Re: [OAUTH-WG] OAuth2 Implementation questions (client secret and refresh tokens)

[Dave Rochwerger <[e-mail]>]

Is this a security issue in the OAuth2 process then (for mobile apps using
the authorization code flow)?

1. The draft says that mobile apps should be considered public clients
because mobile apps can be decompiled and can not keep their credentials
private.
2. In this case then, the draft says for these mobile apps to not
authenticate with the secret and instead for the server to verify the
redirect URI (and make it mandatory).
3. You said that verifying the redirect URI is not good enough to verify the
client's identity.

What am I missing?

Thanks,
Dave


On Wed, Sep 14, 2011 at 12:51 PM, Torsten Lodderstedt <
[e-mail]> wrote:

> Hi Dave,
>
> redirect URI validation does not authenticate a client. For example, a URI
> registered for a private web client could be used by a (malicious) native
> app to assume the web app's identity. The client secret, in contrast, can be
> used to authenticate it.
>
> regards,
> Torsten.
>
> Am 14.09.2011 19:12, schrieb Dave Rochwerger:
>
>  Thanks for the follow up, Torsten.
>>
>> Whilst I have your attention - any thoughts on my second question,
>> about the use of a client secret?
>> If for all clients we mandated registered URIs and verified them
>> (whether they are private and public), what additional security does
>> the client secret actually provide for private clients in the
>> authorization code flow?
>>
>>
>> Thanks,
>> Dave
>>
>> On Wed, Sep 14, 2011 at 7:20 AM, Torsten Lodderstedt
>> <[e-mail]>  wrote:
>>
>>> Hi Dave,
>>>
>>>
>>> On Wed, 7 Sep 2011 17:22:14 -0700, Dave Rochwerger wrote:
>>>
>>>  1. "The user does not have to be present."
>>>> Maybe I should be more clear. What benefit does that have over just a
>>>> long-lived (forever) access token? The cost is the extra complication
>>>> for
>>>> 3rd party developers to have to worry about refresh tokens. I can not
>>>> see
>>>> a benefit in our model (everything over SSL, etc) to use refresh tokens.
>>>> I want to use refresh tokens - but only if there is a reason for them,
>>>> which I can not see at the moment.
>>>>
>>>
>>> The benefit of refresh tokens significantly depends on your access token
>>> design. If your access tokens are just a pointer to a database you lookup on
>>> any API call, the only benefit if token rotation (coming back to this topic
>>> below). But your access tokens could also directly contain all user data you
>>> need to actually authorize API access. That way you could save DB lookups,
>>> which scales much better. In this model, revocation is much can be easier
>>> implement using refresh tokens. I think this is what Eran refered to.
>>>
>>>
>>>  2. "As Eran points out, you'd have to have do a DB lookup to have true
>>>> revocation."
>>>> The act of revoking tokens is not a common occurrence, DB lookups to
>>>> revoke tokens is not a concern as there is more time spent by the user
>>>> navigating the UI (or network latency, etc) than the cost of the DB
>>>> call.
>>>>
>>>> 3. "In this sense you get the best of a long-lived credential, combined
>>>> with good key rotation and authorization re-verification without having
>>>> to re-involve the end-user."
>>>> That all sounds good, but in our situation (all SSL, etc) - what do we
>>>> want key rotation and re-verification for? I fail to see a reasonable
>>>> vector for access token leakage to warrant any of this in our case.
>>>>
>>>
>>> rotation is a mean to detect tokem theft from the device (see also
>>> http://tools.ietf.org/html/**draft-lodderstedt-oauth-**
>>> security-01#section-4.1.2<http://tools.ietf.org/html/draft-lodderstedt-oauth-security-01#section-4.1.2>
>>> ).
>>>
>>> regards,
>>> Torsten.
