as
"IdP Mix-Up" and "Malicious Endpoint" attacks.
The gist of the mitigation is having the authorization server
return the client ID and its issuer identifier (a value defined
in the OAuth Discovery specification
<http://self-issued.info/?p=1496>) so that the client can verify
that it is using a consistent set of authorization server
configuration information, that the client ID is for that
authorization server, and in particular, that the client is not
being confused into sending information intended for one
authorization server to a different one. Note that these
attacks can only be made against clients that are configured to
use more than one authorization server.
Please give the draft a quick read and provide feedback to the
OAuth working group. This draft is very much a starting point
intended to describe both the mitigations and the decisions and
analysis remaining before we can be confident in standardizing a
solution. Please definitely read the Security Considerations
and Open Issues sections, as they contain important information
about the choices made and the decisions remaining.
Special thanks go to Daniel Fett (University of Trier),
Christian Mainka (Ruhr-University Bochum), Vladislav Mladenov
(Ruhr-University Bochum), and Guido Schmitz (University of
Trier) for notifying us of the attacks and working with us both
on understanding the attacks and on developing mitigations.
Thanks too to Hannes Tschofenig for organizing a meeting on this
topic last month and to Torsten Lodderstedt and Deutsche Telekom
for hosting the meeting.
The specification is available at:
·http://tools.ietf.org/html/draft-jones-oauth-mix-up-mitigation-00
An HTML-formatted version is also available at:
·http://self-issued.info/docs/draft-jones-oauth-mix-up-mitigation-00.html
-- Mike
P.S. This note was also posted at
http://self-issued.info/?p=1524 and as @selfissued
<https://twitter.com/selfissued>.
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Identity Services Engineering Work:george.fletc...@teamaol.com
<mailto:george.fletc...@teamaol.com>
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Mobile: +1-703-462-3494 Twitter:http://twitter.com/gffletch
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Chief Architect
Identity Services Engineering Work:george.fletc...@teamaol.com
<mailto:george.fletc...@teamaol.com>
AOL Inc. AIM: gffletch
Mobile: +1-703-462-3494 Twitter:http://twitter.com/gffletch
Office: +1-703-265-2544 Photos:http://georgefletcher.photography
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