[Openid-specs-ab] Defining a Hardened (Mix-up and Cut-and-Paste Proof) OpenID Connect Profile

Torsten Lodderstedt torsten at lodderstedt.net
Thu Apr 14 18:47:48 UTC 2016

Hi William,

Am 14.04.2016 um 18:22 schrieb William Denniss:
> extensions to OP:
>     - issue ID token in the front channel
>     - add c_hash to id token
> I would rephrase these two extensions as "supporting the hybrid flow", 
> c_hash is a REQUIRED claim for the hybrid flow, the conformance test 
> ID for this is: OP-IDToken-c_hash

That's certainly right. I wanted to list all the necessary changes an OP 
Basic needs to implement in order to support this profile.

> ...
>     Apperently, this solution would not work for plain OAuth.
>     Moreover, I personally think this significantly adds complexity
>     (and cost) to both ends, OP as well as RPs, in comparison to other
>     potential mitigations. Experience shows that complexity is the
>     enemy of correct/reliable implementations, which is esp. dangerous
>     when it comes to security for mass market/internet services.
> We have a solution in Connect. We should document that solution.

We have a solution for hybrid OPs, not basic OPs. And as I said, it does 
not work for OAuth. So even if a basic OP is extended to become an 
hybrid OP, there is no solution for the OAuth use cases, which run on 
the same endpoint.

> The Connect solution requires no changes to Connect. The proposal here 
> is simply documenting what people should do if they want to mitigate 
> these potential attacks.

The proposal might not require changes to Connect, but it requires 
significant changes to basic OPs and RPs. That's what I wanted to point 
out. There might be a significant difference between what is documented 
in a spec and deployed reality. Do we have insights, how many _deployed_ 
OPs and RPs in the wild nowadays use hybrid flows (code id_token 

> I don't believe the Connect WG should be constrained to solve problems 
> without using the features of Connect.
>     So from my perspective, there are two questions:
>     1) Can we find a solution for OIDC and OAuth (for code)?
> That solution may well be: use Connect.  At least, if you wanted to 
> solve it today without any spec changes or extensions, that would be 
> my advice.

Hmmm, I don't intend to issue id tokens to OAuth clients and I don't 
want to make of client any bit harder than absolutely necessary. That 
has been the reason for the success of OAuth 2.0 and OpenId Connect.

>     2) Can we make it a simpler one?
> I'm listening.

As you said, checking nonce (even on the backchannel) is sufficient to 
detect/prevent cut and paste. An alternative solution would be to link 
the state to the code and provide the state value for validation to the 
token endpoint.

With respect to mix up, there are several potential solutions:
- clients/RPs use different redirect uris for different ASs/OPs (as 
originally proposed by the researchers, who discovered the attack)
- provide the client/RP with issuer and client_id as response query 
- provide the client/RP with the token endpoint the particular code is 
good for 