>>>
>>>  On Wed, Sep 7, 2011 at 5:08 PM, Phil Hunt wrote:
>>>>
>>>>  See below...
>>>>>
>>>>> Phil
>>>>> @independentid
>>>>> www.independentid.com [11] [e-mail] [12]
>>>>>
>>>>>
>>>>> On 2011-09-07, at 4:57 PM, Dave Rochwerger wrote:
>>>>>
>>>>>  Hi Phil,>>  The client is then forced to periodically reauthenticate
>>>>>> (without the user) before getting a new access token.
>>>>>> What benefit does that have?
>>>>>>
>>>>> The user does not have to be present.
>>>>>
>>>>>  Refresh also gives the authzn server a chance to revoke access.
>>>>>>>>
>>>>>>> Hence it is better to use shorter lived access tokens with long lived
>>>>>> refresh tokens.
>>>>>> That doesn't follow - we can just as easily revoke the single
>>>>>> long-lived access token.
>>>>>>
>>>>> As Eran points out, you'd have to have do a DB lookup to have true
>>>>> revocation. But, by having a short expiration time on the access token
>>>>> (say 1 hour or less), you get quasi-revocation which has to be
>>>>> re-validated after the access token expires and the client has to
>>>>> re-authenticate and provide a valid refresh token. In this sense you
>>>>> get the best of a long-lived credential, combined with good key
>>>>> rotation and authorization re-verification without having to re-involve
>>>>> the end-user.
>>>>>
>>>>> Dave.>
>>>>>
>>>>>> On Wed, Sep 7, 2011 at 4:24 PM, Phillip Hunt wrote:
>>>>>>
>>>>>>  You can also use a long lived refresh token in combination with a
>>>>>>> short access token. The client is then forced to periodically
>>>>>>> reauthenticate (without the user) before getting a new access
>>>>>>> token.
>>>>>>> Refresh also gives the authzn server a chance to revoke access.
>>>>>>> Hence it is better to use shorter lived access tokens with long
>>>>>>> lived refresh tokens.
>>>>>>>
>>>>>>> Phil
>>>>>>>
>>>>>>> On 2011-09-07, at 15:27, William Mills wrote:
>>>>>>>
>>>>>>>  I'll talk to the refresh token question: they give you a hook for
>>>>>>>> extensibility and key rotation. If you want to rotate your
>>>>>>>> encryption keys or extend the data carried in the token in any
>>>>>>>> way then you want to be able to cleanly refresh your tokens. Note
>>>>>>>> that the refresh flow allows you to issue a new refresh token at
>>>>>>>> the same time. It also allows a clean path to convert tokens in a
>>>>>>>> new client if you decide you want SAML tokens instead of MAC for
>>>>>>>> example.
>>>>>>>> If you want those things you want to use refresh tokens. You can
>>>>>>>> have long lived access tokens too, and just use the refresh
>>>>>>>> tokens when you want to do something new with the access tokens.
>>>>>>>> -bill
>>>>>>>>
>>>>>>>> -------------------------
>>>>>>>> FROM: Dave Rochwerger
>>>>>>>> TO: [e-mail] [2]
>>>>>>>> CC: Quizlet Dev Team
>>>>>>>> SENT: Wednesday, September 7, 2011 2:15 PM
>>>>>>>> SUBJECT: [OAUTH-WG] OAuth2 Implementation questions (client
>>>>>>>>
>>>>>>>> secret and refresh tokens)
>>>>>>>>
>>>>>>>> Hi all,
>>>>>>>> I have been implementing OAuth2 based on the various drafts for
>>>>>>>> our new API. Initially, I implemented everything as per the spec,
>>>>>>>> but due to our particular scenario and restrictions we have in
>>>>>>>> place, there are some fundamental questions that I am unable to
>>>>>>>> defend.
>>>>>>>> I am hoping this group could help answer them for me.
>>>>>>>> Our scenario:
>>>>>>>> ==========
>>>>>>>> * We are implementing an API to allow 3rd party developers to
>>>>>>>> access users' protected resources via their applications. The
>>>>>>>> applications will mostly be native phone apps, but some will have
>>>>>>>> web server backends (javascript-only applications are not a
>>>>>>>> concern at the moment).
>>>>>>>> * We want to provide very long-lived (forever) tokens.
