Re: [OAUTH-WG] OAuth Security Topics -- Recommend authorization code instead of implicit

[Torsten Lodderstedt <[e-mail]>]

Hi Mike, 

I agree that OIDC hybrid flows offer additional security over the OAuth implicit grant and are used in the wild. On my slides and in the initial version of the new section, we had included the hybrid OIDC flows because of their known token injection countermeasures.

I nevertheless feel very uncomfortable to recommend those flows and any flow issuing access tokens in the front channel. In the course of the detailed review of the new text we realized two issues: 

1) Since the access token is exposed in the URL, such flows possess a significantly higher risk to leak the access token (e.g. through browser history, open redirection and even referrer headers) than the code grant.
2) There is no viable way to sender constrain access tokens issued in the front channel. Given the WG decided to recommend use of sender constraint tokens (https://tools.ietf.org/html/draft-ietf-oauth-security-topics-09#section-2.2), it seems contradictory to recommend response types not supporting such an approach. 

kind regards,
Torsten. 

> Am 19.11.2018 um 23:13 schrieb Mike Jones <Michael.Jones=[e-mail]>:
> 
> This description of the situation is an oversimplification.  OpenID Connect secures the implicit flow against token injection attacks by including the at_hash (access token hash) in the ID Token, enabling the client to validate that the access token was created by the issuer in the ID Token (which is also the OAuth Issuer, as described in RFC 8414).  (Note that this mitigation was described in draft-ietf-oauth-mix-up-mitigation.)
>  
> Given the prevalence of this known-good solution for securing the implicit flow, I would request that the draft be updated to describe this mitigation.  At the same time, I’m fine with the draft recommending the code flow over the implicit flow when this mitigation is not used.
>  
>                                                                 Thank you,
>                                                                 -- Mike
>  
> From: OAuth <[e-mail]> On Behalf Of Hannes Tschofenig
> Sent: Monday, November 19, 2018 2:34 AM
> To: oauth <[e-mail]>
> Subject: [OAUTH-WG] OAuth Security Topics -- Recommend authorization code instead of implicit
>  
> Hi all,
>  
> The authors of the OAuth Security Topics draft came to the conclusion that it is not possible to adequately secure the implicit flow against token injection since potential solutions like token binding or JARM are in an early stage of adoption. For this reason, and since CORS allows browser-based apps to send requests to the token endpoint, Torsten suggested to use the authorization code instead of the implicit grant in call cases in his presentation (seehttps://datatracker.ietf.org/meeting/103/materials/slides-103-oauth-sessb-draft-ietf-oauth-security-topics-01).
>  
> A hum in the room at IETF#103 concluded strong support for his recommendations. We would like to confirm the discussion on the list.
>  
> Please provide a response by December 3rd.
>  
> Ciao
> Hannes & Rifaat
>  
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