best regards,
>     best regards,
>     Torsten.
>     Am 13.04.2016 um 06:22 schrieb Mike Jones:
>>     It is always mandatory for the server to include the nonce in the
>>     ID Token if provided in the request and it is likewise always
>>     mandatory for the RP to validate the nonce, if present in the ID
>>     Token.  For all response_type values other than “code”, including
>>     a nonce in the request is also mandatory.
>>     -- Mike
>>     *From:*Openid-specs-ab
>>     [mailto:openid-specs-ab-bounces at lists.openid.net] *On Behalf Of
>>     *John Bradley
>>     *Sent:* Tuesday, April 12, 2016 5:31 PM
>>     *To:* William Denniss <wdenniss at google.com>
>>     <mailto:wdenniss at google.com>
>>     *Cc:* openid-specs-ab at lists.openid.net
>>     <mailto:openid-specs-ab at lists.openid.net>
>>     *Subject:* Re: [Openid-specs-ab] Defining a Hardened (Mix-up and
>>     Cut-and-Paste Proof) OpenID Connect Profile
>>     If they send it and don’t detect if it has changed from the
>>     request then that is a definite error.
>>     If they don’t send it, it should be a warning as there are some
>>     cases with a simple preconfigured client that not checking may be OK.
>>     I personally argued at the time to always check it, so I am not
>>     going to push back very hard on making it mandatory to check
>>     other than it is a change to the spec.
>>     John B.
>>         On Apr 12, 2016, at 5:34 PM, William Denniss
>>         <wdenniss at google.com <mailto:wdenniss at google.com>> wrote:
>>         On Tue, Apr 12, 2016 at 2:28 PM, John
>>         Bradley<ve7jtb at ve7jtb.com <mailto:ve7jtb at ve7jtb.com>>wrote:
>>             The AS is all-ready required to include nonce in the
>>             id_token if it is present in the request.  That is a MUST
>>             in the spec.
>>             That should be a test all-ready.
>>         There is: /ID Token has nonce when requested for code flow
>>         [Basic] (OP-nonce-code)///
>>         So for this, it's more that we need to add an RP test to
>>         ensure that RPs are actually sending and verifying nonce.
>>             Basically fragment encoding is not a good idea any more
>>             other than for JS in the browser or for native apps using
>>             view controllers or system browsers.
>>             Servers really should support the form post response mode.
>>             A client that wants to be secure can send nonce and use
>>             “code id_token” if they want offline or “token id_token”
>>             if they don’t want offline.
>>             That is secure.
>>             There are other mitigations we talked about in OAuth with
>>             returning the client_id and iss as parameters rather than
>>             returning a id_token.
>>             The client really only needs to check those two values in
>>             the front channel, but I would not want to change the
>>             spec to say that.
>>         +1 to all the above
>>             John B.
>>                 On Apr 12, 2016, at 4:58 PM, William Denniss
>>                 <wdenniss at google.com <mailto:wdenniss at google.com>> wrote:
>>                 On Tue, Apr 12, 2016 at 12:50 PM, Hans
>>                 Zandbelt <hzandbelt at pingidentity.com
>>                 <mailto:hzandbelt at pingidentity.com>> wrote:
>>                     from an implementers/deployment standpoint I
>>                     would argue that if the id_token is delivered in
>>                     the front channel, I may just as well get the
>>                     access token from there too rather than having to
>>                     deal with the overhead of calling the token
>>                     endpoint, esp. since at_hash is there to protect
>>                     integrity
>>                     so I'd favor response_mode=form_post,
>>                     response_type="id_token token" over hybrid
>>                     I realize that this doesn't leverage the
>>                     possibility of using a client secret to get the
>>                     access token but it doesn't really seem to matter
>>                     security-wise at this point as the id_token was
>>                     delivered without leveraging it as well
>>                     anything wrong with that line of reasoning?
>>                 1) Server won't have offline access with "id_token
>>                 token". 2) The primary case we are dealing with today
>>                 is the code flow, and we are really just suggesting
>>                 clients add security checks onto their existing flows
>>                 by leveraging id_token.
>>                 It's true that you could do many of the same checks
>>                 with "id_token token" to verify that the access token
>>                 was issued by the correct party and avoid
>>                 cut-and-paste and mix-up, if you only needed a
>>                 once-off access token.
>>                 You raise an interesting point regarding client
>>                 confidentiality.
>>                     On 4/12/16 9:23 PM, William Denniss wrote:
>>                         Good point.
>>                         Regarding the OP tests, the following are
>>                         relevant to mitigate the
>>                         cut-and-paste and mix-up attacks:
>>                          1. ID Token has nonce when requested for
>>                         code flow [Basic] (OP-nonce-code)
>>                          2. Request with response_mode=form_post
>>                         [Extra] (OP-Response-form_post)
>>                         1) is important for preventing cut-and-paste
>>                         (the id token needs to
>>                         contain the 'nonce')
>>                         2) is important for preventing mix-up as it
>>                         means the redirect endpoint
>>                         gets the id_token on the response at the
>>                         server, as opposed to in the
>>                         URI fragment.
>>                         Unfortunately, form_post is optional for OPs,
>>                         and sending the nonce on
>>                         the code flow is optional for RPs (though