>>>>>>>> * We are implementing the "authorization code" flow as that seems
>>>>>>>> best suited to us (we don't want the implicit flow because
>>>>>>>> end-users would have to re-authorize every hour).
>>>>>>>> Our architecture:
>>>>>>>> ============
>>>>>>>> * We control both the API server and the authorization server.
>>>>>>>> * All requests to protected resources (ie: to the API server) are
>>>>>>>> always done over SSL.
>>>>>>>> * All requests to the authz server (token and authorize
>>>>>>>> endpoints) are always done over SSL.
>>>>>>>> * We enforce that every client must supply the state parameter
>>>>>>>> (and our guidelines say they must verify the state for CSRF
>>>>>>>> mitigation).
>>>>>>>> * We enforce that every client must register a redirect URI.
>>>>>>>> * We validate the redirect_uri used to request an access token is
>>>>>>>> the same that was used to obtain the auth code.
>>>>>>>> * The only time a request is not made over SSL is the redirect
>>>>>>>> with the auth_code which is very short-lived (30 seconds) and is
>>>>>>>> tied to a verified redirect URI.
>>>>>>>> * We enforce that access tokens must be provided using the
>>>>>>>> Authorization header only (and of course, over SSL).
>>>>>>>> * We have guidelines saying that all mobile apps must use the
>>>>>>>> native browser (and not an embedded web UI).
>>>>>>>> Questions:
>>>>>>>> ========
>>>>>>>> 1. Given the above scenario, what use are refresh tokens?
>>>>>>>> - Access tokens can not leak because every request (to resource
>>>>>>>> and authz server) containing an access token is done over SSL. We
>>>>>>>> control both the authz and resource servers, so tokens in logs
>>>>>>>> (and other suggested reasons in the archives) are not an issue.
>>>>>>>> - Long-lived refresh tokens and short-lived access tokens are
>>>>>>>> supposed to provide security due to possible access token
>>>>>>>> leakage... but in our 100% SSL scenario, if access tokens are
>>>>>>>> able to leak, then so would the client id, secret and refresh
>>>>>>>> token.
>>>>>>>> - Having a long-lived refresh token that can be exchanged for
>>>>>>>> another access token adds a level of complexity (a second HTTPS
>>>>>>>> request every so often) and seems to provide no benefit for our
>>>>>>>> case.
>>>>>>>> 2. What is the point of the client secret (in our scenario)? -
>>>>>>>> We originally were treating the clients as confidential, but
>>>>>>>> after re-reading the native-application section, it seems we
>>>>>>>> really should treat them as public (phone apps can be decompiled
>>>>>>>> and the secret discovered).
>>>>>>>>
>>>>>>>> - The spec says that the authz server should authenticate
>>>>>>>> confidential clients, but public clients are allowed to just send
>>>>>>>> their public client id (and no secret).
>>>>>>>> - The only verification then, is to enforce redirect URI
>>>>>>>> registration and to validate the redirect URI between
>>>>>>>> authorization and token steps.
>>>>>>>>
>>>>>>>> So, the question is, assuming that we, one: "enforce
>>>>>>>> redirect-URI registration" and two: "validate that URI" - why
>>>>>>>> can't we treat all clients as public and not worry about a
>>>>>>>> secret?
>>>>>>>> What is the benefit of having confidential clients (and a secret)
>>>>>>>> at all?
>>>>>>>> Our API source is not available, but the oauth2 server
>>>>>>>> implementation can be seen here:
>>>>>>>> https://github.com/quizlet/**oauth2-php<https://github.com/quizlet/oauth2-php>[4]
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Dave
>>>>>>>>
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>>>>>>>>
>>>>>>>
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>>>>>>>>
>>>>>>>
>>>
>>>
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