>>                         fortunately to it is
>>                         compulsory for OPs to support thanks to
>>                         OP-nonce-code).
>>                         We could add an hardened OP test for:
>>                         – Forcing nonce to be present on the code flow
>>                         We should definitely have RP tests for:
>>                         – sending and validating nonce on the code flow
>>                         – validating the c_hash, iss, aud on the
>>                         hybrid flow
>>                         How we would profile these tests I'm not
>>                         sure; would they go in the
>>                         Basic testing profile, or in a new Hardened
>>                         one?  We could move
>>                         OP-Response-form_post to the Basic profile if
>>                         we wanted to be
>>                         opinionated, or define a new profile.
>>                         The good news is that supporting the features
>>                         required to mitigate
>>                         mix-up & cut-and-paste is not all that hard
>>                         to do in Connect.
>>                         On Tue, Apr 12, 2016 at 10:46 AM, Nick Roy
>>                         <nroy at internet2.edu <mailto:nroy at internet2.edu>
>>                         <mailto:nroy at internet2.edu
>>                         <mailto:nroy at internet2.edu>>> wrote:
>>                         Would it be possible to check for the secure
>>                         behavior in Roland's
>>                         test suite and either not certify
>>                         non-mitigating implementations, or
>>                         offer a risk mitigation add-on cert for those
>>                         that do the right thing?
>>                         Nick
>>                         From: Openid-specs-ab
>>                         <openid-specs-ab-bounces at lists.openid.net
>>                         <mailto:openid-specs-ab-bounces at lists.openid.net>
>>                         <mailto:openid-specs-ab-bounces at lists.openid.net
>>                         <mailto:openid-specs-ab-bounces at lists.openid.net>>>
>>                         on behalf of
>>                         William Denniss <wdenniss at google.com
>>                         <mailto:wdenniss at google.com> <mailto:wdenniss at google.com
>>                         <mailto:wdenniss at google.com>>>
>>                         Date: Tuesday, April 12, 2016 at 11:01 AM
>>                         To: "openid-specs-ab at lists.openid.net
>>                         <mailto:openid-specs-ab at lists.openid.net>
>>                         <mailto:openid-specs-ab at lists.openid.net
>>                         <mailto:openid-specs-ab at lists.openid.net>>"
>>                         <openid-specs-ab at lists.openid.net
>>                         <mailto:openid-specs-ab at lists.openid.net>
>>                         <mailto:openid-specs-ab at lists.openid.net
>>                         <mailto:openid-specs-ab at lists.openid.net>>>
>>                         Subject: [Openid-specs-ab] Defining a
>>                         Hardened (Mix-up and
>>                         Cut-and-Paste Proof) OpenID Connect Profile
>>                         One item that came out of the discussions on
>>                         the sidelines of IETF95
>>                         with folk from this WG (specifically Nat,
>>                         Mike, John, Brian and
>>                         myself) was the need for the Connect
>>                         community to respond to the
>>                         recently <http://arxiv.org/abs/1508.04324v2/>
>>                         documented
>>                         <http://arxiv.org/abs/1601.01229v2/> security
>>                         threats.
>>                             Connect is actually in a much stronger
>>                         place for mitigating these
>>                             attacks (as noted in the papers
>>                         themselves) than pure OAuth, with
>>                             the id_token offering a cryptographic
>>                         binding of the code to the
>>                             issuer, audience and session identifier
>>                         (nonce).
>>                             However, certain steps need to be
>>                         followed, for example using
>>                             'nonce' with the code flow (which is
>>                         optional to implement for
>>                             clients) to protect against
>>                         cut-and-paste, and using the form-post
>>                             response type with the hybrid flow to
>>                         verify that the code was
>>                             issued by the expected IdP, to ensure the
>>                         code is exchanged at the
>>                             correct token endpoint (mitigating mix-up).
>>                             We discussed last week creating a profile
>>                         of Connect that recommends
>>                             those practices to mitigate these classes
>>                         of attack as a response to
>>                             the security researchers' findings. I
>>                         wanted to share that
>>                             suggestion with the list, and continue
>>                         the conversation.
>>                         _______________________________________________
>>                         Openid-specs-ab mailing list
>>                         Openid-specs-ab at lists.openid.net
>>                         <mailto:Openid-specs-ab at lists.openid.net>
>>                         http://lists.openid.net/mailman/listinfo/openid-specs-ab
>>                     -- 
>>                     Hans Zandbelt             | Sr. Technical Architect
>>                     hzandbelt at pingidentity.com
>>                     <mailto:hzandbelt at pingidentity.com> | Ping Identity
>>                 _______________________________________________
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